How to use the paint calculator
- Measure the room's length and width along the floor, and the height of the walls.
- Enter the number of coats, how many doors, and the total area of the windows.
- Tick Include the ceiling if you're painting it too.
- Type the coverage printed on your tin and choose the tin sizes sold where you live.
The formula
wall area = 2 × (length + width) × height
painted area = (wall area − doors − windows) × coats
paint = painted area ÷ coverage per coat
We then suggest a mix of tins (metric: 5 L, 2.5 L and 1 L) or cans (US: gallons and quarts) that covers what you need with the fewest tins, then the least left over (bigger tins are usually cheaper per litre).
Worked examples
Metric: a 4 m × 3 m room with 2.4 m walls has 33.6 m² of wall. Minus one door (1.51 m²) and 1.5 m² of window, two coats at 12 m²/L need 5.1 litres — buy 1 × 5 L + 1 × 1 L.
Imperial: a 12 × 12 ft room with 8 ft walls has 384 sq ft of wall. Two coats at 350 sq ft/gal need 2.19 gallons — buy 2 × 1 gallon + 1 × 1 quart (before subtracting doors and windows).
Why coverage varies
The coverage on the tin assumes a smooth, sealed surface and an even coat. Rough, textured or porous surfaces — brick, render, new plaster — soak up more paint. Strong colour changes often need an extra coat. That's why the coverage box sits right in the calculator: the label on your actual tin is the best guide. Without it, we use 12 m² per litre (the low end of typical UK emulsions) or 350 sq ft per US gallon (the low end of Sherwin-Williams' range).
Tips
- New plaster usually needs a mist coat or primer first — follow the paint maker's advice.
- Buy matching batches. If you need several tins of a mixed colour, buy them together and stir them together for an even colour.
- Keep a little spare for touch-ups later.
Common mistakes to avoid
- Forgetting the second coat. Most colour changes need two coats.
- Using the best-case coverage. The tin's top figure assumes a smooth, sealed wall; rough or new surfaces use more.
- Forgetting the ceiling, trim or doors — they usually need different paint and are worked out separately.
Measuring an awkward shape? See how to measure an irregular area.
Frequently asked questions
How much paint do I need for a 12 × 12 room?
With 8 ft walls, the walls are 384 sq ft. Two coats at 350 sq ft per gallon need about 2.19 gallons — for example 2 × 1 gallon + 1 × 1 quart. Subtract doors and windows for a closer figure.
How many square feet does a gallon of paint cover?
Sherwin-Williams gives roughly 350–400 sq ft per gallon per coat on a smooth surface. We default to the lower 350 sq ft so you’re less likely to run short. Textured or porous surfaces such as brick or fresh plaster can take noticeably more paint.
How many square metres does a litre of paint cover?
It depends on the product. UK wall emulsions commonly cover about 12–14 m² per litre per coat; some trade and budget paints cover less (8–10 m²). We default to 12 m² per litre, the low end of the usual UK range — but the figure printed on your tin always wins, so type it in.
How much paint do I need for a 4 × 4 m room?
The walls of a 4 × 4 m room with 2.4 m walls are about 38 m². Two coats at 12 m²/L need about 6.4 litres before taking off doors and windows. Dulux UK suggests about 5 litres for a room this size, so a 5 L tin is often enough if your paint covers 14 m²/L or more — check the tin.
Do I need two coats?
Two coats is the usual recommendation for walls, especially when changing colour or painting fresh plaster or a dark colour. Check the instructions on the tin; some paints claim one-coat coverage in specific conditions.
Is ceiling paint included?
Only if you tick “Include the ceiling”. The ceiling is shown as a separate amount because ceilings are usually painted with a different paint.
Sources
Last updated 11 November 2026. Found a mistake? Tell us.