Buying paint is a guessing game unless you do the maths. Too little means an extra trip mid-project (and a possible colour mismatch); too much wastes money. This calculator gives you a reliable figure from your room's dimensions.
How paint quantity is calculated
The core figure is wall area: the room's perimeter (twice the length plus twice the width) multiplied by the wall height. We subtract a standard allowance for each door and window, multiply by your number of coats, and divide by the paint's coverage rate to get the volume required.
What affects coverage
Surface texture and colour matter. Rough, porous or previously unpainted surfaces drink more paint, and covering a dark colour with a light one often needs an extra coat. When in doubt, follow the coverage printed on the tin and add a margin.
Frequently asked questions
Measure the perimeter, multiply by wall height for the total area, subtract doors and windows, then multiply by your number of coats and divide by the paint's coverage (about 11 m² or 350 ft² per coat). This calculator does it all for you.
Two coats is standard for an even, durable finish. You may need three for dramatic colour changes or when going from dark to light, and a primer coat helps on bare surfaces.
No — a US gallon is about 3.79 litres, so it covers roughly 3.5× the area. This calculator outputs whichever unit matches your selected system.