Measure the paintable area
Measure each wall's width and height, then multiply them to find its area. Add the wall areas together. If you are painting the ceiling, calculate it separately using room length multiplied by width.
Subtract doors and windows
Measure large openings and subtract their area. For a quick estimate, small openings may be left in as a built-in allowance, but a precise project should measure them. Feature walls, trim and doors often use different products and should be calculated separately.
Allow for multiple coats
Multiply the paintable area by the number of coats. New plaster, major colour changes, porous surfaces and repairs may need primer or additional coats. Do not assume a one-coat claim applies to every surface.
Convert area to paint quantity
Divide the total coated area by the manufacturer's stated coverage per litre or gallon. Coverage varies with the paint, application method, texture and condition of the surface. Round up to available container sizes and keep a modest allowance for touch-ups.
Before buying
- Confirm all measurements use the same unit.
- Read the specific product's coverage label.
- Separate primer, wall paint, ceiling paint and trim paint.
- Consider buying tinted paint in one batch to reduce colour variation.
The paint calculator provides a planning estimate. Surface preparation and real coverage can change the final amount required.