How it is calculated
The calculator adds up the four walls (perimeter × height), subtracts about 17 sq ft for each door and 19 sq ft for each window, multiplies by the number of coats and divides by the coverage printed on the can. It adds extra for the roller, tray and texture, then picks gallons, quarts or a 5-gallon bucket so little is left over.
Wall area = 2 × (length + width) × height − doors × 17 sq ft − windows × 19 sq ftGallons = area × coats ÷ coverage × (1 + extra)Example
A 12 × 10 ft room with 8 ft ceilings, one door and one window has 315 sq ft of walls. Two coats at 400 sq ft per gallon plus 10 % extra need 1.7 gallons: one gallon and three quarts, or simply two gallons.
Tips from practice
- New drywall and patches soak up paint. Prime first, or plan one more coat.
- A big color change, such as white over navy, usually takes three coats.
- Buy tinted paint all at once: batches can differ slightly in color.
Questions and answers
How many square feet does a gallon of paint cover?
Most interior paints cover 350–400 sq ft per gallon per coat on smooth, primed walls. The exact number is on the can; enter it as coverage.
How much paint for a 12 × 12 room?
With 8 ft ceilings, about 1.9 gallons for two coats on the walls, or two gallons. Add about a gallon for the ceiling.
Do I need to subtract doors and windows?
The calculator does it for you. For very large windows, choose “Wall area” and enter your own measurement.
Found a mistake in a formula? Write to [email protected]