Drywall Calculator
Result
Sheets to buy
- Surface area in square feet
- 400 ft²
- Surface area in square metres
- 37.16 m²
Drywall calculator: enter the total length of the walls, the ceiling height and the area of the openings, and it returns how many sheets to order. The wall area is the easy half — length times height, less the doors and windows — and the sheet size is what people get wrong, because a sheet is not a unit of area. A 4 by 8 sheet is 32 square feet, so 400 square feet of wall is 12.5 sheets — 13 once you round up, and 14 once the usual 10% allowance is added. The last sheet is cut for the ceiling line and the last stud bay, and the off-cut from a sheet you have already cut for a door is usually too short to use anywhere else. Ten percent is the normal allowance for a straightforward room, more if the walls are out of square or if you are hanging the boards vertically to save on butt joints, where every cut is a full height. The sheet size is a dropdown rather than a field because the trade has settled on four sizes — 4 by 8, 4 by 10 and 4 by 12 feet, and the 1200 by 2400 millimetre sheet used in China — and a longer sheet means fewer joints to tape, fewer sheets to carry and a heavier board that one person can no longer lift.
What one sheet covers
| Sheet | Coverage per sheet (ft²) | Coverage per sheet (m²) | Sheets per 100 ft² |
|---|---|---|---|
| 4 × 8 ft (1220 × 2440 mm) | 32 | 2.973 | 3.13 |
| 4 × 10 ft (1220 × 3050 mm) | 40 | 3.716 | 2.5 |
| 4 × 12 ft (1220 × 3660 mm) | 48 | 4.459 | 2.08 |
| 1200 × 2400 mm (3.94 × 7.87 ft) | 31 | 2.88 | 3.23 |
The sheet areas are exact — a 4 by 8 sheet is 32 square feet by definition, not by manufacturer — which is the difference between this table and the coverage tables on this site that describe a product. The fourth column is the fast estimate: divide your wall area by 100, multiply by it, add the wastage allowance and round up. The metric sheet is not a round 32 square feet: 1200 by 2400 millimetres is 2.88 square metres, which is 31.0 square feet, and on a large room that 3% is a whole extra sheet.
Formula
Sheets = ceil( ( length × height − openings ) × (1 + wastage ÷ 100) ÷ area of one sheet )
- L
- Total length of the walls being boarded, in feet
- H
- Ceiling height, in feet
- A
- Area of doors, windows and other openings, in square feet
- s
- Area of one sheet: 32 ft² for a 4 × 8, 48 ft² for a 4 × 12
- w
- Wastage allowance as a percentage of the wall area
Use it once you have decided on a sheet size, because that decision is made by the room rather than by the price. A 4 by 12 sheet spans a 8 foot wall in one piece with no butt joint, which is worth more than the sheets it saves — butt joints are the ones that crack. A 4 by 8 sheet is the one an experienced pair can carry up a stairwell and hold overhead, which is why it is still the default. Measure the perimeter and the ceiling height rather than one wall, and deduct the openings only where the board actually stops: a window opening is boarded around, so the deduction is real, but a doorway with a frame is boarded up to the frame and the small triangle above the door is a cut. If you are boarding the ceiling as well, run the page a second time with the room length and the room width, and add the two answers.
Worked examples
A 12 by 15 foot room in 4 by 8 sheets, 10% wastage
- Wall area: 54 × 8 = 432 square feet, less 32 square feet of door and window = 400 square feet (37.16 m²)
- With 10% wastage: 400 × 1.10 = 440 square feet
- A 4 × 8 sheet covers 32 square feet: 440 ÷ 32 = 13.75, rounded up to 14 sheets
These are the page defaults. The wastage allowance is doing visible work here: without it, 400 ÷ 32 = 12.5 rounds up to 13 sheets, and the fourteenth sheet is the one that covers the cuts around the door and the window reveal. Boarding a room almost always wants that extra sheet, which is why the answer is 14 rather than 13.
The same room in 4 by 12 sheets
- Wall area: 54 × 8 = 432 square feet, less 32 square feet of door and window = 400 square feet (37.16 m²)
- With 10% wastage: 400 × 1.10 = 440 square feet
- A 4 × 12 sheet covers 48 square feet: 440 ÷ 48 = 9.17, rounded up to 10 sheets
The same wall, 4 sheets fewer. A 4 by 12 sheet covers an 8 foot wall in one piece, so there is no horizontal butt joint to tape and no butt joint to crack — but a 4 by 12 sheet weighs around 35 kilograms at 12.5 mm, which is a two-person lift and usually a hoist for anything above the first floor. The sheet size is a decision about labour, not about maths.
A 32 by 9 foot garage wall in 4 by 10 sheets, 15% wastage
- Wall area: 32 × 9 = 288 square feet, less 20 square feet of window = 268 square feet (24.9 m²)
- With 15% wastage: 268 × 1.15 = 308.2 square feet
- A 4 × 10 sheet covers 40 square feet: 308.2 ÷ 40 = 7.71, rounded up to 8 sheets
A 9 foot wall is where the sheet size starts to dominate: a 4 by 8 sheet leaves a 1 foot strip to be cut for every column of boards, and those strips are where a bad joint comes from. A 4 by 10 sheet covers the wall in one piece with 1 foot to trim, which is why the answer is 8 sheets rather than the 10 that 4 by 8 sheets would need.
Limitations
The area arithmetic and the sheet areas are exact; the wastage allowance is the estimate. Ten percent is normal for a rectangular room hung horizontally, and it is not enough for a room that is out of square, a wall with a chimney breast or a bay, or boards hung vertically where every cut is a full height rather than a width. The openings are deducted as flat areas, which is right for the glass but optimistic around a reveal: the strip above a door and the strip beside a window are cuts, and a cut produces an off-cut that is often too short to use. Nothing else that goes into the job is counted here. Joint compound and paper tape are the two biggest omissions and both are driven by the joint length rather than the board area, which depends on how the sheets are laid — compound runs about 0.3 kilograms per metre of joint with the tape the same length as the joint, and screws run about 32 per sheet. Corner bead, the metal trim on every external corner, is counted by the metre of corner. Ceilings are not counted at all: run the page again with the room length as the wall length and the room width as the height, and add the sheets. Finally, board thickness (12.5 or 15 mm) and the type (standard, moisture-resistant, fire-rated) change what you buy but not how many, so they are not inputs here.
Frequently asked questions
- How many sheets of drywall do I need for a 12 by 15 foot room?
- Fourteen 4 by 8 sheets: the walls are 432 square feet, less 32 for the door and window, which is 400 square feet, and 10% wastage takes that to 440. One sheet covers 32 square feet, so 440 ÷ 32 = 13.75, rounded up. Without the wastage allowance it would be 13 — the extra sheet pays for the cuts around the door and the window reveal.
- What is the difference between drywall, sheetrock and plasterboard?
- Nothing but the region. Drywall is the North American trade term, sheetrock is the US brand name that became a generic word for it, and plasterboard is the British, Irish and Australian term for the same gypsum board. The sheets are the same product with the same sizes: 4 by 8 feet is the standard panel in all of them, and 1200 by 2400 millimetres is the metric panel size.
- Should I hang the boards horizontally or vertically?
- Horizontally, in almost every case. A board hung across the studs horizontally has its long edges on the studs, which is where the tapered edge is designed to be taped, and the butt joints end up at the corners of the room. Hanging vertically puts a butt joint every 4 feet, which is both harder to finish and the place where cracks appear, and it produces a cut at the ceiling and the floor for every column. Vertical hanging is only worth it on a wall under 4 feet wide or where a horizontal joint would land at an awkward height.
- How much extra should I order for waste?
- Ten percent for a rectangular room with the boards hung horizontally, and fifteen for a room that is out of square, a wall with a lot of openings, a bay window, or boards hung vertically where every cut runs the full height. Order a whole sheet rather than a fraction — the answer is rounded up for that reason — and keep one spare: a sheet that gets broken on site or cut wrong is far easier to replace while the batch is still in the shop.
- Does this calculate the ceiling too?
- No, only the walls. A ceiling is a separate calculation with the same arithmetic: run the page again with the room length as the wall length and the room width as the height, and add the sheets. The 12 by 15 foot room above has 400 square feet of wall and a 180 square foot ceiling, which is another six sheets at 4 by 8 before any wastage — worth doing separately, because the off-cuts from the walls fit a ceiling far better than the other way round.
- What else do I need besides the sheets?
- Joint compound, paper tape, screws and corner bead. The first three are driven by the joint length rather than the board area, which is why they are not calculated here: how many joints you end up with depends on whether the boards are hung horizontally or vertically and where the sheets land. As a starting point, compound runs about 0.3 kilograms per metre of joint, the paper tape is the same length as the joint it covers, and a 4 by 8 sheet takes about 32 screws. Corner bead is counted by the metre of external corner.
References
- Approximate Conversions from U.S. Customary Measures to Metric — the chart behind the feet, inches and square metres on this page — National Institute of Standards and Technology
- The International System of Units (SI) brochure — the square metre the sheet areas are converted to — Bureau International des Poids et Mesures (BIPM)