Paint Calculator
It estimates how much paint a room needs from the wall perimeter, ceiling height, number of coats, and the doors and windows you are not painting.
Paint to buy
2.75 gal
Wall area
540 ft²
Doors & windows
−87 ft²
Paintable area
453 ft²
Gallons needed
2.59
Buy (rounded up)
2.75 gal
In liters
9.8 L
This covers walls only. For the ceiling, work out the floor area and add it separately. Buy all your paint in one batch so the color matches exactly.
What it calculates
It estimates how much paint a room needs from the wall perimeter, ceiling height, number of coats, and the doors and windows you are not painting.
Why it matters
Running out mid-wall means a visible lap mark, and a second trip risks a slightly different batch color.
Who it's for
Anyone painting a room, decorators quoting a job, and landlords estimating turnover costs.
Formula
- A
- Wall area = perimeter × height
- c
- Number of coats
- C
- Coverage per gallon, typically 350 sq ft
Worked example
A room with a 60 ft perimeter and 9 ft ceilings, two coats
- 1Wall area = 60 × 9 = 540 sq ft
- 2Subtract 2 doors (42) and 3 windows (45) = 453 sq ft
- 3453 × 2 coats ÷ 350
2.59 gallons — buy 2¾ gallons
How the paint calculator works
Wall area is the room's perimeter multiplied by ceiling height. Doors and windows are subtracted at standard sizes — about 21 sq ft for a door and 15 for a window. Multiplying by the number of coats and dividing by the coverage rate gives gallons. Coverage varies: 350 sq ft per gallon is typical for a smooth primed wall, but textured or porous surfaces drink noticeably more.
Wall area is the room's perimeter multiplied by ceiling height. Doors and windows are subtracted at standard sizes — about 21 sq ft for a door and 15 for a window.
Coverage varies more than people expect. 350 sq ft per gallon is typical on a smooth primed wall, but bare drywall, textured plaster and porous surfaces can drop that to 250 — adjust the figure below if you know your surface.
Common mistakes
- Measuring floor area instead of wall area.
- Buying for one coat when the color change needs two or three.
- Assuming the coverage on the tin applies to bare drywall or textured plaster, where it does not.
Tips and best practice
- Prime bare or heavily patched walls; it costs less than the extra topcoat you would otherwise need.
- Buy all your paint in one batch so the color matches exactly.
- Dark colors over light usually need three coats, not two.
Frequently asked questions
How much paint do I need for a room?
Multiply the room perimeter by ceiling height, subtract doors and windows, multiply by the number of coats and divide by 350 sq ft per gallon.
How much does a gallon of paint cover?
About 350 square feet on a smooth, primed surface. Textured, porous or unprimed walls can drop that to 250.
How many coats do I need?
Two for most repaints. Three when going dark over light, light over dark, or painting bare drywall without primer.
Does this include the ceiling?
No, it covers walls only. For the ceiling, use the square footage calculator on the floor dimensions and add that area separately.
Related calculators
Methodology & trust
- Formula source
- Standard trade coverage rate of 350 sq ft per US gallon.
- Last updated
- 2026-07-28
- Privacy
- Every calculation runs in your browser. No inputs are sent to a server or stored.
- Accessibility
- Keyboard navigable, labeled inputs and WCAG AA color contrast.