Paint Calculator
Work out how many gallons of paint a room needs from its dimensions and number of coats, with doors and windows subtracted from the wall area. Calculates in your browser.
Paint to buy
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Show the math
Wall area is the perimeter times the height; openings are subtracted, then multiplied by coats and divided by coverage:
wall area = 2 × (length + width) × height
gallons = (wall area − openings) × coats ÷ coverage per gallon
Each door and window is subtracted at the area you enter (21 and 15 sq ft by default, for a 3 × 7 ft door and a 3 × 5 ft window). Coverage starts at 350 sq ft per gallon per coat; replace it with the figure on your paint can. All math runs locally in your browser.
What this does
A paint calculator works out how many gallons a room needs from its wall dimensions and the number of coats, subtracting doors and windows from the wall area.
How to use it
- Enter the room’s wall length and height.
- Set the number of coats.
- Subtract the doors and windows.
- Read the gallons to buy.
How it works
Wall area is the perimeter times the height, minus openings. Coverage starts at 350 sq ft per gallon per coat; replace it with the figure on your paint can. Two coats double the paint.
gallons = wall area × coats ÷ coverage
Understanding your result
The figure already counts your coats and subtracts doors and windows, but real coverage drops on porous, textured, or previously dark walls, so round up rather than down. Buy a little extra from the same batch for touch-ups, since colors can differ slightly between mixes, and budget a separate coat of primer when you are covering a bold color or bare drywall.
Example
A 12×12 room with 8-ft walls, one door and two windows needs about 1.9 gallons for two coats, so buy 2 gallons.
Last updated .
Frequently asked questions
How much wall does a gallon of paint cover?
Check the coverage printed on the can, since it varies with the paint, the surface, and the color change. The tool starts at 350 square feet per gallon per coat; replace that with your paint’s figure.
How many coats should I plan for?
Two coats are standard for an even finish, especially over a color change or fresh drywall. One coat may do for a like-for-like repaint.
How are doors and windows handled?
Each door and window is subtracted from the wall area at the size you enter, 21 sq ft (a 3 × 7 ft door) and 15 sq ft (a 3 × 5 ft window) by default, so you do not buy paint for openings.
Is anything stored?
No. Everything is calculated locally in your browser.
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