Worked example
Paint = [(perimeter × height − openings) × coats × waste factor] ÷ coverageWork from paintable wall area
A 12×12 room has a 48 ft perimeter. At an 8 ft ceiling, the four walls total 384 ft² before subtracting doors and windows.
Subtracting one 20 ft² door and two 15 ft² windows leaves roughly 334 ft² of paintable wall. Openings vary, so measuring them is more accurate than using defaults.
Coats and coverage change the answer
Two coats double the coverage requirement to 668 ft². Adding 10% for touch-ups, roller loss and small measurement errors brings the planning area to about 735 ft².
At 350 ft² per gallon, the mathematical need is about 2.1 gallons. Paint is sold in fixed can sizes, so the purchase quantity rounds up.
- Use the coverage printed on the specific paint
- Allow more for textured or repaired walls
- Primer is a separate product and calculation
When two gallons might be enough
One-coat repainting in a similar color may need far less, particularly on smooth walls. Strong color changes, porous surfaces and deep colors can need an additional coat.
Keep the assumptions visible. A slightly conservative purchase is useful only if unopened cans can be returned or the remainder will be kept for touch-ups.
Review note and sources
What was checked.
The worked example was recalculated from room perimeter, openings, coats, coverage and waste. Product-label coverage remains authoritative.
Sources were checked on September 1, 2026. Current product labels, provider agreements, utility tariffs, and local requirements take precedence.
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Turn the example into your estimate.
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