Paint is the cheapest material in a room and the most annoying to run out of, because the second tin — mixed later, from a different batch — can dry a shade off from the first. This paint calculator works the way painters actually estimate: gross wall area from the room’s perimeter and height, minus the doors and windows you will not paint, multiplied by the number of coats, divided by the coverage your tin claims. It answers in litres and gallons (both, whichever unit you work in), with the ceiling as an option rather than an assumption.
Worked examples — one room, both systems
Metric room, 4 m × 3 m with a 2.5 m ceiling: the walls are 2 × (4 + 3) × 2.5 = 35 m². Deduct one door (1.9 m²) and two windows (1.8 m² each): −5.5 m², leaving 29.5 m² per coat. Two coats at a coverage of 10 m² per litre: 29.5 × 2 ÷ 10 = 5.9 litres. Imperial room, 12 ft × 10 ft with 8 ft ceilings: walls 2 × 22 × 8 = 352 sq ft, minus a 21 sq ft door and two 12 sq ft windows = 307 sq ft per coat; two coats at 350 sq ft per gallon gives 1.75 gallons. Both answers are deliberately a little generous — coverage figures on tins are measured on sealed, smooth surfaces, and real walls drink more on the first coat.
Coverage: the number that decides everything
The default 350 sq ft per gallon (≈10 m²/L) is the middle of the range quality wall paints publish. Rough plaster, bare brick and strong colour changes all pull it down — a first coat over fresh plaster can cover half the label figure, which is why the mist coat exists. If your tin states its own coverage, type it in: the label for the product in your hand outranks any calculator’s default, including this one’s. Dark-over-light and light-over-dark both usually mean a third coat, not thicker paint: change the coats field and see the honest total rather than stretching two coats thin.
The ceiling question
Ceilings are included only if you tick the box, and they are costed at the same coverage — optimistic, since ceiling paint is usually a dedicated product and overhead work wastes more to drips and roller spray. If the ceiling is a different colour or product (it usually is), run the calculator once with the ceiling off for the walls, and once with only the ceiling’s area for its own tin.
What is not estimated
Primer and undercoat are separate products with separate coverages — the calculator will happily compute their quantity if you enter their figures, but it does not assume them. Radiators, woodwork and the wall inside cupboards are not in the room maths. And no calculator can see sheen differences between batches: whatever the total, buy it in one visit, in one batch, and keep the batch code — a photographed tin label is the cheapest colour insurance there is. Exterior work is a different estimating problem (roughcast and weather change everything); our Spanish companion covers facade paint per square metre if that is your job.
How painters sanity-check the tin count
A working painter cross-checks any calculator against the room in one glance: a standard single room takes about 5 litres for two coats on the walls; a double, 7–8; add half as much again when the ceiling joins in. If your result lands far from those anchors, the input to re-examine is coverage — a tin claiming 13 m²/L is describing one coat on a sealed surface, and two coats at that figure on bare plaster is how a wall ends up striped. The other check is tins, not litres: paint is sold in fixed sizes, so convert the total into the combination your store stocks (a 5 L and a 2.5 L often beat three 2.5 L tins on price) and buy them together — batch codes again. Decant the leftovers into a sealed jar with the room’s name and the date on the lid; future you, touching up a scuff, will not remember which white was which.
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Versión en español: Calculadora de Pintura para una Habitación: Litros por Paredes Descontando Puertas y Ventanas, por Manos · Drywall Calculator · Baseboard Calculator.
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