Room Paint Gallons Calculator
Buying the right amount of paint is the difference between one smooth trip to the store and a frustrating mid-job run for one more can. This calculator estimates how many gallons a room needs. Enter the room length and width, the wall height, the total area of doors and windows to leave unpainted, and the coverage your paint provides per gallon, and the tool computes the wall area, subtracts the openings, and divides by coverage to give the gallons required for one coat. The wall area is the room perimeter (twice the length plus twice the width) times the height, and subtracting the openings avoids over-buying for doors and windows you will not paint. Coverage is left editable because it varies with the paint and surface; most interior paints state roughly 350 square feet per gallon on smooth, primed walls, but rough or porous surfaces cover less, so use the figure on your can. The result is for one coat, so double it for the usual two-coat finish, and always round up to whole gallons since paint is sold by the can. Every figure is computed deterministically from the formula shown below, with a worked example that reconciles exactly to the calculator's defaults.
Paint needed is the wall area minus openings, divided by coverage: (perimeter x height - openings) / coverage. A 12 x 14 room with 9 ft walls and 60 sq ft of openings has 408 sq ft to paint, needing 1.17 gallons (round up to 2) at 350 sq ft per gallon.
Paint gallons formula
Wall area = ( 2 x ( length + width ) ) x height
Paintable area = wall area - openings
Gallons (1 coat) = paintable area / coverage per gallon
Cans to buy = round gallons up to the next whole number
Multiply the perimeter by the height for wall area, subtract doors and windows, then divide by the paint's coverage. Round up to whole cans, and double for two coats.
Worked example
Suppose a room is 12 by 14 feet with 9-foot walls, 60 square feet of openings, and paint that covers 350 square feet per gallon.
- Perimeter = 2 x (12 + 14) = 52 ft
- Wall area = 52 x 9 = 468 sq ft
- Paintable area = 468 - 60 = 408 sq ft
- Gallons (1 coat) = 408 / 350 = 1.17
- Cans to buy = round 1.17 up to 2
You need 1.17 gallons for one coat, so buy 2 cans. These are the calculator's default inputs, so the result above matches the widget exactly.
Gallons by paintable area
Gallons needed for one coat at 350 square feet per gallon.
| Paintable area | Gallons (1 coat) |
|---|---|
| 200 sq ft | 0.57 |
| 408 sq ft | 1.17 |
| 700 sq ft | 2.00 |
| 1,050 sq ft | 3.00 |
Coverage varies by paint and surface; use the figure printed on your can.
Room paint gallons calculator: frequently asked questions
How do I work out how much paint a room needs?
Find the wall area by multiplying the room perimeter by the wall height, subtract the area of doors and windows, then divide the paintable area by the coverage of one gallon. A gallon typically covers about 350 square feet in one coat. Round up to whole gallons, and multiply by the number of coats you plan to apply.
How much area does a gallon of paint cover?
Most interior wall paints state coverage of roughly 350 to 400 square feet per gallon for one coat on a smooth, primed surface. Rough, porous or unprimed surfaces absorb more and cover less. The coverage figure is on the can, so this calculator leaves it editable; enter the value your paint specifies for the most accurate result.
Should I subtract doors and windows?
Yes, for a tighter estimate. Doors and windows are not painted with wall paint, so subtracting their area avoids buying more than you need. A standard door is about 20 square feet and an average window around 15 square feet. Enter the total opening area to deduct it from the wall area before dividing by coverage.
How many coats should I plan for?
Two coats are standard for an even finish, especially over a color change or fresh drywall. This calculator estimates gallons for one coat; double the result for two coats. A primer coat may be needed over bare or patched surfaces, which would add another pass. Always round up to whole gallons when buying.
Why round up to whole gallons?
Paint is sold in whole gallons (and quarts), and running out mid-job risks a visible seam or a slightly different batch color. Rounding up ensures you have enough and leaves a little for touch-ups. The calculator shows the exact gallons needed and the rounded-up number of cans to buy.
Official sources
- Area, length and coverage measurement reference: US National Institute of Standards and Technology (NIST). As at 25 June 2026.
Reviewed by the CalculatorHub team, edited by James Graham, 25 June 2026. See our methodology. This is general information, not financial, tax, legal or investment advice.