What the Builder's Opening Will Give Back
The crate is in the hall with the pallet still under it and the delivery note says a hundred and sixty kilos. Nobody has moved it, which is the right decision, because where it can stand is settled by four measurements taken at the opening and one taken under the floor — and if any of them comes up short, the stove goes back on the pallet rather than into the room.
Take the recess at the back, not the front. Old openings splay: the jambs run outwards towards the room, so the width read at the lintel line flatters the width at the back wall by an easy hundred millimetres, and it is the back wall the appliance has to stand clear of. Then the height under the lintel or the arch, read at its lowest point rather than at the middle of the span, and the depth from the face of the breast through to the back. Write all four down before you open the manual, because the manual is about to ask for gaps, and gaps are subtracted from what you have.
Those gaps are not all the same kind of thing, and readers routinely apply the wrong one. A stove manual gives a distance to a combustible surface, which is the safety figure and comes off the appliance's own type test — BS EN 13240 in Europe, UL 1482 in North America. Many manuals also give a second, smaller minimum gap to a non-combustible surface, and that one is not about fire at all. A convecting body needs air to travel up its sides and out across the top, and a stove boxed tightly into brick on three faces runs hotter than the design intends and eventually distorts. A recess that satisfies the fire figure with nothing to spare can still be the wrong recess.
Then there is what the opening is hiding. An oak bressummer over an inglenook is combustible material inside masonry, and the distance applies to it exactly as it would to a stud wall, whatever the render in front of it looks like. So are timber lintels behind a plastered breast, timber pattress plates, joist ends built into the breast in a Victorian terrace, and the batten a previous owner screwed inside the recess to hang a fire surround on. Approved Document J and NFPA 211 both start from the position that timber built into or against a chimney breast counts as timber. Get a board up and a torch in before accepting that an opening is all brick.
Two Hearths, One Floor
Hearth is two components wearing one word, and a retrofit comes apart at the seam between them. The constructional hearth is part of the building: a thick non-combustible slab whose job is to keep heat away from the structure underneath, and which does not care in the slightest what the stove looks like. The superimposed hearth is the piece the customer chose in the showroom — toughened glass, riven slate, honed granite, a plate of steel — and it lies on top of the first one. Buying a beautiful twelve-millimetre slab and bedding it on floorboards produces a hearth in roughly the sense that a doormat is a hearth.
In England and Wales the dimensions come from Approved Document J's hearth diagrams and are not open to taste. A constructional hearth for a solid fuel appliance is at least 125 mm thick, and where it does not sit inside a fireplace recess it is at least 840 mm by 840 mm on plan. The superimposed layer above it is thin and non-combustible, 12 mm being the figure the document works to, and the appliance then has to stand back from its edges: 150 mm at the sides and back, 225 mm at the front, and further forward again where the appliance can be run with its door open. Wherever the manual asks for more than any of that, the manual wins — the appliance was tested and the document was generalised.
What sits below matters as much as what sits on top, and this is the part that decides whether the job is possible at all. The same diagrams keep combustible material out of the 250 mm beneath the hearth's top surface, unless there is at least 50 mm of air between the underside of the slab and the timber. On a suspended ground floor the honest consequence is usually to cut the boards away entirely, drop the slab between the joists onto its own bearing, and leave the void underneath ventilated rather than packed with anything.
North America is not the same route with different numbers on it. For a listed stove the floor protection is whatever the label and the manual say, because that is what the appliance was tested standing upon; for an unlisted one NFPA 211 sets out assemblies described as construction and thermal performance together, which makes the pad something to build up and add together rather than something to buy. Neither of those is the hearth extension table in the residential code's masonry fireplace chapter — 16 in forward and 8 in to the sides for an opening under six square feet, 20 in and 12 in for one at or above it. Those belong to a built-in masonry fireplace. Reaching for them because they are the figures with inches attached is the commonest way a freestanding appliance fails an inspection it looked ready for.
Finally, weigh the thing. Eurocode 1 puts unreinforced normal-weight concrete at 24 kN per cubic metre, so a 125 mm constructional hearth is roughly 3 kN per square metre of dead load before anything at all stands on it — against the 1.5 kN per square metre Category A imposed load a domestic floor is designed to in the UK National Annex. The slab on its own is twice the live load the floor was drawn for, and the appliance has not arrived yet. On a suspended floor that is a structural question with a structural answer: sleeper walls under the slab, an opening trimmed onto trimmers sized for what the cut joists were carrying, or a purpose-made supporting frame. It is not an extra joist and some optimism.
| Route | What it fixes | Where it is written |
|---|---|---|
| England and Wales | A constructional hearth 125 mm thick, 840 mm square where it is not inside a recess, with stated margins from the appliance to the hearth edges | Approved Document J, whose hearth diagrams also govern what may occupy the 250 mm below the hearth's top surface |
| A listed appliance in North America | Floor protection material, thickness and extent, tested with that appliance and valid only in the configuration that was tested | The listing label and installation manual, from testing to UL 1482; NFPA 211 and CSA B365 take the listing as the requirement |
| An unlisted appliance in North America | A minimum protection assembly stated as construction and thermal performance together, so it is added up rather than bought | NFPA 211 with the residential code adopting it — and never the masonry fireplace hearth extension table, which describes a different appliance |
A hearth in section, from the bearing up
- The appliance on its legs — stands back from the hearth edges by the margins in the manual, with the larger margin always at the loading door Wood Stove & Fireplace BTU Calculator
- Superimposed hearth — the thin non-combustible finish the customer chose, which protects nothing on its own and is not a hearth by itself
- Constructional hearth — the thick non-combustible slab that keeps heat off the structure, sized in plan and in thickness by the governing document Concrete Calculator
- Ventilated void and its bearing piers — the air separation between slab and timber, which stays empty because filling it is what removes the separation
- Timber floor, trimmed away — boards and joists cut back around the slab, with the cut ends carried on trimmers rather than on hope
- Sleeper wall and trimmer — the load path for a slab that weighs more per square metre than the floor was ever designed to carry
Where the Flue Passes Timber
Three routes out of the room, and only one of them meets structure. Lining the existing chimney keeps everything inside masonry that is already standing. Coming through the back wall and climbing the outside face puts the run in the weather but well away from anything that burns. Going straight up in twin-wall — through the bedroom, through the loft, out through the roof — is what people choose when there is no chimney at all, and it is the route where a pipe at several hundred degrees passes within a hand's width of dry softwood at every floor it crosses.
The distance that keeps them apart is a property of the chimney system rather than of the appliance, and it is published on the system itself. A European twin-wall product carries a BS EN 1443 designation on every length: temperature class, pressure class, condensate and corrosion classes, then a soot-fire letter with a number after it — so a designation ending G50 is a sootfire-resistant chimney with a 50 mm distance to combustible material. North American factory-built systems listed to UL 103 HT publish the equivalent in inches in their installation instructions. Two products of the same bore from two manufacturers can differ here, which is why the sheet that came in the box is the only acceptable source for the figure.
The opening is then the outer casing plus that distance on both sides, and in a retrofit the binding constraint is what the existing floor already gives you. Joists at 400 mm centres, 47 mm thick, leave roughly 353 mm clear between them. A casing measuring 200 mm across its outside, wanting 50 mm on each flank, needs 300 mm and drops straight in; the same casing at a 60 mm distance needs 320 mm and still fits. Take the casing up to 250 mm and it does not, and now a joist is being cut — which means a trimmer, hangers, and a load path for everything that joist was carrying. That is the point where this stops being a stove job and becomes a structural alteration with a stove at the end of it.
Fitting the opening is not the same as holding it. The distance has to survive at whichever face the pipe has drifted towards, not at the one it was measured from, so it is the plumb of the flue over the whole run rather than the geometry of any single hole that the figure finally rests on. Listed firestop plates on both faces of the floor locate the pipe and close the cavity, but they are not brackets and should not be the only thing setting its position. And whatever the space around the pipe turns out to be, it is not somewhere to push loft insulation back into afterwards; on a ventilated or air-cooled system that space is part of the chimney.
- Take the outer casing dimension off the chimney system's own installation sheet, not off the flue bore stamped on the stove's collar — they are different numbers and the larger one is what the hole has to clear.
- Read the distance to combustible material off the same document: from the designation printed on the pipe if it is a European system, from the listed instructions if it is a UL 103 HT one.
- Measure the actual clear width between joists on site, in the bay the flue is going through, because centres drift and the bay you get is not always the bay on the drawing.
- Where the opening will not sit between two existing joists, have the trimming detailed before the ceiling comes down, since the trimmer size depends on what the cut joist was carrying and that is not a decision to take on the morning of the fit.
- Repeat the exercise at every level the run crosses — the first floor, the ceiling below the loft, and the roof plane — because the tightest bay is rarely the first one.
Put in the outer casing width and depth from the chimney sheet, and the distance to combustible material from the same sheet, and it returns the opening with the distance added at both flanks rather than once across the hole. Read the width out of the breakdown and set it against the clear span you actually measured between joists, not against the nominal centres.
The width of the chimney or flue assembly itself.
The depth of the chimney or flue assembly itself.
The minimum clearance from the chimney/flue to any combustible framing, per the manufacturer's listing.
Required chase opening area
3.89 ft²
Clearance-to-combustibles is appliance- and chimney-system-specific per the manufacturer's listing and the applicable fuel gas/mechanical code (NFPA 211 governs solid-fuel chimneys) — always use the exact clearance from your specific unit's installation instructions, never an assumed value.
- Chase opening width
- 2.33 ft
- Chase opening depth
- 1.67 ft
They open the calculator with your figures already in it
Fireplace/Chimney Chase Framing Clearance Calculator: 3.89 ft² — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
What this calculation does not cover
- The airspace this opening creates has to stay empty for the life of the chase. It is the ventilated gap that keeps framing below its ignition temperature, so blowing attic insulation across it, or packing mineral wool into it because the chase feels cold, voids the very listing the clearance figure came from — the opening measures right and the framing still chars.
- Nothing here firestops the chase. Where it passes a floor or ceiling the gap around the chimney has to be closed at each level with the material the listing names, usually sheet metal or a listed firestop spacer, and a chase left open top to bottom is a vertical shaft that takes a basement fire into the attic in minutes.
- Flue termination height is a separate rule this page does not reach: the chimney has to finish at least 3 ft (0.91 m) above the roof where it passes through and 2 ft (0.61 m) above anything within 10 ft (3 m) of it. A chase framed to exactly the right clearance around a flue that stops short still pushes smoke back into the room every time the wind swings.
Running the Sizing Check Backwards
The stove is bought, so which stove is no longer an interesting question. Whether the one in the hall will be run the way it wants to be run is. Take the output off the plate, ask what floor area that output suits, and set it beside the room it is going into — the check is more useful in this direction, because the answer changes what you do rather than what you buy. A stove comfortably larger than its room does not get sent back; it gets fed smaller loads at a more open air setting instead of full loads shut right down, and it gets reloaded more often. That is a habit to hand over in writing at the end of the job, and it is most of the difference between a flue swept once a year and one swept twice.
What a floor-area estimate cannot see is the shape of the space. It works on a length, a width and a climate adjustment, so a room with a two-storey void above it, an open stair carrying warm air upstairs, and a sitting room that only shares air with the rest of the house when somebody leaves a door open all read identically to it. Treat the result as a bracket, put the manufacturer's own rated heating area next to it, and let the two disagree in the open rather than trusting either on its own.
Give it the plan dimensions of everything the stove will really be asked to warm — doors open, stair included — pick the climate, then set the answer beside the output already stamped on the stove you bought. A wide gap either way is not a reason to change appliance. It is a reason to change how the appliance gets loaded.
The length of the area you want to heat.
The width of the area you want to heat.
Colder climates need proportionally more heating capacity per square foot.
Recommended heating capacity (BTU/hr)
23,900 BTU/hr
A rough planning estimate — actual heating needs also depend on insulation quality, ceiling height, and how open the floor plan is to adjacent rooms.
- Space area
- 598 sq ft
They open the calculator with your figures already in it
Wood Stove & Fireplace BTU Calculator: 23,920 BTU/hr — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Add the equipment this sizes
This result is a specification — 23,900 BTU/hr — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
What this calculation does not cover
- This sizes from floor area alone. There is no ceiling height or volume input, so a vaulted or cathedral room, a double-height space, or a stove on a lower floor with an open stairwell all read the same as a flat-ceilinged room of the same footprint, and all hold considerably more air to heat than this figure assumes.
- Insulation, air-tightness and glazing are not inputs. The climate selector is the only lever in the model, and it moves the answer across the whole 30-60 BTU per square foot band on its own. An uninsulated house with single glazing and a recently air-sealed one of the same footprint in the same town get an identical number here.
- This is a heat-output estimate, not an installation design. It says nothing about clearances to combustibles, hearth and floor protection, flue and chimney sizing, or combustion and make-up air for a tight house. A solid-fuel appliance install is governed by the manufacturer's listing and by local code and has to be inspected on that basis, not on a BTU figure.
- A manufacturer's published maximum output is not directly comparable to this number. Those ratings come from a laboratory test burn on prepared fuel; sustained output on your own cordwood, at your species and moisture content, is lower. Sizing to a peak rating and then damping the stove down to avoid overheating is what builds creosote in the flue.
- A stove is a single point source, and this assumes the heat reaches everything you measured. Closed doors, corridors and rooms off the stove room will stay cold while the stove room overheats, and no allowance is made for fans, ducting or a heat-distribution kit.
Free Area Is Smaller Than the Hole
Air is specified as a free area and bought as a product with a size, and those are two different measurements. What sits between them is blades, frames and mesh. A vent asked for a stated free area and satisfied with a grille of that gross size has delivered somewhere between a quarter and three quarters of what was asked for, depending entirely on what the grille happens to be made of.
Where the product publishes its own tested free area, use that figure and stop. Where it does not, the combustion air provisions of the International Mechanical Code and the International Fuel Gas Code set out what to assume instead, and the spread is wide enough to change the size of hole you cut. Count the whole path as well: an external terminal, a sleeve, an internal grille and an insect screen are four obstructions in series, not one, and the smallest of them governs.
The two regimes a reader is likely to meet disagree by more than an order of magnitude, and it is worth knowing why before splitting the difference between them. Approved Document J asks a closed solid-fuel appliance in England and Wales for 550 mm² of permanently open free area per kilowatt of rated output above 5 kW, so a nominal 8 kW stove wants around 1,650 mm² — a single modest air brick. The indoor air method in NFPA 54 and the International Fuel Gas Code asks instead for one square inch of free area per 1,000 BTU/hr of appliance input, through two openings, one near the ceiling and one near the floor — and never less than 100 square inches at either of them. That same 8 kW is roughly 27,000 BTU/hr, so the rate gives about 27 square inches at each opening and the 100 square inch floor is then what actually governs. The calculator below applies both, the rate and then the floor, and shows the unfloored figure beside the answer. Neither document is wrong. The first sizes a vent to outdoors on top of the background leakage a British dwelling is assumed to have; the second sizes transfer openings between a confined space and the rest of a building, moving room air across almost no pressure difference at all. Feed one document's number into the other's rule and the answer is out by a factor of ten before the floor is even reached.
Modern airtightness is why this section exists. Approved Document J's solid-fuel allowances assume a level of adventitious leakage that a dwelling built or retrofitted to a low design air permeability simply does not have, and the document is explicit that tighter houses need more; the appliance manual will usually be blunter still and require a dedicated supply. Where the stove is genuinely competing with a range hood on boost and a tumble dryer, the answer is a sealed appliance drawing its air from outdoors through a kit it is listed for, or an interlocked makeup air unit — not a larger hole in the same room, which under negative pressure becomes an exhaust path rather than a supply. A carbon monoxide alarm goes in either way, required by the residential codes wherever a solid-fuel appliance is installed, and it is the alarm rather than the arithmetic that will find a spillage nobody predicted.
| Obstruction | Free area to assume | Why that figure |
|---|---|---|
| Metal louvre or grille | 75 per cent of the gross opening | The single figure the IMC and IFGC combustion air provisions tell you to assume, and only where the product's own tested free area is unpublished — a deep-bladed louvre can measure well below it |
| Wood louvre | 25 per cent of the gross opening | The same provisions; timber blades are thicker and set closer, so a wooden vent has to be three times the size of a metal one to pass the same air |
| Insect screen behind either | Counted with the louvre, and only at a mesh the code will accept | Those codes set a minimum mesh size precisely because a finer screen blinds with lint and dust and gives no sign that it has |
| Anything with a published free area | The published figure, in preference to any table | The maker's own test data beats an assumption every time, and most air bricks and wall terminals now print it on the packaging |
This runs the rate in the NFPA 54 and IFGC indoor air method — one square inch per 1,000 BTU/hr, at each of two openings — which sizes a transfer opening for a confined space rather than an outdoor vent for a stove. It does not apply that method's 100 square inch minimum per opening, so add that yourself. Use it for the order of magnitude and for the free-area habit, then take the binding figure from the appliance manual and from whichever of Approved Document J, NFPA 211 or CSA B365 governs where you are building.
The fuel-burning appliance's total rated input, from its nameplate.
Minimum free area per opening
100 in²
This is the NFPA 54/IFGC indoor air method for a confined space, requiring TWO openings of this minimum free area each (one high, one low). Outdoor air methods use different (typically smaller) sizing factors, and unconfined spaces may not require dedicated combustion air openings at all — confirm which method and space classification applies to your installation with the full code section before finalizing opening size, as this is a single-method screening calculation, not a complete combustion air analysis.
- Free area before the 100 in² floor is applied
- 100 in²
They open the calculator with your figures already in it
Appliance Combustion Air Free Area Calculator (Indoor Air Method): 100 in² — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Add the equipment this sizes
This result is a specification — 100 in² — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
What this calculation does not cover
- Free area is not the size of the hole you cut. A grille or louver passes only its open fraction — roughly three-quarters for metal, as little as a quarter for wood — and an insect screen behind it takes more again, so a 100 in² (645 cm²) free-area requirement can need an opening of 130 to 400 in² (840 to 2,580 cm²) depending on what covers it. Size from the louver's published free area, or the appliance is starved through an opening that measures correctly.
- The openings only work if the space on the other side is large enough. This method assumes the adjoining room, plus everything freely communicating with it, holds at least 50 ft³ (1.4 m³) per 1,000 BTU/hr (0.3 kW) of combined appliance input; cutting two grilles into the partition of a small closet does not create combustion air, it shares one shortage between two rooms.
- Nothing here accounts for air being pulled out of the space. A kitchen hood, a clothes dryer or a bath fan can drop the room below atmospheric pressure and reverse an atmospheric flue no matter how generous the openings are, which is how a correctly sized opening still ends with a CO alarm — a house with substantial mechanical exhaust needs the appliance's air supply looked at as a pressure balance, not as an area.
The Bricks in the Box, and the Liner Above Them
Open the stove before it goes anywhere near the opening. Inside the firebox there will be a set of firebricks or vermiculite panels sitting in retaining lugs, and there will be a visible gap at their ends. That gap is not slack workmanship and it is not something that happened in transit. It is there because the panel is going to grow, the steel box around it is going to grow at a different rate, and the difference has nowhere to go but sideways.
The movement is small and it is not nothing. Fireclay refractory runs at somewhere around 5.5 parts per million per degree Celsius, so a 350 mm panel taken 500 degrees above a cold room grows by about a millimetre. A millimetre is trivial right up until it is a millimetre of solid ceramic pressing on a snug steel lug, at which point the panel cracks across a corner and the owner concludes the appliance was faulty. Replacement bricks are cut fractionally under size for the same reason, and forcing a tight one home is a repair that fails on the third fire. Firebox brick for residential appliances is specified in ASTM C1261, and vermiculite board is a different material again with its own data sheet.
The same coefficient governs the liner above, where the numbers grow large enough to have to be designed for. Clay flue liners to ASTM C315 are installed to the practice set out in ASTM C1283 and bedded in a refractory mortar formulated for heat and for a little movement — but a mortar is not a joint. A 4.5 m run through a two-storey stack, taken 450 degrees above the temperature it was built at, lengthens by around 11 mm, and that has to be accommodated somewhere: in the joints the system was designed with, in the gap left at the termination, or eventually in a lifted crown and a top course of brick pushed out of line. Ask for the accommodation detail rather than assuming the mortar will absorb it.
A flexible stainless liner is a different material and must not be run through the fireclay figure at all. Austenitic stainless moves roughly three times as much, which is exactly why those systems hang from a plate at the pot and are left free at the bottom instead of being mortared in at both ends; BS EN 1856-2 is the document covering metal liners and connecting flue pipes. Keep the fireclay coefficient for clay, firebrick and cast refractory, take a published value off the product's data sheet wherever the manufacturer gives one, and give the temperature field the rise from a cold installation up to peak rather than the reading off the flue itself.
It works at either scale. Put in 0.35 m and a 500 degree rise for a firebox panel and you get the millimetre that explains the gap at its ends; put in the vertical run of a clay liner and the rise from a cold install and you get the movement the top of the stack has to swallow every winter.
The total vertical run length of the firebrick liner section being checked.
The difference between installation temperature and expected peak operating temperature.
The specific firebrick material's coefficient of thermal expansion, per °C.
Expansion gap needed
0.515 in
The thermal expansion coefficient varies meaningfully between different firebrick/fireclay formulations — use the specific manufacturer's published value for an accurate result on a critical installation.
They open the calculator with your figures already in it
Firebrick Chimney Liner Thermal Expansion Calculator: 0.5148 in — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Add the equipment this sizes
This result is a specification — 0.515 in — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
What this calculation does not cover
- The whole run is treated as reaching the peak rise at once. A real flue is hottest at the appliance and cools toward the terminal, so a fully heat-soaked liner is a bounding case rather than what a tall stack sees in normal operation.
- This is the liner's own free expansion, not its movement relative to what encloses it. The masonry chimney, outer stack or steel casing expands as well, and the clearance that actually has to be left is the difference between the two, not the figure given here.
- The coefficient is applied as a single constant. A published value is a mean over the temperature band the manufacturer states, and refractory expansion is not linear across a wide range. Permanent, irreversible dimensional change from first firing or from moisture is a separate movement and is not included at all.
- Only the vertical run you enter is expanded. Circumferential growth of the liner and movement in the mortar or joint filler are not calculated, and the result says nothing about how many expansion joints to use, where they go, or how much of its own thickness a compressible joint material will actually take up.
- This is a movement figure, not an installation specification. It does not replace the liner manufacturer's expansion-joint and fixing instructions, and it checks nothing about clearance to combustibles, support and restraint of the liner, flue sizing, or the condition of the existing chimney.
Where a Winter of Wood Actually Sits
The first dimension the woodpile takes from the stove is log length, and it comes off a tape held inside the firebox rather than off the brochure. Measure the clear internal length between the brick panels, take off enough that a log can be laid flat without being wedged, then take off more again for getting it in past the door frame at an angle. What is left is the length to order, and it is also the depth of the store. Buying a hundred millimetres longer than the firebox turns every delivery into an afternoon with a saw.
Then double the store. Dense hardwood split large takes the better part of two summers to come down to a moisture content the appliance can actually burn, which means there are permanently two piles on the property: the one being burned and the one drying for the winter after. A store sized for a single season guarantees the second season is burned wet, and wet wood is what a chimney swept twice a year is usually reporting. Size the structure for two seasons, or build two structures.
Sizing it after that is stacked geometry and nothing more, and the arithmetic is unkind. One row, one log length deep at 400 mm, stacked to 1.2 m because higher than that falls over, holds 0.48 cubic metres per metre of run — so a full cord, 3.62 cubic metres, is a row seven and a half metres long. That is why a two-season store is two or three parallel rows and a long wall rather than one heroic stack, and why the garden usually decides the answer before the appliance does. Each row wants to stand on bearers clear of the ground, stay open at both ends so air travels along it, and be covered on top only — a wrapped pile is a compost heap with a roof. Take the three dimensions after the wood is stacked rather than while it is still a heap on the drive, because a stacked dimension is the only one anybody can argue about afterwards.
- Measure the firebox internally, set the log length from that, and then set the store depth to the log length rather than the other way round.
- Decide how many seasons the structure has to hold before fixing its length, because seasoning time and not annual consumption is what makes a store long.
- Stand it on bearers clear of the ground, leave both ends open, cover the top only, and keep the long sides open to the prevailing wind.
- Measure length, depth and height of the finished row and check the figure against what was ordered, on the ground, before the delivery driver leaves.
Take the length along the row, the depth front to back and the height off the bearers once the wood is stacked, and it converts the row into cords — which is how you find out whether the store you are about to build holds one winter or two.
The length of the stacked woodpile.
The front-to-back depth of the stack — usually the length of one log.
The height of the stacked pile.
Estimated firewood cord needed
1 cord
- Stack volume
- 128 cubic ft
They open the calculator with your figures already in it
Firewood Cord Calculator: 1 cord — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Estimated cost — your price
This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.
What this calculation does not cover
- It treats the pile as a rectangular block of length by width by height. A row that tapers, leans, domes at the top or stands on uneven ground holds less wood than the block implies, and a height taken from the ground rather than from the underside of the lowest log adds the bearers or pallet to the answer.
- The result is stacked volume with the air gaps included, which is how a cord is defined — it is not a measure of solid wood. Nothing here corrects for how tightly the row was built, for crooked or knotty rounds, or for a thrown load that has not been stacked at all; a loose pile of the same outer dimensions holds substantially less wood than a ranked one.
- Species, density and moisture content are not inputs, so a cord of pine and a cord of oak return the same figure. This sizes volume, not heat: it says nothing about usable heat output, burn hours, or whether the wood is dry enough to burn.
- It reports the stack as measured on the day. Wood shrinks across the grain and the pile settles as it seasons, so the same wood re-measured after a season reads as fewer cords, and this calculation does not project that change.
- This is not a determination of what was sold. It converts three measurements into cords; it cannot tell you whether a delivery matched the order, and it does not resolve terms such as face cord, rick or truckload, which carry no fixed depth and vary between sellers.
Six numbers to have on paper before the stove leaves the pallet
Each of these comes off a tape or off a document already in the house. The workspace opens on the flue opening with a placeholder casing and distance loaded — replace both with the figures from your own chimney system's sheet before reading anything off it.
- Outer casing dimension of the chimney system — Across the outside of the twin-wall, from the installation sheet. Not the flue bore stamped on the stove's collar, which is the smaller of the two numbers and the wrong one.
- Distance to combustible material for that system — From the BS EN 1443 designation printed on the pipe, or from the UL 103 HT instructions. It belongs on both flanks, so the opening grows by twice it and not by it.
- Clear width between joists in the bay the flue crosses — Measured on site in that bay, at every level the run passes. Centres drift, and the tightest bay is rarely the first one you open up.
- Connected floor area the appliance is expected to serve — Length and width of what genuinely shares air, run against the output already stamped on the stove as a check on a purchase rather than as a shortlist.
- Rated output in kilowatts and the same figure in BTU per hour — The air routes are written in different units and answer different questions. Carrying both stops one document's number being fed into the other document's rule.
- Internal firebox length, and the stacked dimensions of the store — The firebox sets the log length, the log length sets the store depth, and the store has to hold two seasons because one of them is still drying.
Opens the calculators above on one screen with the dimensions from this article already filled in. Quantities only — this site publishes no price list, because local prices vary too much to publish honestly.