How the paint math works
Minus 21 sq ft per door and 15 sq ft per window
Gallons = paintable area × coats ÷ coverage per gallon
Paint is sold in gallons and quarts, so the result is rounded up to whole quarts. If you’d need three or more extra quarts, the calculator buys another gallon instead, because a gallon usually costs about the same as three quarts and leaves you touch-up paint.
Worked example
A 12 × 12 ft room with 8 ft walls has 384 sq ft of wall. Subtract one door and two windows (51 sq ft) and you have 333 sq ft. Two coats at 350 sq ft per gallon is 1.9 gallons, so buy 2 gallons.
Coverage changes with the surface
The number on the can assumes a smooth, sealed wall. Textured walls, bare drywall, fresh plaster and dramatic color changes all use more paint. Lower the coverage setting if your walls fall into one of those groups.
Common questions
How much does a gallon of paint cover?
Most interior wall paint covers about 250 to 400 sq ft per gallon per coat. The label gives the exact figure. Rough or porous surfaces get less.
Do I need two coats of paint?
Usually, yes. Two coats give even color and sheen, especially over a different color. Some one-coat paints can manage over a similar color in good condition.
When do I need primer?
Prime new drywall, patched areas, stains, glossy surfaces and big color changes such as dark to light. Primer is included here as one coat over the same area.
How much paint for a 12×12 room?
With 8 ft walls, one door and two windows, about 1.9 gallons for two coats, so 2 gallons. Add roughly another half gallon if you paint the ceiling.
Related calculators
See how we calculate for every formula and constant used on this site.