How to use the paint calculator
- Measure the room length, width and wall height.
- Count doors and windows (adjust their areas if they are unusually large).
- Choose the number of coats — two is standard, especially over a color change.
- Enter the coverage printed on your paint can.
Formula
Wall area = 2 × (length + width) × height − doors × door area − windows × window area. Gallons = (wall area + ceiling) × coats ÷ coverage per gallon.
Worked example
A 12 × 12 ft room with 8 ft walls, one door and one window: 384 − 20 − 15 = 349 sq ft. Two coats at 350 sq ft/gal = 1.99 gal → buy 2 gallons.
Tips for using the paint calculator
- Use the coverage printed on the can; textured walls and dark color changes need more paint.
- Plan two coats for most color changes, and prime bare drywall or patched areas first.
- Buy all gallons from one batch, or mix them together, to avoid slight color differences.
Paint Calculator FAQs
How much paint for a 12×12 room?
About 2 gallons for two coats on the walls with one door and one window, assuming 350 sq ft per gallon. Add about 0.8 gallon for two coats on the ceiling.
Do I need primer?
Primer is recommended over new drywall, stains, bare wood and dramatic color changes. Calculate it like a single coat using the primer's coverage.
How much does it cost to have a room painted?
See our paint cost calculator for DIY materials and a professional cost range.
Is old paint a lead hazard?
In homes built before 1978, sanding or scraping old paint can release lead dust. Contractors must follow the EPA RRP rule; see our painting guide.
