Project estimating guide
Paint Coverage and Coats: How Many Gallons to Buy
Use the paint product’s stated coverage for one coat and multiply the measured surface by the number of coats. A room’s floor area does not substitute for its wall area.
Reviewed September 6, 2026Calculate the actual painted surface
For a 12 × 15 ft room with 8 ft walls, wall area is 2 × (12 + 15) × 8 = 432 ft². If openings total 32 ft², the net wall surface is 400 ft². A separately included ceiling adds another 180 ft².
Apply coverage once per coat
At an illustrative 350 ft² per US gallon, 400 ft² of wall and two coats need 2.286 gallons before allowance. With 10% allowance that becomes 2.514 gallons, so three 1-gallon cans are needed.
400 ft² at an example 350 ft²/gal
Coverage is an input, not a guarantee for every paint and substrate.
| Coats | Gallons before allowance |
|---|---|
| 1 | 1.143 |
| 2 | 2.286 |
| 3 | 3.429 |
Separate products and purchase sizes
Calculate primer, trim, ceiling paint, and different wall colors separately. Their coverage and package sizes can differ. Compare final quantities of complete cans rather than dividing the whole house’s area by one coverage rate. Keep touch-up reserves visible and check whether the quoted paint price is per quart, gallon, or another container.
Sources and product references
- BEHR — paint calculator
Manufacturer estimating context. Enter coverage from the specific paint label.
- NIST — SI units: volume
Unit relationships; supplier product coverage and prices remain separate inputs.