How it works
Start with the area you're actually painting: the wall area minus any doors and windows you're skipping. Multiply that by the number of coats, since each coat covers the same area again, then divide by the paint's coverage rate to get the volume needed.
Coverage rate is the amount of wall a can of paint spreads across — typically around 350 sq ft per gallon in the US or 10 m² per litre in Australia and other metric countries, though it varies by product and surface. The can's label always has the manufacturer's own figure, which is worth using over a generic default when precision matters.
The result is not rounded up automatically, because paint is sold in fixed can sizes that differ by brand — round the figure up to whatever size can you're buying.
Worked example
Two examples below — one in US units, one in metric.
- Wall area is 400 sq ft with no deductions.
- Two coats: 400 × 2 = 800 sq ft of coverage needed.
- Divide by the coverage rate: 800 ÷ 350 = 2.29 gallons.
Buy 2.29 gallons of paint, rounded up to whatever can size is available.
- Wall area is 40 m² with no deductions.
- Two coats: 40 × 2 = 80 m² of coverage needed.
- Divide by the coverage rate: 80 ÷ 10 = 8.0 litres.
Buy 8.0 litres of paint, rounded up to whatever can size is available.
Common questions
How many coats do I actually need?
Two coats is standard for most interior walls, especially over a different existing color. A single coat can work for touch-ups or when painting the same color, and dark-to-light color changes sometimes need three.
Should I round up the amount I buy?
Yes — paint is sold in fixed can sizes, so round up to the next full gallon or litre can, and buy a little extra for touch-ups later. Running short mid-wall means an awkward return trip and a slightly different batch.
Why does coverage rate vary by paint?
Coverage depends on the paint's formulation and the surface texture — a smooth primed wall might get 400 sq ft per gallon, while a rough or porous surface can drop that closer to 250. Check the can's label for the manufacturer's actual coverage rate rather than relying on a generic default.