Painting guide
How Much Paint Do I Need for a Room?
Measure room walls, subtract openings, account for coats and coverage, and estimate the gallons of paint to buy.
Short answer
Measure the wall area, subtract major doors and windows, multiply by the number of coats, then divide by the coverage listed on the paint can. Round up to full gallons and consider a small buffer for touch-ups.
Use the Paint CalculatorThe formula
- Wall area = 2 x (room length + room width) x wall height - doors and windows
- Painted coverage = wall area x number of coats x (1 + buffer percentage)
- Gallons to buy = painted coverage / coverage per gallon, rounded up
Worked example
A 12 ft by 12 ft bedroom with 8 ft walls
- Wall area before openings: 2 x (12 + 12) x 8 = 384 sq ft.
- Subtract 40 sq ft for a door and window: 384 - 40 = 344 sq ft.
- Two coats plus a 10% buffer: 344 x 2 x 1.10 = 756.8 sq ft of coverage.
- At 350 sq ft per gallon: 756.8 / 350 = 2.16 gallons.
Buy 3 gallons, then confirm the coverage and finish on the selected product label.
Measure walls, not the floor
Paint quantity starts with the wall surface. For a simple rectangular room, add the room length and width, double that number for all four walls, and multiply by wall height. Rooms with varied heights or alcoves are easier to estimate wall by wall.
Subtract larger openings such as doors, sliding doors, or broad windows. Small openings often do not materially change the purchase after rounding and leaving some paint for touch-ups can be helpful.
Coverage and coats change the answer
Coverage is not one universal number. Sherwin-Williams states that a gallon typically covers about 350 to 400 square feet, while noting that application method and the surface type or condition affect product requirements.
A major color change, porous new drywall, texture, repair patches, or a product requiring primer can change the project. Calculate wall paint separately from ceiling paint, trim paint, and primer.
- Use the coverage stated for the paint you will actually purchase.
- Count each coat of wall paint; two coats doubles the covered surface.
- Keep leftover matching paint labeled for later touch-ups when practical.
Common paint estimating mistakes
Do not use floor square footage as wall square footage. Do not assume all products cover the same amount, and do not combine primer, wall paint, ceiling paint, and trim paint into one number.
The calculator provides a shopping estimate, not a guarantee of finish quality or a replacement for product instructions.
References and verification
Use these references together with the instructions and coverage or yield information on the product you select.