Paint Calculator: Gallons Needed for Walls
Enter paintable wall area in square feet, subtract doors and windows, and use the coverage printed on your paint can.
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How to measure
Measure each wall width and height in feet. Multiply each pair, add the wall areas and subtract doors or windows that will stay unpainted. If painting the ceiling, measure it separately. Use the coverage number for the exact color, finish and surface.
Formula and assumptions
Gallons = ((wall area − openings) × coats ÷ coverage per gallon) × (1 + allowance / 100).
Worked example
For 380 sq ft of wall with 35 sq ft of openings, two coats, 400 sq ft per gallon and 10% allowance, the estimate is 1.90 gallons.
Before you buy
Coverage depends on porosity, color changes and application method. Round up to container sizes sold for your exact paint.
Why can the real quantity differ?
Unprimed drywall, textured surfaces and a strong color change may reduce coverage. A primer is a separate product and should not be counted as a finish coat.
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Editorial review: September 2026. This is a planning estimate; confirm dimensions and product specifications before purchase.