How this calculation works
Estimate coating volume from the surface actually painted, the number of coats and the coverage stated for the product. Subtract windows and doors only if they will not receive that paint. Coverage is not universal: surface texture, porosity and application method affect consumption, so use the manufacturer figure applicable to the intended product and surface.
Worked example
This example uses illustrative inputs. Your current result is shown above.
For 100 m² gross wall area, 10 m² openings, two coats, 10 m²/L coverage and 10% waste, net area is 90 m². Paint required is 90 × 2 × 1.10 ÷ 10 = 19.8 L, rounded upward to 20 whole litres.
Assumptions and limits
- Coverage is per coat and applies to the selected paint; primer and topcoat must be estimated separately if their coverage differs.
- Openings cannot exceed gross area. The whole-litre recommendation does not optimise available tin sizes or combine different colours.
Questions people ask
Should primer count as one of the coats?
Only when its coverage and product are the same as the estimate being made. Usually calculate primer and finish coats separately.
Can I subtract every door and window?
Subtract only surfaces excluded from the paint application. Frames, reveals and doors that will be coated need their own measured allowance.