Wall Area Calculator
Get the wall area of a room with doors and windows deducted. This is the number that paint, wallpaper, plaster and putty calculators all build on.
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Formula and method
Gross wall area is twice the sum of length times height and width times height. Openings are calculated from door and window counts and sizes and subtracted to give the net area.
Measure the walls, not the floor
Wall area is perimeter times height, less openings. A 4 × 5 m room with a 2.7 m ceiling has 48.6 m² of wall and 20 m² of floor — the floor figure is not a rough approximation, it is a 2.4× undercount.
Take height readings at both ends and in the middle of each wall. Ceilings are rarely level, and a 2.5 cm difference around four walls is almost half a square metre on its own.
What to deduct and what to leave
Deduct windows and doors fully: a 1.2 × 1.4 m window is 1.68 m², a 0.9 × 2.0 m door is 1.8 m². Do not deduct the reveals if you are painting them — count them separately and add them back, because they are usually 0.15 to 0.25 m² each.
Leave an open archway in the area if you are painting its reveals, or deduct the opening and add the reveals. Deducting both the opening and forgetting the reveals is the classic way to come up short on paint.
Checking the total against materials
Sanity-check the result backwards: 43 m² of wall at 10 m² per litre is 4.3 litres per coat, so about 9 litres for two coats. If a contractor's estimate says 15 litres, either the area or the coverage is wrong, and it is cheaper to find out now than to buy a third tin.
The same check works for wallpaper (rolls by drops), tiles (10 % waste) and plaster (kg per square metre per millimetre). One area figure feeds all of them, so getting it right once saves four separate errors.
How to measure the inputs
- Measure room length and width, then measure floor to ceiling height.
- Count doors and windows and measure each width and height.
- Note the result — you can reuse it in the paint, wallpaper and plaster calculators.
Worked example
Room length 13.12 ft; Room width 9.84 ft; Room height 8.53 ft; Door count 1; Door width 2.62 ft; Door height 6.56 ft; Window count 1; Window width 3.28 ft; Window height 3.28 ft → Net wall area: 363.82 ft²
Reference data
| Room, m | Wall area at 2.7 m, m² | Minus window and door, m² |
|---|---|---|
| 3 × 4 | 37.8 | 33.7 |
| 4 × 5 | 48.6 | 44.5 |
| 5 × 6 | 59.4 | 55.3 |
| 6 × 8 | 75.6 | 71.5 |
One window of 1.5 × 1.5 m and one door of 0.9 × 2.1 m are subtracted — 4.1 m² in total.
Practical tips
- Sketch the room and write each measurement on the drawing.
- Measure both ends of every wall: lengths usually differ slightly.
- Keep the perimeter for skirting board ordering.
Sources
- СП 54.13330.2022 — Здания жилые многоквартирные
Checked and updated October 22, 2026