BuildCalcs Paint Calculator UK
Estimate litres of paint required for the four walls of a rectangular room using the coverage stated for your chosen paint.
Reviewed: 20 September 2026 · How we check calculations
How much paint should you buy?
Real-world coverage varies with surface texture, porosity, colour change and application method. The calculated litres are therefore a planning estimate.
Related calculators
How the room paint estimate is calculated
The calculator finds the perimeter of the room, multiplies it by wall height and deducts the entered door and window area. That net wall area is multiplied by the number of coats and divided by the paint's stated coverage in square metres per litre. For a 4 m by 3 m room with 2.4 m walls, the gross four-wall area is 33.6 m² before openings are deducted.
Use the coverage printed on the tin
Coverage varies between products, finishes and surfaces. Fresh plaster, bare masonry and porous repairs can absorb considerably more paint than a previously painted smooth wall. A major colour change can also require an extra coat. The manufacturer's figure for the exact paint is therefore a better input than a generic average.
Buying practical tin sizes
The calculated litres are a theoretical requirement, not a recommendation to buy that exact volume. Round up to available tin sizes and keep enough of the same batch for touch-ups where appearance matters. Ceilings, woodwork, radiators and feature walls using a different product should be calculated separately. Primer, mist coats and specialist sealers are not automatically included.
Measure each paint system separately
If one wall is a feature colour, remove its area from the main-room calculation and calculate it separately using the coverage and coat count for that paint. The same approach works for rooms with substantially different wall heights. This avoids using one average coverage figure for products that may behave differently.