How to use the Paint Coverage
- Enter wall lengths and ceiling height, or the total area if you know it.
- Subtract large windows and doors, leaving small ones in as an allowance.
- Enter the coverage rate from the tin and the number of coats.
- Round up to available tin sizes and keep a little for touching up.
How the calculation works
Wall area is perimeter × height, and paint quantity is that area divided by the coverage rate, multiplied by the number of coats. Coverage is typically quoted around 10 to 13 m² per litre for emulsion on a prepared surface, but that figure assumes a sealed, smooth substrate — new plaster, bare timber and textured finishes can absorb 30 to 50% more on the first coat, which is why a mist coat is used to seal fresh plaster before the finish coats.
Two coats should be assumed for any colour change and for all fresh work; a like-for-like refresh in the same colour sometimes needs one. Dark colours over light, and strong reds and yellows in particular, often need three because their pigments have lower opacity. Rollers, brushes and trays retain paint too, which is part of why the real-world figure lands above the arithmetic.
Litres = (wall area × coats) / coverage per litre ; wall area = perimeter × height − large openingsSource: Manufacturer coverage rates typically 10–13 m²/L for interior emulsion; two-coat practice per British Standard BS 6150 painting guidance.
Worked example
A 4.5 m × 3.6 m room, 2.4 m high, two coats, minus a 2.2 m² patio door, at 12 m²/L coverage.
- Perimeter = 2 × (4.5 + 3.6) = 16.2 m.
- Wall area = 16.2 × 2.4 = 38.88 m², minus 2.2 = 36.68 m².
- Two coats = 73.36 m².
- Litres = 73.36 / 12 = 6.11.
About 6.1 litres, so buy a 5 L tin plus a 2.5 L — roughly 7 L if the walls are freshly plastered and absorb more on the first coat.
Frequently asked questions
How many coats do I need?+
Two for any colour change or new work. Strong reds and yellows, and dark shades over pale walls, often need three because of lower pigment opacity.
Why did I use more paint than calculated?+
Coverage figures assume a smooth sealed surface. New plaster, bare timber and textured finishes absorb considerably more, and rollers and trays retain a surprising amount.
Should I subtract windows and doors?+
Subtract large openings such as patio doors and picture windows. Leave small ones in — the paint saved is roughly what you lose to cutting in and spillage.
Do I need primer as well?+
On bare or repaired surfaces, yes: a primer or a thinned mist coat seals the substrate so the finish coats cover evenly. Painting straight onto fresh plaster wastes topcoat and risks patchy finish.
Last reviewed August 31, 2026. We review this page whenever the underlying formula, tax year, published rate or standard changes.