The spacing is set by the sheet, not the stud
Sixteen inches looks like an arbitrary number until you notice what it divides into. A standard sheet of plywood, OSB or plasterboard is 4 feet by 8 feet — 48 inches by 96 inches. Divide either dimension by 16 and you get a whole number: three bays across the width, six along the length.
That is the whole reason the spacing exists. Every sheet edge lands on the centre of a stud, so two adjacent sheets can be fixed to the same member with room for both. Framing at, say, 15 inch centres would be perfectly strong and completely unusable, because every sheet joint would fall in mid-air and you would be cutting and adding noggins all day.
Twenty-four inch centres follow the same logic — 48 and 96 both divide by 24 — and use a third fewer studs. The trade is that the sheet material has to span further, so thin plasterboard can telegraph a slight waviness between studs, and the framing carries less redundancy. As a rough guide, 16 inch is the default for load-bearing walls and anywhere the finish matters, and 24 inch is common for non-load-bearing partitions and in engineered systems designed for it.
In metric-framed construction the same idea gives 400 mm and 600 mm centres, sized to the 1200 by 2400 mm sheet. They are close cousins of 16 and 24 inch, but they are not identical: 16 inches is 406.4 mm and 24 inches is 609.6 mm. Do not mix the two systems on one wall.
Counting the studs
This is where estimates go wrong, and it is worth being slow about.
Studs are placed at centres, but they are counted as objects. A 12 foot wall is 144 inches. Divide by 16 and you get 9 — but 9 is the number of *bays*, the gaps between studs. Every bay has a stud on each side, and the studs in the middle are shared, so the number of studs is one more than the number of bays.
Number of studs = span ÷ spacing + 1
So a 12 foot wall at 16 inch centres takes 10 studs, not 9. Same wall at 24 inch centres is 6 bays and 7 studs. Get this wrong and you are one stud short on every wall in the building — the classic fencepost error, and it always under-orders.
There is a second case people miss. If the span does not divide evenly, you still need a stud closing the far end. A 12 foot 6 inch wall at 16 inch centres gives 150 ÷ 16 = 9.4 bays. You cannot have four tenths of a bay hanging open, so you frame 10 bays and 11 studs, with the spacing pulled in slightly to suit. Spacing is a maximum, not a target — tighter is always allowed, wider is not. The lumber calculator on this site does exactly this: it rounds the bays up and then adds the closing member.
What the arithmetic does not count
The formula gives you the studs in the field of the wall. Real framing needs more, and none of it comes out of a spacing calculation.
- Corners: a corner needs enough timber to fix the sheet material on both walls and to catch the internal finish. Depending on the detail used that is two or three studs at every external corner.
- Wall intersections: where a partition meets a through wall, the through wall needs backing for the partition to fix to.
- Openings: every door and window adds a pair of king studs, a pair of jack or trimmer studs supporting the header, and cripple studs above and below. A single window can easily add six pieces, and removing the studs the opening displaced does not cancel that out.
- Headers and plates: top plates, sole plates and headers are all linear metres of the same timber and are usually a bigger number than people expect. A wall has at least two plates running its full length, often three.
- Blocking and noggins: fire blocking, mid-height noggins and backing for fixtures such as rails, cabinets and radiators.
A practical habit is to work out the field studs from the spacing, then walk the plan and add for every corner, junction and opening, then apply the waste factor to the total. Adding a percentage and hoping it absorbs the openings does not work, because openings are not a random loss — they are a countable quantity you can simply count.
The waste factor
Ten percent is the usual allowance on framing timber, and it covers bowed and twisted pieces that get rejected on sight, cutting waste, and the odd mistake. It is applied to the piece count and rounded up, because you buy whole boards.
If your lengths do not suit the stock available — framing 9 foot walls out of 8 or 10 foot timber, for instance — the real waste is much higher than 10 percent, and the fix is to choose stock lengths that cut cleanly rather than to inflate the percentage.