How to Calculate Paint for One Room

Updated October 30, 2026

Paint for one room is wall area, minus the parts you do not paint, times the number of coats, divided by the coverage printed on the can. Gallons come last. Starting from “a gallon does a bedroom” is how people run out on the second coat or store a gallon they never open.

Measure the walls you will actually paint

Measure the floor length and width, then the wall height from floor to ceiling. Eight feet is common. A basement, a drop ceiling, or a room with a soffit may not be. You want the height of the wall surface, not the empty space above a soffit you are leaving alone.

The calculator treats the room as four walls: 2 × (length + width) × height. That is gross wall area. The ceiling is not included. A closet is not included unless you run the numbers again for that smaller room.

Subtract doors and windows

A standard interior door is about 21 square feet, roughly 3 by 7. A typical window is about 15 square feet, roughly 3 by 5. The calculator subtracts those amounts for each door and window you count.

A sliding door or a picture window is larger than that default. The form does not take a custom opening size. If 15 square feet is close, you can count the big window as one window and accept a little extra paint. If it is a wide slider, measure it, divide by 15, and round up to a window count that gets near the real area. Guessing low is how the gallon count comes up short. Leftover paint from guessing high is the cheaper mistake, as long as the cans are the same tint.

A 12 by 10 foot room with 8 foot walls

Two doors, two windows, which matches a lot of bedrooms.

• Gross walls: 2 × (12 + 10) × 8 = 352 sq ft.
• Openings: (2 × 21) + (2 × 15) = 72 sq ft.
• Paintable walls: 352 − 72 = 280 sq ft.

That 280 is one coat on the walls. It is not the ceiling, and it is not the inside of the closet.

Coats, coverage, and waste

Two coats is the usual plan on new drywall or on a color change. Multiply the paintable area by the coats: 280 × 2 = 560 square feet of painting.

Divide by the coverage on the can, in square feet per gallon. 350 is a common interior figure and the number the calculator starts with. It is not a property of your walls. Flat paint on a thirsty surface, or a deep color over white, often covers less. Type the number from the can you are buying.

560 ÷ 350 = 1.60 gallons before waste.

The waste choices are 0, 5, 10, 15, and 20 percent.

• 5% — smooth walls, a similar color, and careful cutting-in.
• 10% — the default. It covers the brush work at the trim, a porous patch, and the paint that stays in the tray. 1.60 × 1.10 = 1.76 gallons.
• 15% or 20% — a large color change, rough texture, or touch-up you expect to pull from the same cans the next day.

The calculator then rounds up to the next whole gallon. 1.76 gallons becomes 2 gallons. Stopping at one gallon leaves the second coat unfinished, and the second coat is the one people see. An unopened gallon can often go back. A wall that flashes because you switched cans in the middle of a wall cannot.

Ceiling, primer, and the closet

This tool is walls only. The ceiling of the example room is 12 × 10 = 120 square feet. At 350 square feet per gallon and one coat, that is about 0.34 gallons before waste. It does not fit inside the 2 gallon wall result. If the ceiling is the same product, add that area yourself.

Primer is a different can. Do not assume the wall gallons include it. A closet with the same wall height is another small room. Run the calculator again, or the closet stays the old color.

Buy the gallons in one trip

Tinted paint varies by batch. Buy the gallons together so the store mixes one formula, and keep the formula sticker. If you enter a price per gallon, the cost line is your price times the recommended gallons. It is not a sale price, and it does not include tape, rollers, or primer.