See how many gallons of paint you need from your wall area, number of coats, and the coverage on the can.
To get wall area, multiply each wall's width by its height and add them; subtract large doors and windows (~20 sq ft each). Estimate only.
Paint is estimated from the total area to cover — wall area times the number of coats — divided by the coverage printed on the can (commonly around 350 square feet per gallon). Primer and rough or porous surfaces reduce coverage, so round up.
Gallons = (wall area × coats) ÷ coverage per gallon
For a tighter estimate, yes — subtract about 20 sq ft per door and 15 per average window. Leaving them in just gives you a little extra paint.
Two is standard. Dark colors over light (or the reverse) may need a primer coat plus two finish coats.
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Last reviewed: September 2026. Free to use, no signup — Paint Calculator by ToolWise Digital.
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