Lumber and timber: board feet, studs, rafters, sheets, decks and stairs

What the lumber calculators cover

Lumber quantities start from a unit conversion and end in a cut list. The measure group comes first: board feet from thickness, width and length, board feet against cubic volume, the dressed size behind a nominal name, and the weight of a piece from its size, species and moisture. Then come the walls, the roof, the sheets, the stairs, the trim and the outdoor structures, each counted in the unit it is cut or sold in. The last group tests a number against a published method and does not design the member.

The roof group is about geometry: a common rafter's length from its run and pitch, the bird's mouth, the hip and valley cuts, dormer, gambrel and mansard rafters, and the chord lengths of simple trusses. Where the answer is a cut, the page gives the angle or the length so it can be marked out; where it is a count, the count is rounded up.

How the answers are worked out

The board foot is a unit of volume: thickness in inches times width in inches times length in feet, divided by twelve. Nominal sizes are names and not measurements, so the dimensional size calculator gives the dressed size a nominal two-by-four reaches after milling and drying, and the weight calculator takes its wood properties from the Wood Handbook of the USDA Forest Products Laboratory.

Sheet goods are counted from the sheet size of the market, one module in North America and another in metric markets, divided into the area and rounded up with a waste allowance. Trim is counted in stock lengths: the baseboard calculator names the stick lengths merchants stock in North America and the different run of lengths metric merchants stock, so the same room can take a different number of sticks in each.

Framing counts follow the spacing. The stud calculator cites IRC R602.3 and notes that AS 1684.2 and BS 8103 give equivalent spacing rules in Australia and the United Kingdom, and the bore and notch page tests a hole or a notch against a published limit before the frame needs a different detail.

Units and documents by market

The market setting chooses the system the answers appear in: inch-pound units for the United States, metric for the other three markets, with every calculator restating its answer in the other system. The site's market notes call the material lumber in the United States and Canada and timber in the United Kingdom and Australia. The stair calculator is the one page here that checks all four markets, citing IRC 2024, Approved Document K, the ABCB Housing Provisions Standard under the NCC, and the National Building Code of Canada 2020. The table shows what the other calculators cite for each market.

Lengths, waste and the cut list

Counts round up because lumber, trim and sheets are bought whole. The trim and decking calculators count whole stock lengths, the rafter calculators give the length and the cut of each member so that a cut list can be set out, and the deck board calculator takes the width of the board and the gap left between boards as inputs. A waste allowance is an input on the sheet and trim calculators, and each states the allowance it starts from.

Where to start, by job

Start from the job and not from the unit. A wall begins with the stud count and the plates; a floor with the joist run and the rim board; a roof with the pitch, which sets every rafter length and cut, or with the truss spacing when the roof is bought on a supplier's drawing; a deck with the joists, the ledger and the boards; stairs with the total rise, which the stair calculator divides into equal risers before it sets the going against the rules. Trim comes last, counted room by room in stock lengths. The measure group at the top is where any of these meets a unit it does not recognise.

What the calculators leave to others

The checks group tests a figure against a published method, such as the bending stress in a rectangular beam or the buckling load of a slender column, and the designer's detail, not a calculator, decides the member. The deck joist span page is likewise a check: it shows bending and deflection at a span and spacing for comparison with the allowable. The I-joist depth page describes itself as a pre-check and not a design, and the load duration page applies a factor from the National Design Specification table to a design value you supply. Moisture is what the shrinkage page isolates: it estimates how far timber moves across the grain between the moisture it is fixed at and the moisture it reaches in service, and it names the Eurocode 5 service classes it runs between.

Whole jobs, taken off

A job takeoff asks for the dimensions once and returns every line the job needs, each from its own calculator. These are the ones where lumber is a main line.

• Timber deck

  A rectangular deck: the boards, footings, fixings and finish from its dimensions.

• Framed exterior wall

  A new timber-framed wall: studs, insulation, sheathing, wrap and cladding, with every build-up on one page.

• Stud partition

  A single run of internal partition: the studs, the jointing compound and the outlets.

• Baseboard and casing

  Skirting around a room and casing around its openings, as lengths with a stick count pooled across the room.

• Staircase replacement

  A new timber flight: risers and treads checked against your own rules, the handrail up the pitch and a guard where needed.

• Garden room or backyard office

  A detached timber-framed room on a base: the frame, the sheathing and the roof deck that depend on lumber.

Read an answer backwards

When you start from what you hold and want to know what it will cover, these pages run the same calculators in reverse.

• How much subfloor will my plywood sheets cover?
• How big a deck will my deck boards cover?
• How long a wall will my studs frame?

Worked examples

A job worked through with its inputs and its working shown, so you can see the figures before you enter your own.

• Attached deck

  A deck against a house, from ledger and footings to boards and rail, each bought in the order the work runs.

• Stud partition with a door

  A partition framed with a door in it, so the studs at the opening are counted instead of assumed.

• Privacy fence

  A timber fence run: posts, rails and boards, and the concrete that holds the posts.

• An exterior wall's plates and headers, in lineal feet

  Plates, headers and the extra studs of a 24 ft exterior wall with two windows and a door

• A straight partition wall, in studs

  Studs for a 20 ft interior basement wall with one door

• A backyard deck, in deck boards

  Deck boards for a 16 by 12 ft deck off the back of a house

Reference tables

• Production rates by operation

  How long each operation takes one person, in person-hours per unit of work.

• Bulk densities of construction materials

  What a cubic metre of loose material actually weighs, in both systems, as this site's calculators compute it.

Guides

The practice behind the numbers: how the work is done, what goes wrong and how to check it.

• Framing a Load-Bearing Wall

  A bearing wall read as a load path from the ground up: what the sole plate stands on, and how studs, headers and plates hand weight down.

• Cutting and Setting Rafters

  A carpenter's field guide to rafter layout, treating every plumb, seat and cheek cut as one triangle solved from run and rise.

• Framing a Gambrel, Mansard or King Post Roof

  Gambrel, mansard and king post frames worked from the break line down: chord lengths, the thrust the change of angle makes, and what it does to the order.

• Framing a Skylight Opening

  One catalogue code decides the opening, the header, the curb and the saddle above it — worked in that order, from the unit outward.

• Ordering Roof Trusses

  A fabricator builds what the enquiry sheet says. On a vaulted extension in the lee of a taller roof, span, heel and the drift case decide it.

• Checking Truss Shop Drawings

  Forty-one sheets and one seal: what to read on a truss design drawing, from the load block to the plate callout, before the pack reaches the saw.

• Building a Timber Deck

  A carpenter's walk down the load path of a timber deck — footing, post, beam, joist, board — each member sized by what the one above hands down.

• Resurfacing a Deck on Its Existing Frame

  The frame is the half you inherit. Whether it takes composite at the spacing it already has is a board question first and a joist question second.

• Building a Pergola

  A pergola is read from underneath, so the rafter and purlin grids get settled against the yard's stock lengths before the first tail is cut.

• Founding and Anchoring a Shed or Pergola

  A flat-pack kit settles everything except the runner grid under it, the post positions it has to land on, and the hold-down it ships without.

• Building Stairs That Pass Inspection

  A one-millimetre layout error repeats once per riser, so control the whole flight from a single datum and prove the spread before the inspector arrives.

• Re-Treading a Staircase Over Existing Stringers

  A tread cap adds thickness to every step and to neither floor, so the bottom riser grows and the top one shrinks by exactly the cap thickness.

• Trimming Out a Room: Crown, Stock Lengths and Board Feet

  Crown runs the whole ceiling line, so corners and stock lengths set the order — and the hardwood shed prices the same room by the board foot instead.

• Fitting Skirting and Architrave

  No corner is square, so the joints carry the job: where to scribe, where to mitre, and how to hold one margin round every door lining in the house.

• Building Closet Shelving From Lumber

  A reach-in closet is bought in sticks and quoted in board feet, so the shelf run, cleats and rod get tallied twice before any order goes in.

• Sizing a Floor That Does Not Bounce

  A floor can pass span over 360 and still move underfoot. Span, depth and everyday mass set its frequency, and frequency is what the client feels.

• Timber Connections That Carry

  Timber members rarely break — their joints do, so this field guide sorts carpentry connections by the way each one actually gives way.

• Checking an Exposed Timber Beam

  One glulam justified twice: full section with snow's duration factor, then the residual section an hour of fire leaves behind.

• Building With CLT and Glulam

  Mass timber erection is finish work: every CLT panel and glulam beam you set is the ceiling below it, so damage is permanent.

Terms

• Timber / lumber
• Nominal size (the 2x4 problem)
• Joist, rafter and centres
• Stud and framed partition
• Wall plate, top plate and sole plate
• Noggins and blocking
• Roof truss and cut roof
• Plywood
• OSB, chipboard and particle board
• Hardwood and softwood
• Decking
• Stair rise and going

How the numbers are worked out

Every calculator names the method it uses and the standard or guidance behind it. These are the methods the most of the calculators above rest on, and the published documents they cite.

Methods

• Slope, Pitch and True Surface Area

  Converting a plan footprint into the sloped area a roof actually has — the factor that decides whether a shingle order is right or a third short. Applies to 9 of the calculators above.

• Spacing, Pitch and the Count That Follows

  Why a run divided by a spacing gives the number of gaps and not the number of things, why the division rounds up rather than to nearest, and what the leftover bay at the end does to both. Applies to 9 of the calculators above.

• Perimeter Runs, Stock Lengths and Mitres

  Why a room's area tells you nothing about how much skirting it needs, why adding a door can increase the trim order, and why a crown moulding mitre is neither forty-five degrees nor half the corner angle. Applies to 6 of the calculators above.

• Timber: Load Duration, Notching, Char and Panel Action

  Why the same joist is stronger against a wind gust than against a bookcase, why a notch is limited by geometry rather than by stress, and why a heavy timber beam can outlast an unprotected steel one in a fire. Applies to 6 of the calculators above.

• Stair Geometry, Ramps and Guarding

  Why you choose the number of risers rather than their height, why the spread across a whole flight is held to three-eighths of an inch, and why a cable railing has to be checked deflected rather than at rest. Applies to 5 of the calculators above.

• Deflection, Creep and Vibration: What Governs a Span

  Why doubling a span multiplies its deflection by sixteen, why a stronger grade of steel does nothing for it, and why a floor that passes every deflection check can still be unacceptable to walk on. Applies to 4 of the calculators above.

Standards cited

• International Residential Code

  United States. Cited by 8 of the calculators above.

• Approved Documents to the Building Regulations (England)

  England (Wales, Scotland and Northern Ireland have their own). Cited by 2 of the calculators above.

• International Building Code

  United States. Cited by 1 of the calculators above.

• National Electrical Code (NFPA 70)

  United States. Cited by 1 of the calculators above.