How to calculate how much paint you need
Add up the area of your walls, subtract doors and windows, multiply by the number of coats, then divide by the paint's coverage rate: Paint = (Wall area − openings) × coats ÷ coverage per litre. Wall area for a rectangular room is 2 × (length + width) × height.
How many coats?
Most jobs need two coats for an even finish. Go to three when painting a light colour over a dark one, or onto fresh plaster. A primer/undercoat may be needed on bare or patchy surfaces — count it as an extra coat.
Tips to buy the right amount
- Buy a little extra (round up) so you have enough for touch-ups and don't run out mid-wall.
- Coverage drops on rough, porous or textured surfaces — reduce the coverage number for those.
- Painting the ceiling too? Add its area (length × width) to the wall area.
Does this include the ceiling?
No — it covers the four walls. Add the ceiling area (length × width) manually if you're painting it.
What coverage should I use?
Check the tin. Typical emulsion is 10–12 m² per litre (about 350 ft² per US gallon) per coat; primers and textured paints cover less.
Metric or US gallons?
Switch units to feet and the coverage/price flip to US gallons (1 gal ≈ 3.785 litres).