Calculators

Paint Calculator

Gallons and quarts of paint for a room's walls and ceiling, less doors and windows, for any number of coats.

How it works

The wall area of a rectangular room is its perimeter times its height. Subtract about 21 sq ft for each door and 15 sq ft for each window, add the ceiling if you are painting it, and multiply by the number of coats. Dividing by the coverage on the can gives gallons. The result is rounded up to the quart, and three quarts become a gallon because they cost about the same.

gallons = (2 × (L + W) × H − openings + ceiling) × coats ÷ coverage
L, W, H
Room length, width and wall height, in feet
Coverage
Square feet one gallon covers in one coat, usually 350–400
Openings
21 sq ft per door, 15 sq ft per window

Worked example

A 12 × 14 ft bedroom with 8 ft walls, one door and two windows, two coats at 350 sq ft per gallon.

  1. 1Walls: 2 × (12 + 14) × 8 = 416 sq ft
  2. 2Less openings: 416 − 21 − 2 × 15 = 365 sq ft
  3. 3Two coats: 730 sq ft ÷ 350 = 2.09 gallons
  4. 4Rounded up to the quart: 2 gallons and 1 quart

Buy 2 gallons and a quart, about $105 at $45 a gallon.

Frequently asked questions

How many square feet does a gallon of paint cover?

Most interior paints cover 350–400 sq ft per gallon on smooth, primed walls. Textured walls, bare drywall, masonry and dark-to-light colour changes soak up more, so plan on 250–300 sq ft or add a coat.

Do I need two coats of paint?

Usually. Two coats give an even colour and sheen, and hide roller marks. One coat can be enough when repainting in the same colour with a good paint. Paint-and-primer products still generally need two coats over a big colour change.

How much paint do I need for trim and doors?

Much less than for walls. A quart of semi-gloss is enough for the baseboards, door and window casings of an average room. A standard interior door takes about a quart for two coats on both sides.

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