Paint Calculator
Enter your room's dimensions — the gallons and paintable area update as you type, and the link in your address bar always reflects your numbers.
How this calculator works
We start with the gross wall area of the room's perimeter: 2 × (length + width) × height. From that we subtract standard openings — about 21 sq ft per door and 15 sq ft per window — to get the actual paintable area, floored at zero so an unusually small room with lots of openings never goes negative. Multiplying paintable area by the number of coats gives the total area of paint you'll actually be applying. We then divide by 350 sq ft, a commonly cited gallon-coverage figure for smooth, primed interior walls, and round up, since paint is only sold in whole cans. That 350 figure is a typical manufacturer estimate rather than an exact physical law — textured or unprimed surfaces will cover less, so check your specific paint can when it matters.
Worked example
A 12×10 ft room with 8 ft walls, 1 door, 2 windows, and 2 coats: gross wall area is 2 × (12 + 10) × 8 = 352 sq ft, minus 21 for the door and 30 for the two windows, leaving a 301 sq ft paintable area. Two coats means 301 × 2 = 602 sq ft of total paint coverage needed. Divide by 350 and round up for 2 gallons. Drop to a single coat and the same room only needs 301 ÷ 350 ≈ 1 gallon.
Frequently asked questions
Why does this calculator assume 350 sq ft per gallon?
350 sq ft per gallon per coat is a typical figure printed on many interior paint cans for a smooth, primed wall. It's a manufacturer estimate, not a physical constant — rougher textures like orange-peel or stucco, and porous unprimed drywall, soak up more paint and will cover noticeably less than 350 sq ft. If your paint can states a different coverage rate, trust the can over this default.
Should I paint the ceiling too?
This calculator only covers the four walls. A ceiling is a separate flat area — usually just length × width of the room — and it's common to paint it in a different (often flatter, whiter) finish than the walls. Work out ceiling paint as its own calculation rather than folding it into the wall total here.
Do I need primer separately?
Yes, in most cases. If you're painting new drywall, patched areas, or making a big color change (like going from a dark color to a light one), a separate coat of primer is standard practice. Primer isn't included in the gallon count above — treat it as its own can and its own coat.
Why does the result round up to a whole gallon?
Paint is sold in fixed container sizes — gallons and quarts — not by the fluid ounce. Rounding up to the next full gallon reflects what you can actually buy at the store; a result of 1.2 gallons still means you're walking out with 2 gallons (or 1 gallon plus a quart) in hand.
Does the door and window deduction match my exact doors and windows?
Not exactly — the calculator subtracts about 21 sq ft per door and 15 sq ft per window, which are common averages for standard residential sizes. A large picture window or an oversized double door will eat into your paintable area more than that; for unusual openings, measure them and adjust the paintable-area result by hand.