Paint Calculator
Calculate how many litres of paint you need to cover your walls. Enter room dimensions and number of coats.
How to use this tool
- Enter room width, room length, wall height, number of coats and paint coverage in the fields above.
- Results update instantly as you type — or click Calculate.
- Read your paint needed and the full breakdown beneath it.
Estimate how many litres of paint you need before your next painting project.
⚠ This tool provides general estimates for education only and is not financial, tax or legal advice. Figures may not reflect your situation — verify with a qualified professional.
Formula
Wall perimeter = 2 × (width + length)
Total wall area (m²) = perimeter × height
Paint needed (L) = area × coats ÷ coverage (m²/L)
litres = (2 × (w + l)) × h × coats / coverage
How it works
This calculator estimates the volume of paint required to cover all four walls of a rectangular room. It computes wall area from the room perimeter and ceiling height, then divides by the paint's stated coverage rate and multiplies by the number of coats.
Doors and windows are not subtracted from the wall area in this model, so the estimate is intentionally slightly generous — a common practical approach to avoid running short. Actual coverage can vary with surface porosity, paint brand, and application method.
Worked example
- Room: 4 m wide × 5 m long × 2.5 m high. 2 coats. Coverage 10 m²/L.
- Perimeter = 2 × (4 + 5) = 18 m.
- Wall area = 18 × 2.5 = 45 m².
- Paint needed = 45 × 2 coats ÷ 10 m²/L = 9.0 L.
Wall area = 45.0 m²; Paint needed = 9.0 L
Common mistakes to avoid
- Using gross wall area without subtracting doors and windows — a standard door (1.8 m2) and window (1.2 m2) can reduce paint needed by half a litre or more per coat.
- Assuming a coverage rate of 10-12 m2/L for rough or previously unpainted surfaces — bare plaster and textured walls absorb more paint and may yield only 6-8 m2/L.
- Forgetting to account for the number of coats — a single coat over a dark colour rarely provides full coverage; two coats are the norm.
Key terms
- Coverage rate
- The area one litre of paint covers in a single coat, typically stated on the tin in m²/L; influenced by surface texture and paint viscosity.
- Coat
- A single layer of paint applied over the entire surface; most projects require 2 coats for uniform colour and opacity.
- Wall area
- The total paintable surface area of the walls in a room, calculated from perimeter × height.
- Perimeter
- The total linear distance around the four walls of a room: 2 × (width + length).
Frequently asked questions
- How much paint do I need for a room?
- Calculate the perimeter × wall height = wall area in m², multiply by the number of coats, then divide by the paint coverage (typically 10 m²/L).