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Paint Calculator
One gallon of interior paint covers about 350 square feet per coat — but the number that matters is your paintable area after doors and windows come out, times the number of coats. Enter the room below and get gallons to buy, primer, and what it should cost.
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Gallons are rounded up to whole cans — paint is sold by the gallon (and quart). If the exact figure is just over a whole gallon, a quart of the same paint often covers the difference for less.
How it works
The calculator works the way painters estimate:
Paintable area = wall area − (doors × 21 sq ft) − (windows × 15 sq ft)
Gallons = paintable area × coats ÷ coverage per gallon
The deductions are the industry defaults: a standard interior door with its trim accounts for about 21 sq ft of wall, an average double-hung window about 15 sq ft. Coverage of 350 sq ft per gallon is the conservative end of what major brands print (350–400); use 400 only on smooth, previously painted walls in a similar color, and 300 or less on rough texture, brick, or bare drywall without primer.
Primer is a separate coat with the same coverage math. It is not upsell: on new drywall, primer is what stops the mud joints from flashing through the finish coats, and on a dark-to-light change it is cheaper than a third coat of tinted paint.
Worked example
A 12×12 ft bedroom with 8 ft ceilings, 1 door and 2 windows: walls are 2 × (12 + 12) × 8 = 384 sq ft; minus 21 for the door and 30 for the windows leaves 333 sq ft of paintable wall. Two coats at 350 sq ft per gallon: 333 × 2 ÷ 350 = 1.90 gallons, so buy 2 gallons, plus 1 gallon of primer if you're priming (333 ÷ 350 rounds up to 1). At $45/gal paint and $28/gal primer, that's $118 in materials.
Frequently asked questions
How much paint do I need for a 12x12 room?
About 2 gallons for two coats on the walls of a 12x12 room with 8 ft ceilings, one door and two windows (333 sq ft of paintable wall). Add 1 gallon of primer for new drywall or a big color change, and roughly 1 more gallon if the ceiling is getting painted too.
How many square feet does a gallon of paint cover?
Plan on 350 sq ft per gallon per coat for interior wall paint — the range printed on cans is 350–400. Rough, porous or heavily textured surfaces drink more; use 300 sq ft/gal there. Primer covers a similar 200–350 sq ft per gallon depending on the surface.
Do I really need two coats?
For a durable, even finish, yes in most cases. One coat can work for a refresh in the same color with a premium paint-and-primer product, but color changes, patched walls and new drywall need two finish coats — and the second coat is where the uniform sheen comes from.
How much paint for just an accent wall?
Measure that one wall (width × height) and divide by 350 per coat. A 12 ft × 8 ft accent wall is 96 sq ft — about 0.27 gallons per coat, so two coats run just over half a gallon. A single quart covers roughly 87 sq ft per coat; for a full accent wall, buy the gallon and keep the rest for touch-ups.
Should I subtract doors and windows?
Yes — real paintable area excludes them, and on a typical room they remove 40–80 sq ft, which can be the difference between one gallon and two. The standard deductions are 21 sq ft per door and 15 sq ft per average window.
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About the author
Harrison Alley is a web publisher and data enthusiast based in Dallas, Texas. He built Urbarama’s remodeling calculators and maintains them.
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