Paint Calculator — How Much Paint Do You Need?
Getting the right amount of paint before you start saves you a second trip to the hardware store — and avoids running out mid-coat. This calculator works out the paintable wall area, applies your chosen waste allowance, and tells you how many cans to buy.
How we calculate
Wall area = 2 × (length + width) × height. Doors (~1.9 m²) and windows (~1.2 m²) are subtracted. Paint = (paintable area ÷ coverage per litre) × coats × (1 + waste %). Cans = ⌈paint litres ÷ can size⌉.
Worked example
A 4 m × 3 m room with 2.4 m ceilings, one door, one window, two coats at 12 m²/L with 10% waste: wall area ≈ 33.6 m², paintable area ≈ 30.5 m², paint needed ≈ 5.6 L, so 3 × 2.5 L cans.
Common mistakes
- Not accounting for doors and windows — this typically overstates paint needed by 10–15%.
- Using only one coat for a colour change over a very different base colour.
- Forgetting primer when painting over bare plaster or a dark colour.
- Buying single large cans when the remainder cannot be resealed easily.
Frequently asked questions
How much paint do I need for a standard bedroom?+
A typical 4 m × 3 m bedroom with 2.4 m ceilings, one door, and one window requires around 5–6 litres for two coats — roughly 2–3 × 2.5 L cans. Actual coverage depends on the paint brand and surface texture.
How do I calculate paint for an irregularly shaped room?+
Measure each wall individually (height × width), add the areas together, then subtract the area of doors (about 1.9 m² each) and windows (about 1.2 m² each). Enter the total wall area rather than room length and width.
How much extra paint should I buy for waste?+
A 10% waste allowance covers normal roller and brush losses on smooth walls. Increase to 15% for textured surfaces or rough masonry, which absorbs significantly more paint per coat.
Do I need primer as well as topcoat paint?+
Primer is needed when painting bare plaster, new plasterboard, heavily patched surfaces, or when making a dramatic colour change. One coat of primer followed by two coats of topcoat gives the best finish and longest-lasting result.