The requirement
Divide the run length by the centre spacing. Round up to whole spaces. Then add 1. A 5.4 m wall at 400 mm centres is 5400 ÷ 400 = 13.5 spaces. Round up to 14, add 1, and you need 15 studs. The same sum works at 450 mm and 600 mm centres, and for joists.
One caveat before the maths. This is a takeoff sum, not a span table. It only counts pieces at a given spacing. It does not say how far a stud or joist can span. Span limits come from the timber species, grade, load and your local building code. Use your code’s span tables for that.
The sum is the same for studs along a wall or joists along a floor. Here it is in full: Quantity = (run length ÷ centre spacing) + 1.
- Run length is the total length of the wall or floor. Use the same units as the spacing, usually millimetres.
- Centre spacing is the on-centre (o.c.) distance between uprights. 400 mm, 450 mm and 600 mm are the common spacings.
- The + 1 closes both ends. Every run of equal spaces needs one more upright than it has spaces.
The run usually will not divide evenly into the spacing. When that happens, round the number of spaces up, not down. The last gap ends up a little tighter than the rest. That is normal.
Two extras go on top of this base count. Both are easy to forget.
- Corners. An external corner usually takes an extra full-height stud or two beyond the on-centre count. That gives the linings on both faces something to fix to.
- Openings. A window or door needs a full-height king stud plus a shorter jack (trimmer) stud on each side. These carry the header or lintel above the opening. That makes 4 extra studs per opening.
Neither extra replaces an on-centre stud. They are added pieces on top of the run-length sum.
Work it out by hand
Take a 5.4 m wall or floor run. Compare the 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. Dropping from 600 mm to 400 mm centres on the same 5.4 m run adds 5 more studs or joists. That is 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 same formula and the same rounding rule. Floors have no corners, but openings still count. A stairwell or a chimney opening gets doubled trimmer joists on each side. Add them on top of the base on-centre count, just like the king and jack studs.
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 piece can span depends on species, grade and load. Check the span tables in your local building code 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.