Paint Calculator
How much paint you need for a room, minus doors and windows.
Good to know. Changing colour takes two coats. The second is where the colour actually shows up.
Approximate. Real-world values vary.
How it works
In plain words
Multiply the total wall length by the height to get the wall area. Take off the doors and windows. Multiply by the number of coats and divide by how far one litre (or gallon) goes.
The formula
area = wall length × height − doors × 1.8 m² (20 ft²) − windows × 1.44 m² (15 ft²) paint = area × coats ÷ coverage
Worked example
Walls 16 m long and 2.5 m high are 40 m². Minus a door (1.8 m²) and a window (1.44 m²) leaves 36.76 m². Two coats at 10 m² per litre need 7.4 litres.
Sources
- Coverage printed on paint tins Typically 10–12 m² per litre, or 350–400 ft² per US gallon.
- NIST Special Publication 811, Guide for the Use of the International System of Units, Appendix B Exact conversion factors for US customary units.
Sources checked September 29, 2026. 5 automated tests check its answers on every change.
About the paint calculator
Running out of paint halfway through a wall is annoying, and different batches can differ slightly in colour. Buying far too much is a waste. This calculator helps you get close.
Start by measuring the length of each wall you are painting and adding them together, then the height from floor to ceiling. For a rectangular room, the total wall length is simply twice the length plus twice the width. The calculator takes off a standard-sized door and window for each one you enter.
Coverage is printed on the tin, usually as square metres per litre or square feet per gallon. It varies by product and surface: 10 to 12 m² per litre is typical for emulsion on smooth, previously painted walls. Fresh plaster, rough surfaces and strong colour changes soak up more, so use a lower figure or plan for an extra coat. A primer or mist coat may be needed on new plaster.
Two coats is the norm for an even finish. Round up when you buy, and keep a small amount for touch-ups later. For ceilings, calculate the area with the area calculator and use ceiling paint coverage.
Questions people ask
How much paint do I need for a room?
Measure the wall area, subtract doors and windows, multiply by the number of coats, and divide by the coverage on the tin. A typical 5 × 3 m bedroom needs around 7–8 litres for two coats.
How many coats of paint do I need?
Usually two. You may need three for a big colour change or a light colour over a dark one, and a mist coat on new plaster.
How far does a litre of paint go?
Typically 10–12 m² per coat on smooth, sealed walls. Check the tin for your product.