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 is counted as 21 sq ft and each window as 15 sq ft. A gallon covers about 350 sq ft per coat by default, adjust it for your paint and surface. 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. A gallon covers about 350 sq ft per coat, so two coats roughly 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 colours can differ slightly between mixes, and budget a separate coat of primer when you are covering a bold colour or bare drywall.
Example
A 12×12 room with 8-ft walls needs about 2 gallons for two coats.
Sources & methodology
Last updated .
Frequently asked questions
How much wall does a gallon of paint cover?
A gallon typically covers about 350 square feet in one coat. Coverage varies with the surface, the colour change, and the paint, so the default is adjustable.
How many coats should I plan for?
Two coats are standard for an even finish, especially over a colour change or fresh drywall. One coat may do for a like-for-like repaint.
How are doors and windows handled?
Each door is subtracted as about 21 sq ft and each window as about 15 sq ft from the total wall area, so you do not buy paint for openings.
Is anything stored?
No. Everything is calculated locally in your browser.
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