Paint Calculator
Enter your room's length, width, and wall height, plus the number of doors and windows and how many coats you plan to apply, to find out how many litres of paint to buy and your total paintable wall area.
How this calculation works
The tool first finds the room's wall perimeter (2 × (length + width)) and multiplies it by the wall height to get total wall area. It then subtracts an allowance for doors (1.8 m² each) and windows (1.5 m² each) to get the actual paintable area, since you don't paint over openings.
Litres needed = paintable area × number of coats ÷ your paint's coverage rate (m² per litre). Coverage rate varies by paint brand, finish, and the surface's porosity, so check the tin or product data sheet for the true figure rather than guessing.
Ceilings are not included in this estimate — measure the ceiling area separately (length × width) and divide by your coverage rate if you're painting it too.
Worked example
Typical paint coverage by finish
Coverage rates printed on the tin assume a smooth, primed surface applied at the recommended thickness. Real-world coverage is often lower on textured or absorbent walls.
| Paint finish | Typical coverage | Best for |
|---|---|---|
| Matte / flat emulsion | 10–12 m²/L | Ceilings, low-traffic walls |
| Eggshell / satin | 9–11 m²/L | Living rooms, bedrooms |
| Semi-gloss | 8–10 m²/L | Kitchens, bathrooms, trim |
| Gloss | 8–9 m²/L | Doors, skirting, high-wear areas |
| Primer / undercoat | 6–9 m²/L | Bare plaster, colour changes |
How to measure a room for painting
- Measure the length and width of the room at floor level with a tape measure, in metres.
- Measure wall height from floor to ceiling; use the average if the ceiling slopes.
- Count doors and standard windows rather than measuring each opening individually, unless they're unusually large.
- Note any areas you won't paint (built-in wardrobes, tiled sections) and subtract them manually if significant.
- Round your final litre figure up to the next full tin size, then add roughly 10% for waste and touch-ups.
Why coats and primer matter
A single coat rarely gives even, opaque colour — brush and roller marks, patchy absorption, and the underlying colour usually show through. Two coats is the standard recommendation for most interior walls, and it's baked into this calculator's default.
If you're painting over a much darker or very different colour (for example, white over dark red), consider a grey or tinted primer coat first. Primer seals porous surfaces so your topcoat covers more evenly and often uses less paint overall, even though it adds an extra coat to the total.
Buy a buffer, not the exact amount
Always round up to the nearest tin size and buy about 10% more paint than the calculator suggests. This protects against measurement error, extra touch-ups, higher absorption than expected, and the risk of a discontinued colour or batch mismatch if you need to buy more later.