How room area is calculated
A room calculator turns three measurements, length, width and height, into the quantities you need before buying flooring, paint, plaster or insulation. Floor area is length × width, the ceiling has the same area, the perimeter is the distance around the room, and the wall area is that perimeter multiplied by the ceiling height.
This calculator assumes a rectangular room with flat walls and a flat ceiling. It shows the floor area and wall area in both square meters and square feet, so you can quote either to a supplier.
The formulas
- Floor area = Length × Width
- Ceiling area = Floor area
- Perimeter = 2 × (Length + Width)
- Wall area = Perimeter × Height
- Wall volume (optional) = Wall area × Wall thickness
Every input has its own unit (mm, cm, m, ft or in) and is converted to meters before any math. The foot is defined as exactly 0.3048 m (NIST), so 1 m² is about 10.764 ft². Areas are rounded to two decimal places and the wall volume to three.
Worked example (metric)
A bedroom measures 5 m × 4 m with a 3 m ceiling, and its walls are 230 mm thick.
- Floor area: 5 × 4 = 20.00 m² (215.28 ft²). The ceiling is also 20.00 m².
- Perimeter: 2 × (5 + 4) = 18.00 m.
- Wall area: 18 × 3 = 54.00 m² (581.25 ft²).
- Wall volume: 54 × 0.23 = 12.420 m³.
Worked example (imperial)
A 12 ft × 10 ft room with an 8 ft ceiling:
- Floor area: 12 × 10 = 120 ft², shown as 11.15 m² (120.00 ft²).
- Perimeter: 2 × (12 + 10) = 44 ft, shown as 13.41 m.
- Wall area: 44 × 8 = 352 ft², shown as 32.70 m² (352.00 ft²).
The perimeter is reported in meters only, so convert back if you need feet (13.41 m ÷ 0.3048 = 44 ft), or use the length converter.
What each figure is used for
| Result | Typical use |
|---|---|
| Floor area | Flooring, tiles, carpet, underlay, floor screed |
| Ceiling area | Ceiling paint, plasterboard, insulation above |
| Perimeter | Skirting boards, coving, edge trims, expansion gaps |
| Wall area | Paint, wallpaper, plaster, wall tiles |
| Wall volume | Rough quantity of brick or block in the walls |
Things the calculator leaves to you
- Openings. Wall area and wall volume include doors and windows. For paint or plaster, subtract them. For a wall volume estimate, subtract the opening area × wall thickness for each one.
- Wall thickness and room size. If you measured the room from inside, the wall volume uses the inner perimeter, so it slightly understates the material in the corners. That's why it's labelled approximate.
- Wastage. None of the figures add a wastage allowance. Material-specific calculators such as the tile calculator add one for you.
- Irregular shapes. Bay windows, alcoves, chimney breasts and sloped ceilings aren't modelled. Measure them separately and add or subtract the extra area.
Measuring tips
- Measure length and width at floor level, wall to wall, ignoring skirting boards.
- Measure height in more than one place in older buildings, where floors and ceilings are rarely level. Use the largest value for buying materials.
- Round up to the nearest centimeter or quarter inch. Small rounding errors grow once they're multiplied across four walls.
Related calculators
- Paint calculator: turns wall area into liters of paint, after subtracting doors and windows.
- Tile calculator: how many tiles cover your floor area, with a wastage allowance.
- Plaster calculator: plaster volume and cement and sand for the walls.
- Area converter: convert m², ft², yd² and more.
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