Paint Calculator
Work out how much paint a room needs — wall area, coats, doors and windows, litres or gallons, and tins to buy.
Openings to subtract
Paint
Enter the room size — the paint estimate updates as you type.
- Wall area
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- Ceiling area
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- Doors + windows deducted
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- Paintable area
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- Area over all coats
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- Paint needed
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- Tins to buy
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- Estimated cost
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An estimate, not a quote — check the spread rate printed on the tin and allow extra for bare or porous surfaces.
How it works
The calculator uses the standard decorator's formula:
paint = ((perimeter × wall height) − openings + optional ceiling) × coats ÷ spread rate,
where the perimeter of a rectangular room is 2 × (length + width). So a 5 × 4 m
room with 2.5 m walls has 45 m² of wall; take off one door (1.8 m²) and one window (1.5 m²)
and 41.7 m² is left to paint. Two coats means covering 83.4 m², and at a typical emulsion
spread rate of 10 m² per litre that is 8.34 litres — four 2.5 litre tins.
Defaults are the usual trade averages: a door is 1.8 m² (about 20 sq ft), a window 1.5 m² (about 15 sq ft), and the spread rate is 10 m² per litre in metric or 350 sq ft per US gallon in imperial. Every one of those is editable, so you can type the exact figures from your tape measure and the back of the tin. Tins are always rounded up — you cannot buy 3.34 of them.
Treat the answer as an estimate rather than a quote: real coverage depends on the paint, the size of the colour change, and how thirsty the surface is — bare plaster, filler and fresh wood soak up far more than a previously painted wall, so buy a little extra for touch-ups. Everything is arithmetic done in your browser with plain JavaScript. Your measurements are never uploaded, stored or sent anywhere, and there are no ads or sign-ups.
Frequently asked questions
How much paint do I need for a room?
Multiply the room perimeter by the wall height, subtract the doors and windows, multiply by the number of coats, then divide by the paint's spread rate. A 5 × 4 m room with 2.5 m walls has 45 m² of wall; take off one door (1.8 m²) and one window (1.5 m²) and you have 41.7 m². Two coats is 83.4 m², and at 10 m² per litre that is 8.34 litres — four 2.5 litre tins.
How much wall does one litre or one gallon of paint cover?
A typical emulsion or latex covers about 10 m² per litre, which is roughly 350–400 sq ft per US gallon. Gloss, primer and deep colours cover less, and bare plaster, filler or new wood can soak up 20–30% more on the first coat. The exact spread rate is printed on the tin, and this calculator lets you type it in instead of using the default.
Do I really need two coats of paint?
Usually yes. Two coats is the standard for an even finish, and you need three when going from a dark colour to a light one or painting over bold accent walls. One coat only works for a refresh in the same shade. Set the number of coats in the calculator and the litres and tin count update instantly.