Paint Calculator

Enter the room's length, width, and wall height, subtract doors and windows, pick your coats, and get the paint to buy in gallons and quarts — based on 350 sq ft of coverage per gallon.

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How the paint math works

Wall area is the room's perimeter times the wall height: 2 × (length + width) × height. From that you subtract the openings you won't paint — the standard estimating allowances are 21 sq ft per door and 15 sq ft per window — then multiply by the number of coats, because a second coat is a second full pass over every square foot. Divide by coverage (350 sq ft per gallon is the standard planning figure) and round up to the nearest quart, since that's the smallest can worth buying.

The formula

paintable area = [2 × (L + W) × H − doors × 21 − windows × 15 (+ L × W for the ceiling)] × coats
gallons = area ÷ 350, rounded up to the next quart

L, W, and H are in feet. The ceiling, if you're painting it, adds the floor area (L × W) per coat — though note that ceilings are usually done in flat ceiling paint, so treat that as a separate can, not extra wall color.

Worked example

A 12 × 15 ft bedroom with 8 ft walls, 2 doors, and 2 windows: walls are 2 × (12 + 15) × 8 = 432 sq ft, minus 42 (doors) and 30 (windows) = 360 sq ft per coat.

Two coats makes 720 sq ft of painting. At 350 sq ft per gallon that's 720 ÷ 350 = 2.06 gallons — rounded up to the next quart, buy 2 gallons + 1 quart. (Since a quart often costs nearly half a gallon's price, many painters just buy 3 gallons and keep the extra for touch-ups.)

Why two coats, and why coverage is a range

Two coats is the default for a reason: a single coat almost never hides evenly. The old color ghosts through, roller overlap shows as stripes in raking light, and future touch-ups "flash" against single-coat walls. The 350 sq ft/gallon figure is honest but slippery — rough texture, bare drywall, and porous surfaces drink paint and can pull real coverage down toward 250, while smooth previously-painted walls stretch past 400. And when you buy, err slightly high on purpose: paint colors are mixed per batch, and a can tinted next month may not match today's walls exactly. The leftover quart from the original batch is the only touch-up paint that's guaranteed to match.

Frequently asked questions

How much paint do I need for a 12x12 room?

A 12 × 12 room with 8-ft ceilings has 384 sq ft of wall; subtract a door and two windows and you're near 330 sq ft. Two coats is about 660 sq ft of painting, which at 350 sq ft per gallon means buying 2 gallons.

How many coats of paint do I need?

Plan on two. One coat almost never hides the old color evenly — sheen varies, roller marks telegraph, and touch-ups flash. The main exceptions are repainting with the identical color or using a premium paint-and-primer over a very similar shade, and even then a second coat is cheap insurance.

Do I need primer before painting?

Prime bare drywall (it drinks paint otherwise), patched or repaired spots, glossy surfaces after sanding, stains, and any time you're going from a dark color to a light one — a tinted primer can save you a third coat of expensive finish paint. Repainting a wall in good condition with a similar color usually needs no primer.

How many square feet does a gallon of paint cover?

About 350 sq ft per coat is the standard planning number, but it swings from roughly 250 on rough or porous surfaces like texture and bare drywall to 400+ on smooth, previously painted walls. When in doubt, use 300-325 and you'll land safe.

Should I buy extra paint?

Yes — buy slightly more than the math says. Paint is mixed in batches, and a gallon tinted next month can be a visibly different shade, so leftover paint from the original batch is the only perfect touch-up material. A quart or so left in the can is the goal, not a mistake.

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