How this calculator works
- Step 1Walls2 (L + W) × HThe perimeter times the height.
- Step 2Openings− doors and windowsAreas left unpainted.
- Step 3Paintarea × coats ÷ coverageCoverage from the tin.
The walls of a rectangular room form one long strip: the perimeter of the floor times the wall height. Take away the doors and windows to get the area to paint.
Each coat covers that area again. Divide by the coverage printed on the tin, in square metres per litre, to get the litres.
Worked example: a 4 × 3 m bedroom
- Litres
(2 (L + W) H − openings) × coats ÷ coverage
- Wall area2 × (4 + 3) × 2.737.80 m²
- Less openings37.80 − 3.534.30 m²
- Two coats34.30 × 268.60 m²
- Paint68.60 ÷ 106.86 L
Read the result
A 10 × 8 m hall with 3.2 m walls and 12 m² of openings needs 20.64 L for two coats.
- Round uptin sizesBuy whole tins that cover the litres.
- PrimerseparateNew plaster usually needs a primer first.
Assumptions and limits
- Rectangular roomFour flat walls.
- Tin coverageUse your product’s figure.
- No wastageSpills and rollers soak up some paint.
- Same paintCeiling included only if chosen.
Other building estimates
Paint covers walls. Floors, bricks and concrete have their own quantities.
For floor and wall tiles, use the tile calculator. The brick calculator covers new walls, and the area calculator handles odd-shaped rooms.
Common mistakes
Mistake: Forgetting the second coat.
Instead: Multiply by the number of coats.
Mistake: Using coverage for smooth walls on rough plaster.
Instead: Lower the coverage figure.
Checking the method
A separate script adds each wall as its own rectangle in exact fractions and matches every golden area and litre figure.
Read how we test our calculators, or browse more everyday tools.