Hong Life CalculatorsUnderstanding the paint calculator — concept and principle
This calculator estimates how much paint you need for the walls of a rectangular room, and optionally the ceiling, in litres and in whole cans. It deducts standard openings for doors and windows, multiplies by the number of coats, and adds a small allowance for waste.
How it is calculated
- Wall area = 2 × (length + width) × wall height.
- Paintable area = wall area − doors × 1.9 m² − windows × 1.5 m² (never below 0), plus length × width if the ceiling is included.
- Paint (L) = paintable area × coats ÷ coverage × 1.1. The factor 1.1 adds 10% for waste, absorption and touch-ups.
- Cans = paint ÷ can size, rounded up.
Coverage is the area one litre covers in one coat, in m² per litre, and is printed on the tin. Lengths are in metres.
The principle
Paint is applied as a film of roughly constant thickness, so the volume needed is proportional to the area covered. Coverage is just the inverse of film thickness: 10 m²/L corresponds to a wet film of 0.1 mm. Each coat repeats that film, so the coats multiply the area. Openings are subtracted with typical sizes (a standard interior door is about 0.9 × 2.1 m, a window about 1.2 × 1.25 m) rather than measured ones, which is accurate enough for ordering paint.
Worked example
- Room 5 × 4 m, walls 2.5 m high, 1 door, 2 windows, coverage 10 m²/L, 2 coats, no ceiling, 2.5 L cans.
- Wall area = 2 × (5 + 4) × 2.5 = 45 m².
- Openings = 1 × 1.9 + 2 × 1.5 = 4.9 m², so paintable area = 40.1 m².
- Paint = 40.1 × 2 ÷ 10 = 8.02 L; with 10% extra, 8.02 × 1.1 = 8.82 L.
- Cans = 8.82 ÷ 2.5 = 3.53, rounded up to 4 cans (10 L).
| Coverage (m² per litre) | Paint needed (L) |
|---|---|
| 2.5 | 35.29 |
| 5 | 17.64 |
| 7.5 | 11.76 |
| 10 | 8.82 |
| 12.5 | 7.06 |
| 15 | 5.88 |
| 20 | 4.41 |
How to read the result
Buy the rounded-up cans, and if a larger can is cheaper per litre, compare a 10 L tin against several small ones. Many interior wall paints quote roughly 10–14 m² per litre on smooth, sealed surfaces; rough, porous or fresh plaster absorbs far more, and a strong colour change or covering a dark colour often needs a third coat. Keep a little leftover from the same batch for future touch-ups.
Common mistakes
- Using the tin's best-case coverage for textured or unprimed walls.
- Counting a single coat when two are needed for even colour.
- Forgetting large openings such as patio doors or built-in wardrobes, or a sloped ceiling.
- Mixing paint from different batches on one wall without boxing (combining) it first.
Frequently asked questions
Should I buy primer separately?
New plaster, bare wood and stained or very dark surfaces usually need a primer or a thinned mist coat; it has its own coverage figure.
How much paint do I need for a ceiling?
Set "Include ceiling" to 1. Ceiling paint is often applied in two coats too; some ceiling paints have different coverage.
Why add 10%?
Paint is lost in rollers, trays and absorption, and walls are rarely perfectly smooth. 10% is a common allowance.
Sources: Manufacturer technical data sheets for coverage (m² per litre) printed on the product · BS 6150: Painting of buildings, Code of practice, BSI