SettingsSettings for this calculationUS
The overall length of the building, along which joists are spaced.
The direction the joists are SPACED along, not the direction they span. Getting the two the wrong way round produces a plausible-looking number for the wrong building: joists spaced along a short wall and spanning the long way is a different order of timber from the reverse, and usually a different depth of joist as well.
The building width, which each ceiling joist must span.
The distance each joist crosses, which is what decides its size as well as its length. If an internal wall or a beam picks the joists up part-way, the span is to that support and not across the building — and a joist spanning 2.4 m onto a wall is a much smaller section than one spanning 4.8 m, so the support is worth identifying before ordering.
The on-center spacing between joists.
Centre to centre, which is what the timber is spaced at and what the boarding above expects — a 1,200 mm board wants joists at 400 or 600 so its edges land on timber. Spacing also sets the count: the number of joists is the run divided by the spacing plus one, and it is the plus one that gets forgotten on a long run.
A wing's width between its walls, which each of its joists spans; 0 for none.
An L, T or U-shaped house has a wing off the main block: its ceiling joists span its own width and are spaced along its own length, so it is counted as its own run.
From the wing's end wall to the main block's wall line.
The run its joists are spaced along, a joist at its end wall and one at every whole or part bay to the main wall line.
A second wing's span; 0 for none.
Measured like the first wing.
A second wing's length; 0 for none.
Measured like the first wing.
Total ceiling joist lumber needed
507 ft
- Number of joists
- 26
- Whole bays between joists
- 24
- Short bay at the last joist
- 12 in
They open the calculator with your figures already in it
Ceiling Joist Spanning Lineal Lumber Aggregator: 507 ft — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- Joist count = ceil(building length ÷ spacing) + 1, so a joist lands at each end wall; total linear length = joist count × building width (span)
Inputs used
- Building Length
- 33 ft
- Building Width (Joist Span)
- 19.5 ft
- Joist Spacing (o.c.)
- 16 in
- First Wing's Span (0 for none)
- 0 ft
- First Wing's Length (0 for none)
- 0 ft
- Second Wing's Span (0 for none)
- 0 ft
- Second Wing's Length (0 for none)
- 0 ft
Intermediate steps
- Number of joists
- 26
- Whole bays between joists
- 24
- Short bay at the last joist
- 12 in
What this calculation does not cover
- A wing is counted as its own run, its joists spanning its width and spaced along its length to the main block's wall line; where the two meet, the trimming, the beam or the bearing that picks the joists up is the framing plan's.
- The total is the in-place length of the joists, not the lumber to buy. It assumes every joist runs the full building width as one continuous piece, so it excludes off-cut waste, the extra material where a span longer than the available stock length has to be made up from two members, and the overlap itself where joists lap over a central bearing wall. Minimum lap length at the bearing is jurisdictional.
- Nothing here tests whether the joists can carry the span. No section size, species, grade, deflection limit or attic loading is asked for, so an 8 m clear span at 24 in centres is aggregated just as readily as a span well inside a published span table. Allowable ceiling joist spans shorten as spacing widens and as attic storage loading is added, and the governing deflection limit is stricter for a plastered or boarded ceiling than for an unfinished one; both are jurisdictional.
- The two building dimensions are not interchangeable, so the answer depends on which one you enter as length. A 12 m by 8 m building at 24 in centres returns 160 m of lumber, while the same building entered as 8 m by 12 m returns 168 m. Joist direction is fixed by the bearing walls and, in a pitched roof where the ceiling joists also act as rafter ties resisting outward thrust at the wall plate, by the rafter direction; the calculator has no notion of that thrust or of the tie connection at each end.
- Only plain field joists are counted, with nothing added and nothing taken away. Blocking, strutting or herringbone bracing at mid-span, rim and end joists, hangers, strongbacks and ceiling binders are all outside the total, and no joists are subtracted where a stairwell, loft hatch or vaulted section interrupts the run. The span above which intermediate rows of strutting become a requirement is set by local rules.
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.
Part of a bigger job
This trade is one line of a job takeoff. Run the whole job and every other trade comes back with it, off the same measurements.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-10-11 · v1.1.0
Regulatory standards & verification citations1
- Joist count = ceil(building length ÷ spacing) + 1, so a joist lands at each end wall; total linear length = joist count × building width (span)
Cite this page
Your workspace
Most jobs need more than one number. Add the calculators you need next and they open right here, underneath this one — your figures stay on screen and nothing is lost to a page change.
Tools and safety for this job
To cut framing lumber, frame joists and a deck and frame the roof structure. Generic types, no brands, no prices.
Protection this work requires
- Hardwood dust is a recognised carcinogen: extract at the tool and wear a P2/N95 respirator when cutting or sanding indoors.
- Saws, grinders and breakers run above 85 dB, where hearing damage accumulates and does not recover: defenders or plugs for every cut, not just the long ones.
- Boards, blocks and bagged material cause most lasting back injuries on small sites: two people or a lifter for full sheets, and never a bag on one shoulder up a ladder.
- Most fatal falls on small jobs are from under three metres: use a tower or a ladder tied at the top, and never work off the top two rungs.
Frame the roof structure: Rafters, joists, purlins and trusses, and the ties and straps that hold them, are sized by an engineer, a truss maker or accepted span tables, and this site never sizes them. A count of rafters or trusses at a chosen spacing is a quantity to order against; what each member must be is not something a count can say.
Show the 12 tools this job needsHide tools
Essential
Circular saw
Claw hammer
Combination or framing square
Cordless drill/driver
Spirit level
Tape measure
Recommended
Framing nailer
Impact driver
Mitre saw
String line and pins
Tower scaffold or trestles
Optional
Hand saw
Also needed as materials: circular saw blades, drill and driver bits, framing nails, fuel cells, impact driver bits, mitre saw blades.
what each carpentry tool is for, and the spec that decides which to buy where one does.