A stud wall is set out at centres, and the count is the wall length divided by the spacing plus one — which is the part people get wrong in both directions. Then come the openings, which need studs on either side, a head, a sill, and trimmer studs to carry the load around; then the plates top and bottom; then the noggins that stop the studs twisting and give the board something to land on. This counts all of them and shows the wall as a line with every stud marked, so you can see whether the spacing lands where the sheet edges need it to.
Wall, spacing and openings
Spacing divides the wall, and openings interrupt it. Both change the count.
A row of noggins every 1.2 m vertically; one row for a standard wall.
Result
One more stud than you think
The classic off-by-one. A 5.4 m wall at 400 mm centres has 13.5 bays, which rounds up to 14 bays of studwork — and 14 bays need 15 studs, because there is a stud at each end as well as between. Counting bays instead of studs leaves you one short at every wall, and the error is invisible until you are fixing the board at the last corner.
The second thing to know is that centres are set out from one end, not centred on the wall. Starting at one end and working along means the last bay is whatever is left over, which is fine because a stud wall does not care about symmetry. It does matter where the board lands: plasterboard is 1200 mm wide, so studs at 400 mm centres put a stud at every third-board joint.
What an opening costs you
An opening is not a hole, it is a structural interruption, and it needs four studs at minimum: one either side as a jamb, one across the top as a head, and one at the bottom as a sill. Anything wider than about 1.2 m, or anything in a load-bearing wall, needs a proper lintel and often doubled jamb studs, because the load that used to go down through that piece of wall now has to go around it.
The opening also removes studs from the field, which is why the calc adds four per opening rather than the exact arithmetic: some of the field studs fall inside the opening and are not needed, and four is a reasonable net figure for a standard door. For a wide opening or a load-bearing wall, count it properly rather than using the rule of thumb.
They stop the studs twisting under load, and they give the plasterboard a horizontal edge to land on where two sheets meet part-way up the wall.
Put them at the board joint if you can. A sheet edge that lands on nothing will crack at that line within a year.
Spacing and what hangs on the wall
Four hundred millimetres is the standard for a reason: it is a third of a plasterboard width, so every vertical board joint lands on a stud without any special arrangement. Six hundred is acceptable for a non-load-bearing partition with thicker board, and three hundred is specified where heavy things are being fixed — a wall-hung boiler, a tall run of kitchen units, grab rails.
If something heavy is going on the wall, the honest answer is to put a stud, a noggin or a plywood panel exactly where it is needed rather than to close the spacing of the whole wall. A 600-mm wall with a sheet of 18 mm ply in the right place is stronger where it matters than a 300-mm wall with nothing.
| Wall | Stud size | Centres | Board | Note |
|---|---|---|---|---|
| Internal partition | 63 x 38 mm | 600 mm | 12.5 mm | Non-load-bearing |
| Internal partition | 75 x 38 mm | 400 mm | 12.5 mm | The safe default |
| Load-bearing internal | 100 x 38 mm | 400 mm | 12.5 mm | Check with the designer |
| External timber frame | 140 x 38 mm | 600 mm | Sheathing | Engineered, not rule of thumb |
| Wall for heavy fittings | 75 x 38 mm | 400 mm | 18 mm ply patches | Ply where it matters |
| Bathroom, for a rail | 75 x 38 mm | 400 mm | 18 mm ply | Full-height ply is cheapest |
A worked example
- Field studs5.4 m at 0.4 m is 13.5, so 14 bays and 15 studs.
- OpeningsOne opening needs four studs of trimming — two jambs, a head and a sill.
- Total19 studs in all, which at $6 each is $114 of studs.
- PlatesTwo lengths of wall is 10.8 m of plate.
- NogginsOne row at 1.2 m spacing is 4 noggins, and the wall takes 5 board widths.
Nineteen studs for a five-and-a-half-metre wall looks high until you remember that the wall has two faces, an opening and a top and bottom plate: the studwork is the cheap part of a partition, and the board and the finishing are the expensive part.
Setting out and building
Mark the plates together, not separately: lay the top and bottom plate side by side, mark both at once with a tape and a square, and the studs will line up. Mark an X on the side of each stud line so you know which side of the mark the stud goes, and every stud goes on the same side.
Build the wall flat on the floor and stand it, if there is room. It is faster, it is square, and the fixings are easier. Nail through the plate into the end of the stud rather than skew-nailing wherever you can. Then check it is plumb before fixing the top plate, because a wall that is out of plumb at the stud stage is out of plumb for the life of the building.
The board is the next calculation: the drywall sheet figure turns the wall into sheets, and the wall area figure handles the surface that is being covered once it is up. If the wall is part of a bigger job, the labour hours figure is where the studwork turns into days.
Do
- Mark the top and bottom plate together so the studs line up.
- Mark which side of the line the stud goes, and keep it consistent.
- Put a noggin wherever two board edges meet part-way up.
- Add ply where something heavy is going rather than closing the spacing everywhere.
- Check plumb before fixing the top plate.
Avoid
- Counting bays instead of studs and being one short at every wall.
- Assuming an opening is just a gap rather than four studs of trimming.
- Letting a board edge land on nothing and cracking at that line.
- Marking the plates separately, so the studs do not line up.
- Fixing the top plate before the wall is plumb.
Where the numbers stop being true
The opening allowance is four studs, which suits a standard door in a non-load-bearing partition. A wide opening, a wide opening in a load-bearing wall, or an opening with a lintel needs more, and a structural opening should be designed rather than estimated.
Nothing here accounts for corners, junctions or intersections. A corner needs extra studs to give both walls something to fix to, and a T-junction needs a stud or a backing block at the point where the second wall arrives. On a room with four corners that is another eight studs or so.
And there is no allowance for waste, for the studs that arrive warped, or for the ones you cut short. Timber is sold in lengths and it moves: order ten percent over on a job of any size.
How the figures are produced
Field studs are the wall length divided by the centres, rounded up, plus one for the far end. Openings add four studs each for jambs, head and sill. Plates are two lengths of wall. Noggins are the wall length divided by the noggin spacing, rounded down, times the number of rows. Board widths are the wall length divided by 1.2 metres, which is a standard plasterboard width.
Questions people ask
Why 400 mm centres?
Because it is a third of a 1200 mm plasterboard width, so every board joint lands on a stud with no special arrangement.
Can I use 600 mm?
On a non-load-bearing partition with 15 mm board, usually yes. Not where anything heavy is being fixed.
How many studs does a doorway need?
Four as a rule of thumb: two jambs, a head and a sill. Wide or structural openings need more, and a lintel.
Do I need noggins?
Yes, at least one row. They stop the studs twisting and they give the board a horizontal edge to land on.
How much extra timber should I order?
About ten percent. Timber warps, cuts go wrong, and a short order costs more in time than it saves in material.
What size stud should I use?
75 x 38 mm at 400 mm centres is the safe default for an internal partition. Anything structural should be specified.