How much paint do i need?
One gallon covers roughly 350 square feet in a single coat on a smooth, primed surface. Two coats are standard for a colour change, so plan on about 175 square feet of finished wall per gallon. That figure is an industry starting point, not a specification for the product you are buying.
Short version
- 350 sq ft per gallon is the general trade figure, not a product specification.
- Texture, porosity and a big colour change all reduce it.
- The real number is the spread rate on your product's technical data sheet.
- Buy for two coats and round up to the next whole gallon.
How to work out your own number
Multiply the room perimeter by the wall height for gross area. Subtract about 20 square feet per door and 15 per average window. Multiply by the number of coats, then divide by the spread rate published for your product.
Round up to the next whole gallon and keep the remainder for touch-ups — a later batch will not match exactly.
What makes coverage worse than the label says
Unprimed or porous drywall absorbs the first coat. Textured surfaces have far more real surface area than their measured area suggests. Rough exterior substrates such as heavy stucco or split-face block can halve it.
Common mistakes
- Treating the general figure as the specification for a particular product.
- Buying for one coat over a different colour.
- Estimating walls and ceilings together when they take different products.
- Forgetting primer on bare or patched surfaces.
Sources
- Sherwin-Williams. Paint calculator (accessed 2026-08-17).
- Benjamin Moore. Paint calculator (accessed 2026-08-17).
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