How the wall area calculator works
A rectangular room is a box, so its walls unfold into one long strip whose length is the perimeter and whose height is the ceiling height. Multiplying the two gives the gross wall area. Openings are then subtracted to get the net figure that most materials are priced on.
Net wall area = 2 × (length + width) × height − doors × 1.9 m² − windows × window sizeThe floor area is length × width, and because the ceiling sits directly above it the two are identical in a room with a flat ceiling. The volume, floor area × height, is the number heating engineers and ventilation guides ask for.
Worked example
A living room of 5 m × 4 m with a 2.4 m ceiling has a perimeter of 18 m and a gross wall area of 18 × 2.4 = 43.2 m². Two internal doors take off 3.8 m² and two 1.5 m² windows another 3.0 m², leaving 36.4 m² of net wall. The floor and ceiling each measure 20 m², and the volume is 48 m³. In imperial units a 16 × 13 ft room with 8 ft walls gives 464 ft² gross and 392 ft² net after two doors and two 15 ft² windows.
Which figure to use for which job
| Job | Area to use | Typical allowance |
|---|---|---|
| Wall tiles, cladding, panelling | Net wall area (only the tiled zone) | +10% for cuts |
| Plasterboard or skim coat | Gross wall area | Openings are cut out of full boards |
| Flooring, underlay, screed | Floor area | +5–10% depending on layout |
| Ceiling boards or paint | Ceiling area = floor area | Add sloped sections separately |
| Skirting, coving, cornice | Perimeter | Minus door widths for skirting only |
| Heating, air changes | Room volume | Used by heat-loss and fan sizing |
Measuring rooms that are not rectangles
Split an L-shaped room into two rectangles, run each through the calculator and add the wall figures, then subtract twice the length of the imaginary line between them multiplied by the height, since that line is not a real wall. For an attic room, measure the knee walls as normal and treat each gable end as a rectangle plus a triangle: the triangle is half its base times its height. A bay window adds its own small run of wall: measure around the bay, not across it.
Frequent measuring errors
- Mixing units. One square metre is 10.764 ft², not 10.
- Measuring over skirting. Take lengths at waist height, wall to wall, and ignore trim.
- Uneven ceilings. Old houses can vary by several centimetres; use the highest reading.
- Deducting openings for board. Sheet materials are cut around openings, so the offcuts are waste; order boards on the gross area, finishes on the net area.
- Ignoring built-ins. Fitted wardrobes and kitchen units hide wall you do not need to finish; measure their width and deduct them like a large door.
Frequently asked questions
How do I calculate the square footage of walls in a room?
Add the four wall lengths to get the perimeter and multiply by the ceiling height. A 12 × 14 ft room with 8 ft walls has 52 ft × 8 ft = 416 ft² of wall before doors and windows are removed.
How many square metres of wall does a 4×4 m room have?
The perimeter is 16 m, so with a 2.5 m ceiling the walls total 40 m². Subtract about 1.9 m² per door and the size of each window for the net figure.
Is the ceiling area the same as the floor area?
Yes for a flat ceiling: both are length × width. Sloped or vaulted ceilings are larger and should be measured along the slope.
How big is a standard door and window for area calculations?
An interior door is about 0.9 × 2.1 m (1.9 m²) or 3 × 7 ft (21 ft²). Windows vary widely, so measure yours; 1.2 × 1.25 m (1.5 m²) and 3 × 5 ft (15 ft²) are common sizes.
How do I convert square metres to square feet?
Multiply by 10.764. So 20 m² equals 215 ft², and 36.4 m² of wall equals about 392 ft².
Last reviewed: 2026-09-27 · How we calculate