How to use this calculator
- Enter or choose units.
- Enter or choose room length.
- Enter or choose room width.
- Enter or choose wall height.
- Enter or choose doors.
- The paint needed updates as you type. Use Copy result or Share to keep it.
Method
Area = 2(L + W) × H − Doors − Windows (+ L × W for the ceiling); Paint = Area × Coats ÷ Coverage
- L, W, H
- room length, width and wall height
- Coverage
- area one litre or gallon covers in one coat
Worked example
- Walls = 2 × (4 + 3.5) × 2.5 = 37.5 m²; less 1.9 m² door and 2 × 1.5 m² windows = 32.6 m².
- Paint = 32.6 × 2 coats ÷ 10 m²/L = 6.52 L.
- 6.52 ÷ 4 L = 1.63 → 2 cans.
Two coats on a 4 × 3.5 m room with 2.5 m walls take about 6.5 litres, so two 4-litre cans.
What the result means
Primer, a big colour change or fresh plaster usually needs an extra coat. Check the coverage on your actual product; it ranges widely.
Assumptions and limits
- Coverage is the same for every coat.
- Trim, skirting and doors themselves are not painted.
- Textured or porous surfaces can halve the real coverage.
Questions
How much does a litre of paint cover?
Many emulsions state 10–14 m² per litre per coat on smooth walls; a US gallon typically covers 350–400 ft².
Should I subtract doors and windows?
Yes for large openings. For a small room, leaving them in gives a useful margin.
Please note: Quantities are estimates. Confirm requirements with your engineer or supplier before ordering materials.