Multiply each wall’s width by its height and add the results. Large doors and windows can be deducted.
Use the coverage rate on the product to convert total area into litres and account for the number of coats.
Rough or highly absorbent surfaces may use more paint, so a small reserve is practical.
The calculation
Wall area is room perimeter multiplied by height. Subtract doors and windows; add the ceiling separately if it will be painted. Liters equal net area × coat count ÷ the product's stated coverage in m²/l per coat. Keep filler, primer and finish-paint estimates separate.
Worked example
A 4 × 5 m room at 2.8 m high has 18 × 2.8 = 50.4 m² of wall. Subtracting 6 m² of openings leaves 44.4 m². At two coats and an illustrative 10 m²/l, the result is 8.88 liters. This is an arithmetic example, not a product specification. Allow separately for texture and losses, then round to available pack sizes.
Before purchasing
- Recheck dimensions and the openings to subtract.
- Confirm that the coverage figure applies to one coat.
- Plan the allowance and available pack sizes separately.
What if the pack is sold in kilograms?
Do not apply an m²/l figure directly to kilograms. Use the stated m²/kg coverage or the manufacturer's conversion information. Water-based paints do not all have the same density.