Practical guide · Reviewed September 14, 2026
Calculating paint with coverage and coats
The paint calculation is for a rectangular room. Wall area is 2 × (length + width) × height. Subtract the combined area of doors and windows. Add length × width only if the ceiling uses the same paint.
Worked example
A 4 × 3 m room with 2.4 m walls has 33.6 m² of wall area. Subtract 2 m² of openings: 31.6 m². Two coats and 10% allowance give 69.52 m² of coverage demand. At 10 m²/L, the estimate is 6.952 L. With 2.5 L tins, buy 3 tins.
Use the product label
Coverage depends on the paint, surface, application and condition of the substrate. Enter the manufacturer's stated coverage for one coat and review whether it applies to your surface. The default is an example.
Different products stay separate
If the ceiling is a different colour or finish, do not include it in the same paint quantity. This release does not offer an independent ceiling-only row; calculate it separately with your supplier. Primer and trim need their own estimate and are not included.
Units
Metric uses litres and m²/L. Imperial uses US gallons and ft²/US gal, not UK imperial gallons. Changing measurement units converts dimensions, pack volume and coverage; it does not change the price of that physical pack.
Arithmetic examples are illustrative. Verify measurements and the product label before purchasing.
Use these numbers in a project ↗