Free UK construction calculators Metric and imperial 2026 UK prices
Work out the area of the walls in a room, or of any set of walls, in square metres or square feet. Take off doors and windows, add the ceiling if you need it, and press Calculate to see the area to finish, with paint, plaster, plasterboard, wallpaper and skirting worked out for you.
| Surface | Size | Area |
|---|
Each wall before doors and windows are taken off; the last line is the total to finish.
The result shows the wall area before and after the openings, the ceiling, the total to finish in both square metres and square feet, and the length of skirting board. It also turns the area into paint, skim plaster, plasterboard and wallpaper, and lists each wall on its own.
Measure each wall along the floor from corner to corner, and the height from the floor to the ceiling. In an older house, measure the height at both ends of a wall and use the larger figure, because floors and ceilings are rarely level. Measure doors and windows across the opening, including the frame if you are not painting or papering it.
For a simple rectangular room you only need three numbers: the length, the width and the height. Chimney breasts add a little: measure the depth of each side of the breast and add both depths to the wall width. Alcoves are covered by measuring the wall straight across, since the extra sides balance out the breast.
One wall: area = width × height
A rectangular room: wall area = 2 × (length + width) × height
Area to finish = wall area − doors − windows · Ceiling = length × width
For example, a bedroom 4 m long and 3.5 m wide with a 2.4 m ceiling has 2 × (4 + 3.5) × 2.4 = 36 m² of wall. Take off one 762 × 1981 mm door (1.51 m²) and one 1.2 × 1.2 m window (1.44 m²), and 33.05 m² is left to paint. Adding the 14 m² ceiling makes 47.05 m².
Whether you take openings off depends on the job. For paint and plaster, take them off: you are not covering them. For wallpaper, a common rule is to ignore small windows because you lose as much paper cutting round them as you save, so the calculator only takes off half a window width when it counts the wallpaper drops. Doors are always taken off.
| Opening | Typical size | Area |
|---|---|---|
| Internal door | 762 × 1981 mm | 1.51 m² |
| Wide internal door | 838 × 1981 mm | 1.66 m² |
| Small window | 600 × 900 mm | 0.54 m² |
| Medium window | 1200 × 1200 mm | 1.44 m² |
| Patio doors (typical) | About 1800 × 2100 mm | 3.78 m² |
For a sloping ceiling or a gable wall, use Separate Walls: measure the triangle's base and height and enter half the base as the width, which gives the same area.
| Room size | Wall area | After openings | Paint (2 coats) | Wallpaper rolls |
|---|---|---|---|---|
| 3 × 3 m (small bedroom) | 28.8 m² | 25.85 m² | 4.0 L | 6 |
| 4 × 3.5 m (double bedroom) | 36.0 m² | 33.05 m² | 5.1 L | 7 |
| 5 × 4 m (living room) | 43.2 m² | 40.25 m² | 6.2 L | 8 |
| 6 × 4 m (large lounge) | 48.0 m² | 45.05 m² | 6.9 L | 9 |
These use standard 10.05 × 0.53 m wallpaper rolls with a plain or random-match pattern, and paint at 13 m² a litre per coat.
For a full painting quote, use our paint calculator; for boards and screws, see the plasterboard calculator.
One square metre is 10.764 square feet, and one square foot is 0.0929 m². UK materials are sold by the square metre, but many people still measure rooms in feet. Choose Feet in the calculator to enter feet and read the area in square feet, with square metres shown alongside. Remember that you cannot convert an area by multiplying by 3.28 (the feet in a metre): the factor is squared, which is why it is 10.764.
| From | To | Multiply by |
|---|---|---|
| m² | ft² | 10.764 |
| ft² | m² | 0.0929 |
| m² | yd² | 1.196 |
| yd² | m² | 0.836 |
For exact conversions to the decimal place, the UK weights and measures rules define the yard as exactly 0.9144 m, which fixes all these factors.
Add the length and width, double it, and multiply by the ceiling height: 2 × (length + width) × height. Then take off the area of each door and window.
With a 2.4 m ceiling, 2 × (4 + 3) × 2.4 = 33.6 m² before doors and windows. With one door and one window taken off, about 30.7 m².
Yes for paint and plaster. For wallpaper, take off doors but you can ignore small windows, because cutting round them wastes paper.
For two coats at about 13 m² a litre, 30 × 2 ÷ 13 = 4.6 litres, so a 5 litre tin.
Divide the area by 10 for a 2 mm skim with 25 kg bags of Thistle Multi-Finish. A 33 m² bedroom needs about 4 bags, or 5 with the ceiling.
Split it into a rectangle and a triangle. The triangle is half the base × the height. In the calculator, enter a separate wall half the base wide.
Multiply by 0.0929, or divide by 10.764. 100 ft² is 9.29 m².
762 × 1981 mm (2 ft 6 in × 6 ft 6 in) is the most common. 686, 838 and 914 mm widths are also standard.
Paint coverage from Dulux Trade Vinyl Matt (up to 17 m² a litre). Plaster coverage from British Gypsum Thistle Multi-Finish (10 m² per 25 kg bag at 2 mm). Standard UK wallpaper roll 10.05 × 0.53 m. Plasterboard 2400 × 1200 mm. Reviewed November 2026.