Paint Coverage Calculator
Estimate how many gallons of paint the walls of a rectangular room need after subtracting doors and windows, for any number of coats.
Updated Formula covered by automated tests
Buying paint is a guessing game that usually ends in either a second trip to the store or three unopened cans in the garage. This estimator works out the wall area of a rectangular room from its width, length and ceiling height, subtracts standard-size doors and windows, and divides by the coverage printed on the can to tell you how many gallons each coat takes and how many whole cans to buy.
It is meant for interior walls, the most common weekend project. The default coverage of 350 square feet per gallon is a typical label figure for interior latex on smooth, previously painted drywall; change it to whatever your product’s label says. Ceilings, trim and doors are not included, because they are often painted in a different sheen or color and are better estimated separately.
The formula
- width, length
- Floor dimensions of the room in feet
- height
- Wall height from floor to ceiling in feet
- 21, 12
- Square feet deducted for a standard 3 × 7 ft door and a 3 × 4 ft window
- coverage
- Square feet one gallon covers in one coat, from the paint label (default 350)
- coats
- Number of full coats; two is usual for a color change
A rectangular room has two pairs of matching walls, so its total wall area is the perimeter of the floor, 2 × (width + length), multiplied by the wall height. Openings are subtracted using typical sizes because measuring every frame rarely changes the number of cans you buy. The paint needed then scales directly with area and number of coats.
Coverage on the label assumes a smooth, sealed surface and is measured as spread rate, not as what a roller actually leaves behind on textured or porous walls. Fresh drywall, bare plaster, stucco and dark-to-light color changes absorb or hide poorly, so primer or an extra coat is often needed. Rounding up to whole gallons builds in a margin, and the leftover is useful for touch-ups.
Worked example
Nate is repainting a 12 by 14 ft bedroom with 8 ft ceilings. It has one door and two windows, he plans two coats, and the can says 350 sq ft per gallon.
- 1 Wall area: 2 × (12 + 14) × 8 = 2 × 26 × 8 = 416 sq ft.
- 2 Subtract openings: 416 − 21 − (2 × 12) = 416 − 21 − 24 = 371 sq ft.
- 3 Per coat: 371 ÷ 350 = 1.06 gallons.
- 4 Two coats: 1.06 × 2 = 2.12 gallons, which rounds up to 3 gallons.
Result: Nate needs about 2.12 gallons. Three gallon cans cover it with room to spare; two gallons plus one quart (0.25 gal) would also work, since the shortfall is only about half a quart.
Before you rely on the result
- The room is assumed to be a simple rectangle with walls of one height; vaulted ceilings, stairwells and angled walls need to be measured and added by hand.
- Door and window sizes are standard estimates, and trim, closets and ceilings are not included.
- In homes built before 1978, sanding or scraping old paint can release lead dust. Follow EPA lead-safe work practices or hire a certified renovator.
Frequently asked questions
Do I need primer before painting?
Use primer on new drywall, bare wood, patched areas, stains, glossy surfaces and when covering a dark color with a light one. Many "paint and primer in one" products are fine over sound, previously painted walls in a similar color. If you prime, estimate primer separately using the coverage on its own label.
How do I estimate paint for the ceiling?
The ceiling area is simply width × length: 12 × 14 = 168 sq ft for the bedroom in the example. At 350 sq ft per gallon that is about half a gallon per coat. Flat ceiling white is usually bought separately from wall paint.
Is it better to buy gallons or a five-gallon bucket?
For a single room, gallons are easier to carry and store. For a whole house in one color, a five-gallon container is usually cheaper per gallon and avoids slight color differences between batches; if you do use several gallons, mix them together in a larger bucket before starting (called boxing).
Why did I run out even though the math said I had enough?
Textured walls, porous surfaces, thick roller nap and paint left in the tray and roller can easily use 10 to 20 percent more than the label rate. Lowering the coverage figure in the calculator to 300 sq ft per gallon is a sensible adjustment for textured or unprimed walls.