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Paint Calculator: How Much Paint Do I Need?

Work out how many gallons of paint a room needs from the walls, doors, windows and coats, and what it will cost. It rounds up to the cans to buy; the result is an estimate, and the label on the can has the final word.

Your numbers

ft
ft
ft
sq ft

A 36 × 80 in door is 20 sq ft.

sq ft

A 3 × 5 ft window is 15 sq ft.

Adds length × width, with the same paint and coats.

sq ft

Sherwin-Williams gives 350 to 400 sq ft per gallon; BEHR PREMIUM PLUS 250 to 400. Check the can.

$

Optional.

Paint needed

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Cans to buy
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Wall area to paint
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Area × 2 coats
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Paint cost
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An estimate for quoting and buying, not the manufacturer's figure for your surface: rough, porous or new drywall takes more, and so does a big color change.

Primer, trim and doors are not included.

The calculation runs in your browser and ParrotNotes does not store your numbers. They sit in the page address so you can bookmark or share the result.

How the paint is worked out

Start with the wall area. For a rectangular room it is the perimeter times the height: 2 × (length + width) × height. Subtract the doors and windows, add the ceiling if you are painting it, then multiply by the number of coats. Divide by the coverage per gallon and you have the gallons.

The calculator then rounds up to cans you can buy: the least paint that covers the job, in 5-gallon buckets, gallons and quarts, with the fewest cans when two mixes hold the same amount.

Worked example: a 12 × 14 ft bedroom

With 8 ft walls the room has 2 × (12 + 14) × 8 = 416 sq ft of wall. One door (20 sq ft) and two windows (15 sq ft each) take off 50 sq ft, leaving 366 sq ft. Two coats make 732 sq ft of painting; at 350 sq ft per gallon that is 2.09 gallons, so buy 2 gallons and 1 quart.

Add the 168 sq ft ceiling and the job is 534 sq ft per coat, or 3.05 gallons for two coats. Ceilings are usually a different paint, so run them as a second calculation if the product differs.

How far a gallon goes

Sherwin-Williams says a gallon of paint typically covers about 350 to 400 sq ft, and a gallon of primer about 200 to 300. BEHR lists 250 to 400 sq ft per gallon, per coat, for its PREMIUM PLUS interior paint. The range is wide because the surface matters: smooth, previously painted walls reach the top of it; textured, porous or new drywall sits at the bottom.

The calculator starts at 350 sq ft per gallon. Read the coverage on the can you are pricing and put it in; if you are quoting a rough surface, use the low end.

Tips for the walkthrough

Measure each room on its own and note anything that changes the paint: stains that need primer, a dark color going light, textured walls, high ceilings that need a ladder or scaffold. Count the doors and windows, and the trim you will paint in a different finish.

Then write down what the customer told you: the colors and sheen, which rooms first, furniture to move, when it has to be done and what they expect to spend. Those notes decide the quote as much as the gallons do.

Sources, checked September 2026. All figures are estimates:

More for home-services and building-materials sales: Door-to-door sales, Building materials sales, Quote follow-up email.

The gallons are only half the quote

The gallons go in the quote; what the customer said about colors, finish, furniture and timing goes in ParrotNotes: record the walkthrough and get the summary and next steps.

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