Paint Calculator
Work out how many gallons of paint to buy for a room, plus the paintable area and estimated cost, in seconds.
Reviewed by the CalcCafe editorial team · Last updated 1 July 2026 · How we test our tools
Example
A room with 400 ft² of walls, 2 doors and windows, painted with 2 coats at 350 ft²/gallon has a paintable area of 400 − 2×20 = 360 ft². That needs 360 × 2 ÷ 350 = 2.06 gallons, rounded up to 3 gallons. At $35 a gallon the paint costs about $105.
How it works
Paintable area = wall area − (doors + windows) × 20 ft². Gallons = ⌈paintable area × coats ÷ coverage⌉, rounded up to whole cans. Estimated cost = gallons × price per gallon.
Good to know
Buying paint is one of those jobs where guessing usually means a second trip to the store — or half a can left drying out in the garage. This calculator takes the wall area you measured, subtracts a rough allowance for the doors and windows you will not be painting, multiplies by the number of coats, and divides by how far a gallon stretches. The answer is rounded up to whole cans, because you cannot buy two-thirds of a gallon, and it adds an estimated cost so you can budget before you reach the register.
To measure wall area, add up the length of every wall and multiply by the ceiling height. A standard 12 by 12 room with 8-foot ceilings works out to about 384 ft² of wall before openings. The tool assumes roughly 20 ft² for each door or window, which is a common planning figure; tall picture windows or double doors take out more, so adjust the count upward if your openings are large.
Coverage varies by product and surface. Most interior wall paints claim 350 to 400 ft² per gallon on a smooth, previously painted wall, but bare drywall, rough plaster, and deep color changes all drink more paint and often need an extra coat or a primer. Two coats is the norm for an even, durable finish; a single coat rarely hides brush marks or color shifts cleanly.
Treat the result as a shopping estimate, not a guarantee. It does not account for primer, trim, ceilings, or spray-versus-roller waste, and it rounds every partial gallon up. When in doubt, buy the extra can — leftover paint is invaluable for touch-ups, and matching a color later is far harder than keeping a sealed spare.
Frequently asked questions
How much paint do I need for one room?
For a typical 12 by 12 room with 8-foot ceilings you have roughly 380 ft² of wall. Two coats at 350 ft² per gallon works out to about 3 gallons once you round up. Enter your own measurements above for a figure tailored to your room.
How much wall area does one gallon of paint cover?
Most interior wall paints cover about 350 to 400 ft² per gallon on a smooth, previously painted surface. Bare drywall, rough textures, and dramatic color changes reduce coverage and often call for an extra coat or a coat of primer first.
Is my data uploaded anywhere?
No — this calculator runs entirely in your browser. Your inputs never leave your device, and it works offline once loaded.
Is this calculator free?
Yes, completely free with no sign-up and no limits.
People also ask
How do I calculate how much paint I need?
Multiply the total wall area by the number of coats, subtract about 20 ft² per door or window, then divide by the paint's coverage per gallon and round up to whole cans. This tool does all of that automatically as you type.
Do I need two coats of paint?
Two coats is standard for an even, long-lasting finish and reliable color, especially over a different shade or on porous surfaces. One coat can work for minor refreshes of the same color, but it often leaves streaks or patchy hiding.
How many square feet does a 5-gallon bucket of paint cover?
At about 350 ft² per gallon, a 5-gallon bucket covers roughly 1,750 ft² in a single coat, or around 875 ft² if you apply two coats. Actual coverage depends on the product and surface.
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Sources & references
These tools follow our methodology and provide educational estimates only — verify important figures with a qualified professional.