How the paint calculator works
Paint is bought by the gallon, and the label lists how many square feet one gallon covers, usually 250 to 400 ft² on smooth walls.
Paintable area = Wall area − 20 ft² per door − 15 ft² per window
Gallons = Paintable area × Coats ÷ Coverage
Two coats is the norm for good, even colour, especially when changing colours. Rough or porous surfaces like new drywall, stucco and bare wood soak up more paint, so use a lower coverage figure or apply primer first. The suggested purchase rounds up to whole quarts so you are not caught short mid-wall.
Worked example
A 12 × 12 ft bedroom has 48 ft of wall at 8 ft high: 384 ft². Subtract one door and two windows (50 ft²) for 334 ft². Two coats at 350 ft² per gallon need 1.91 gallons, so buy 2 gallons.
Frequently asked questions
How much paint do I need for a 12 × 12 room?
About 2 gallons for two coats on the walls with 8 ft ceilings, not counting the ceiling.
How many square feet does a gallon of paint cover?
Typically 350 to 400 ft² per coat on smooth, primed walls.
Do I need primer?
Use primer on new drywall, patched areas, stains and big colour changes.
Is one coat enough?
Rarely. Two coats give even colour and better durability.
Updated October 2026. Results are estimates for education, not financial or medical advice. All math runs in your browser; nothing you type is sent anywhere.