How the paint calculator works
Wall area = 2 × (length + width) × height − doors × 21 − windows × 15
Gallons = paintable area × coats ÷ coverage per gallon
Example: a 12 × 12 ft room with 8 ft walls
The walls are 2 × (12 + 12) × 8 = 384 square feet. Subtracting one door (21) and two windows (30) leaves 333 square feet. Two coats means 666 square feet of painting. At 350 square feet per gallon that's 1.9 gallons, so buy 2 gallons, about $80 at $40 a gallon.
Tips for buying paint
- Plan on two coats, especially when changing color. Primer counts as a separate coat.
- Rough or porous surfaces like new drywall, brick and textured plaster soak up more paint. Use a lower coverage figure, such as 250 to 300 square feet per gallon.
- If you'd need three or more quarts, a gallon is usually cheaper.
Frequently asked questions
How much paint do I need for a 12 x 12 room?
About 2 gallons for two coats on the walls of a 12 × 12 room with 8 foot ceilings, one door and two windows. Add about another gallon if you're painting the ceiling with the same paint.
How many square feet does a gallon of paint cover?
Most interior paint covers 350 to 400 square feet per gallon for one coat on a smooth, primed wall. The exact figure is printed on the can.
Do I need two coats of paint?
Usually yes. A second coat gives even color and better durability, especially over a different color or fresh patches.
Last reviewed: 2026-10-31