CALCULATION METHOD
How much paint do I need?
Find wall area from room perimeter times wall height, then subtract doors and windows. Multiply the paintable area by the number of coats and touch-up allowance, and divide by the coverage printed on the paint label.
Wall area = 2 × (length + width) × height
Paintable walls = wall area − doors − windows
Finish gallons = paintable area × coats × (1 + allowance) ÷ coverage
Ceiling area = length × width
12 × 10 room paint example
A 12 × 10 ft room with 8 ft walls has 352 sq ft of gross wall area. Deducting one 20 sq ft door and two 15 sq ft windows leaves 302 sq ft. Two coats plus 5% touch-up require 634.2 sq ft of wall coverage, or about 1.59 gallons at 400 sq ft per gallon.
Paint calculator for walls
The wall result is kept separate from the ceiling so different colors and finishes are not incorrectly combined. Enter the number of identical rooms to estimate a repeated apartment, office, or renovation layout.
Paint gallon calculator
The exact gallon requirement is useful for comparison, while the shopping result rounds each paint type into whole gallons and quarts. A quart equals one-quarter gallon.
Ceiling paint calculator
Enable ceiling paint to include room length times width using the same number of finish coats and coverage rate. Disable it when only the walls will be painted.
Primer and paint cost calculator
Primer uses its own editable spread rate and one coat. Material cost uses the rounded gallon and quart containers for wall paint, ceiling paint, and primer, rather than charging for fractional containers.
Sherwin-Williams paint calculator guidance for typical paint and primer coverage ranges; Sherwin-Williams painting FAQ for area divided by label coverage; Behr’s calculator for room, door, window, ceiling, and touch-up inputs. Accessed September 11, 2026.