Paint Calculator

This calculator totals the wall area of a rectangular room, subtracts standard openings — 21 sq ft for each door and 15 sq ft for each window, the estimating standards used by painters — and divides by typical paint coverage of 350 sq ft per gallon per coat.

Rough, porous or unprimed surfaces can drop real coverage to 250–300 sq ft per gallon, and a strong color change usually needs the full two coats. The gallons-to-buy figure is rounded UP to whole gallons, since paint is sold in whole containers.

The formula

Net area = 2(L + W) × H − (doors × 21 + windows × 15); Gallons = Net area × coats ÷ 350

Worked example

A 12 × 10 ft room with 8 ft walls has 352 sq ft of wall; after one door and two windows, 301 sq ft remains. Two coats at 350 sq ft per gallon need 1.72 gallons (6.51 liters) — buy 2 gallons.

The example is pre-loaded in the result panel — press Calculate with your own numbers to replace it.

Frequently asked questions

How far does a gallon of paint actually go?

Manufacturers rate most wall paints at 350–400 sq ft per gallon on smooth, primed drywall. This calculator uses 350 to be safe. Bare drywall, stucco and brick absorb much more — prime first or add a third coat to the estimate.

Should I subtract doors and windows?

Yes for estimating paint, using roughly 21 sq ft per standard door and 15 sq ft per average window. The trim around them still gets painted, but usually with a smaller amount of trim paint, not wall paint.

Do I need two coats?

Almost always for an even finish and true color, especially over primer, patches or a different old color. One coat can work for a refresh in the same color and sheen.

Category: Construction & Home calculators

Gross wall area	352 sq ft
Paintable area (after openings)	301 sq ft
Paint needed	1.72 gal
Paint needed	6.51 liters
Whole gallons to buy	2