How to Calculate Paint Coverage: Litres Per Room Guide
Buying too little paint means a second trip to the store mid-job. Buying too much means leftover tins sitting in the garage for years. Getting the calculation right isn't complicated — it mostly comes down to measuring your walls accurately and knowing the coverage rate of the paint you're using.
Step 1 — Calculate your wall area
Start by measuring the total surface area you're painting. For a standard rectangular room, the formula is:
For a 5m × 4m room with 2.5m ceilings:
If you're also painting the ceiling, add that separately:
Step 2 — Subtract doors and windows
Large openings reduce the paintable area, so it's worth subtracting them — especially if the room has several windows or a wide doorway. As a rough guide:
- Standard door: approximately 1.8 m²
- Standard window: approximately 1.2–1.5 m²
For a room with one door and two windows, that's roughly 1.8 + (2 × 1.35) = 4.5 m² to subtract, bringing the 45 m² wall area down to about 40.5 m². For a quick estimate, many painters simply skip this step and let the waste percentage absorb the difference — which works fine for rooms with only one or two openings.
Step 3 — Find your paint's coverage rate
Coverage rate tells you how many square meters one litre of paint will cover per coat. It's printed on every tin, but typical ranges are:
| Paint type | Coverage per litre |
|---|---|
| Standard interior emulsion | 10–12 m² |
| Premium interior paint | 12–14 m² |
| Primer / undercoat | 8–10 m² |
| Exterior masonry paint | 6–10 m² |
When in doubt, use 10 m²/L as a conservative default for interior walls — it's better to slightly overestimate than to run short on a second coat.
Step 4 — Calculate total litres needed
Divide your paintable area by the coverage rate, then multiply by the number of coats:
Using our 40.5 m² room with a standard paint at 11 m²/L and two coats:
Add 10% for waste (roller absorption, brush losses, touching up):
Round up to the nearest tin size — in this case, a 10L tin would be the practical choice.
How many coats do you need?
Two coats is standard for most interior repaints onto a previously painted surface. You may need more in these situations:
- Painting over a dark color with a lighter shade — consider a primer coat first
- New bare plaster — needs a mist coat (diluted paint) before normal coats
- Bright or saturated colors on white walls — may need three coats for full coverage
Putting it together
The steps are straightforward once you have your measurements — the most common mistake is forgetting to multiply by the number of coats, which can leave you with half the paint you actually need. If you'd rather skip the manual calculation, our paint calculator takes your room dimensions and coat count and works out the litres and tin quantity automatically.
FAQ
How much paint do I need for a room?
Calculate the total wall area (perimeter × ceiling height), subtract large openings like doors and windows, then divide by the paint's coverage rate — typically 10–12 square meters per litre. Multiply by the number of coats and add 10% for waste.
How many square meters does 1 litre of paint cover?
Most standard interior paints cover around 10–12 square meters per litre per coat. Premium paints may cover slightly more. Always check the coverage rate on the tin, as it varies by brand and finish.
How many coats of paint do I need?
Two coats is standard for most interior repaints. If you're painting over a dark color, going from light to dark, or using a new color on bare plaster, you may need a primer coat plus two finish coats.
How do I calculate wall area for painting?
Multiply the room perimeter (the total of all wall lengths) by the ceiling height. For a 5m × 4m room with 2.5m ceilings, that's 2 × (5+4) × 2.5 = 45 square meters before subtracting doors and windows.