The requirement
This is a takeoff sum, not a span table. It answers “how many studs or joists does this run need at this spacing” — pure layout and quantity. It has nothing to say about how far a stud or joist can actually span, which depends on the timber species, grade, load and your local building code. For that, use your code’s span tables. This page only counts pieces.
The formula is the same for studs along a wall or joists along a floor:
Quantity = (run length ÷ centre spacing) + 1
- Run length — the total length of the wall or floor, in the same units as the spacing (usually millimetres).
- Centre spacing — the on-centre (o.c.) distance between uprights: 400 mm, 450 mm and 600 mm are the common increments.
- + 1 — every run of equal spaces needs one more upright than it has spaces, because an upright closes both ends.
If the run doesn’t divide evenly into the spacing — it usually won’t — round the number of spaces up, not down. The last gap ends up a little tighter than the rest; that’s normal and expected.
Two things get added on top of this base count and are easy to forget:
- Corners. An external corner typically takes an extra full-height stud or two beyond the on-centre count, so linings on both faces have something to fix to.
- Openings. A window or door needs a full-height king stud plus a shorter jack (trimmer) stud on each side, to carry the header/lintel above the opening — 4 extra studs per opening.
Neither of these replaces any of the on-centre studs — they’re additional pieces on top of the run-length calculation.
Work it out by hand
Take a 5.4 m wall or floor run and compare three common spacings.
1. Convert the run to millimetres. 5.4 m = 5400 mm.
2. Divide by the spacing, and round up.
| Spacing | 5400 ÷ spacing | Spaces (rounded up) | + 1 = quantity |
|---|---|---|---|
| 400 mm | 13.5 | 14 | 15 |
| 450 mm | 12.0 | 12 | 13 |
| 600 mm | 9.0 | 9 | 10 |
Tighter spacing means more pieces: going from 600 mm to 400 mm centres on the same 5.4 m run adds 5 more studs or joists — half as much again.
3. Add the extras for a stud wall. Say the 400 mm-centre wall above has one external corner and one 900 mm window:
- Base count (from the table): 15 studs
- Corner: + 2 studs
- Window opening (king + jack stud each side): + 4 studs
- Total: 21 studs for that wall
4. Joists work the same way, with different extras. A floor joist run uses the identical formula and rounding rule — there are no corners, but openings still apply: a stairwell or chimney penetration gets doubled trimmer joists on each side of the opening, added on top of the base on-centre count exactly like the king/jack studs above.
Do it in 5 seconds
Related
- Pillar: Concrete Calculations: The Complete On-Site Guide — every concrete sum in one place.
- Spacing tells you how many pieces you need. How far each one can actually span depends on species, grade and load — check your local building code’s span tables for that number.
Sources
- American Wood Council, Wood Frame Construction Manual — on-centre stud and joist layout, and the run-length-over-spacing-plus-one convention.
- APA – The Engineered Wood Association, span and spacing technical notes — standard on-centre spacing increments and framing-around-openings practice.
- Corner and opening framing details (number of studs at a corner, king/jack stud practice) vary by builder and jurisdiction — the counts above are common working defaults, not a code minimum.