Paint
calculator
Walls minus doors and windows, times the number of coats, divided by what a gallon covers, that's the whole formula. Enter the room and get the cans to buy.
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Helix Oxford Scientific
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The coverage math
A gallon of interior paint covers roughly 350 square feet in one coat on smooth, primed walls. Wall area is the room's perimeter times ceiling height, minus standard deductions of about 20 sq ft per door and 15 per window.
334 × 2 ÷ 350 ≈ 1.9 gallons → buy 2
When coverage drops
Textured, porous or unprimed surfaces drink 25–50% more; dramatic color changes (especially over dark walls) need an extra coat or a tinted primer, often cheaper than a third coat of premium paint. Two coats is the realistic default for uniform color; one-coat claims survive only ideal conditions.
Buy slightly over, not under, leftover paint handles touch-ups for years, and a second batch of custom-tinted color may not match perfectly. Ceilings are a separate job: length × width ÷ 350 per coat, and the square footage calculator gets that number quickly.
Paint FAQ
About 350 sq ft per coat on smooth primed walls, the industry standard. Textured or porous surfaces reduce that meaningfully, sometimes to 250 sq ft.
Two is the realistic default for even color. Going light-over-dark or using deep accent colors can require three, or better, one tinted primer coat plus two finish coats.
Add up the wall lengths (perimeter), multiply by ceiling height, then subtract about 20 sq ft per door and 15 per window. That's the paintable area this calculator uses.
Yes, custom-tinted batches can vary slightly, so buying enough at once ensures a match, and leftovers cover touch-ups. The calculator rounds to whole cans, adding a quart when it's close.