Square footage calculator
This calculator works out the floor area of a room, the wall area after windows and doors are removed, the perimeter and the volume. Enter length, width and ceiling height to see all four at once.
Those numbers drive material estimates: flooring and tile from the floor area, paint and wallpaper from the wall area, ventilation and heating from the volume.
Floor area
17.5 m²
188.37 sq ft
Wall area without openings
42.7 m²
Perimeter
17 m
Room volume
47.25 m³
For an irregular room, measure rectangular parts separately and add the areas.
The formulas
- Floor area = length × width. A 5 × 3.5 m room is 17.5 m², about 188 sq ft.
- Perimeter = 2 × (length + width). The same room measures 17 m around.
- Wall area = perimeter × ceiling height − window and door area.
- Volume = floor area × ceiling height. 17.5 m² × 2.7 m = 47.25 m³.
Typical openings to subtract
Measure each opening when you can; these averages are for a quick estimate. Subtract openings for paint and wallpaper only — flooring is unaffected by them.
| Opening | Typical size | Area |
|---|---|---|
| Interior door | 0.8 × 2.0 m | 1.6 m² (17 sq ft) |
| Entry door | 0.9 × 2.1 m | 1.9 m² (20 sq ft) |
| Single window | 0.9 × 1.4 m | 1.26 m² (14 sq ft) |
| Double window | 1.3 × 1.4 m | 1.82 m² (20 sq ft) |
| Patio door block | 2.1 × 2.1 m | 4.4 m² (47 sq ft) |
Turning area into materials
- Flooring and tile: take the floor area and add 5–10% for cuts, up to 15% for diagonal layouts.
- Wallpaper: divide the wall area by the coverage of one roll and add extra for pattern repeat.
- Paint: multiply the wall area by the spread rate on the tin and by the number of coats.
- Skirting and trim follow the perimeter, not the area.
Rooms that are not rectangles
Split an irregular room into rectangles, calculate each area and add them together. Alcoves and bays are separate rectangles you add, while structural columns are areas you subtract.
For an L-shaped room, two rectangles are usually enough to describe the whole floor.
Frequently asked questions
How do I calculate square footage of a room?
Multiply length by width. A 12 × 10 ft room is 120 sq ft. In metres, a 4 × 3 m room is 12 m², which is about 129 sq ft.
How do I calculate wall area?
Multiply the room perimeter by the ceiling height and subtract windows and doors. A 5 × 3.5 m room with a 2.7 m ceiling has 45.9 m² of wall before openings.
Should windows and doors be subtracted?
Yes for paint and wallpaper, otherwise you buy noticeably too much. Flooring estimates ignore them.
How much extra flooring should I buy?
Usually 5–10% above the floor area for cuts, and up to 15% for diagonal installation.
How many square feet are in a square metre?
One square metre equals about 10.764 square feet.
Updated: 2026-08-09