Paint Calculator
Estimate litres or gallons of paint for a room from wall sizes, openings, coats and coverage.
How it works
Add one row for each wall size, with a count for identical walls, and one row for each door or window size. Enter the coverage from your tin and the number of coats. The openings are subtracted from the walls, and the paint is rounded up to a whole litre or gallon.
paintable area = Σ(wall width × height × count) − Σ(opening width × height × count)
paint exact = paintable area × coats ÷ coverage
paint to buy = ceil(paint exact)Example
A room has two walls 4 m × 2.5 m and two walls 3 m × 2.5 m: 20 + 15 = 35 m². A door of 0.9 × 2 m (1.8 m²) and a window of 1.2 × 1.2 m (1.44 m²) take off 3.24 m², leaving 31.76 m². Two coats is 63.52 m². At 10 m² per litre that is 6.352 L, so buy 7 L.
Common mistakes
- Using the same coverage for every paint. Spreading rates differ by product, colour and surface. Read the tin.
- Forgetting the second coat. Coverage on the tin is per coat, so two coats need twice the area.
- Leaving out the ceiling. It only counts if you add it as a wall row (its length and width as the width and height).
About this calculator
List each wall as a width and height, with a count for identical walls, then list the doors and windows to take off. The calculator subtracts the openings from the wall area, multiplies by the number of coats, divides by the coverage you enter, and rounds the result up to a whole litre or gallon. The exact figure is shown as well.
Coverage is a number you enter, per coat. The preset is 10 m² per litre in metric and 350 ft² per US gallon in imperial, as typical starting figures only; your tin states the real one. Ceilings are not included unless you add them as a wall. Trim, textured or porous surfaces, and paint left in the tin are not modelled.
Frequently asked questions
How much paint do I need for a room?
Add up the wall areas, subtract doors and windows, multiply by the number of coats and divide by the coverage per litre or gallon. A room with 31.76 m² of paintable wall painted twice at 10 m² per litre needs 6.352 L, so 7 L.
What coverage should I enter?
Use the figure on your tin. The preset values, 10 m² per litre and 350 ft² per US gallon, are only typical starting points. Rough, porous or dark surfaces usually need more paint than the tin figure suggests.
Is the ceiling included?
No. Add it as a wall row if you are painting it, using its length and width as the width and height. The wall rows only count what you enter.
Which gallon does imperial mode use?
The US gallon, which is 3.785 litres. A UK gallon is larger, at 4.546 litres. If you buy in litres, use metric mode with the litres figure from the tin.
Why is the answer rounded up to a whole litre or gallon?
Paint is sold in fixed tin sizes, so you buy at least the exact amount. The exact figure is shown too, so you can match it to the tin sizes in the shop.