Three different numbers from the same room
A room 5 m × 4 m × 2.6 m gives three quantities that are easy to confuse and are used for entirely different purposes:
- Floor area = L × W = 20 m² — flooring, tile, rug sizing.
- Wall surface area = perimeter × height = 2(L+W) × H = 46.8 m² — paint, wallpaper.
- Volume = L × W × H = 52 m³ — air volume, for heating, cooling and ventilation.
The common error is using floor area to order paint, or wall area to size a rug. They differ by more than a factor of two in a typical room, and the order is either short or left over.
Paint arithmetic
Coverage is the figure to memorise: roughly 10 m² per litre of emulsion, or 350 ft² per gallon (about 37 m² per US gallon). The calculation is then:
litres = (wall area − doors/windows) × 1.1 ÷ 10
The subtraction is worth doing. A 20 m² room has a typical door and window totalling about 3 m², so painting it needs roughly (46.8 − 3) × 1.1 ÷ 10 ≈ 4.8 litres — 5 litres, not 4.7, and in practice two coats means two 2.5-litre tins plus a little spare rather than one.
That 1.1 factor is wastage: cutting in, roller loading and the patch that always needs a second go. Skipping it is how people end up one coat short.
Flooring and wastage
Flooring and tiling need 10% wastage and a few percent more on a diagonal lay or in a room with many corners. For a 20 m² room, 22 m² of material. The flooring calculator and the tile calculator handle the pattern-based layouts properly.
One ordering note worth knowing: buy flooring after the room is measured and preferably after any wet work is done, since a new screed or plaster can change dimensions by a centimetre or two — enough for a rigid material to refuse to fit.
Why volume is not the same as area
Volume matters for three things specifically:
- Heating: the volume of air is what a heater must warm, though the wall and floor surfaces dominate heat loss in most homes — so the real calculation is surface area by orientation, not air volume.
- Ventilation and humidity: cubic metres per hour of fresh air required depends on floor area and occupancy, not volume.
- Acoustics: reverberation time depends on volume and surface treatment, which is why a carpeted room and a tiled one with the same volume sound completely different.
Measuring properly
Three habits that make the numbers usable:
- Measure the wall, not the room. For paint and flooring, measure the surfaces you will cover.
- Measure twice, in the same units. Metric or imperial throughout — mixing produces the 0.3048 errors that turn 5 m into 5 feet.
- Note down-floor irregularities. Skirting, door recess, alcoves and chimney breasts all subtract from a wall or add to a floor, and they are exactly where a naive L × W goes wrong.
Frequently asked questions
What is the difference between room area and volume?
Area is L × W — the floor you cover with flooring or paint a ceiling across. Volume is L × W × H — the air space, which matters for heating, ventilation and acoustics. They answer different questions and using one in place of the other is the most common error.
How much paint do I need for a room?
Subtract doors and windows from the wall area, add 10% for wastage, and divide by about 10 m² per litre (or 350 ft² per gallon). A typical 20 m² room needs about 5 litres for two coats.
How much flooring should I buy?
Floor area plus about 10% wastage, or more for a diagonal or herringbone lay. Buy after the room is finished rather than before, so the dimensions are final.
Do I need to subtract doors and windows?
Yes, for paint and wallpaper — a door and window are typically 3 to 4 m² of a wall, enough to change the number of tins you buy. For flooring you would not subtract them, since flooring is measured as floor area with wastage instead.