Paint calculator

Buying the right amount, not the round amount

The answer this page gives you is denominated in gallons and quarts because that is how the store sells paint, and the gap between the exact figure and the buy figure is worth a glance before you shop. If the exact number reads 2.1 gallons, buying two gallons plus a quart beats buying three gallons: same coverage, roughly fifteen dollars less, and one less half-empty can in the garage in five years. If the exact number reads 2.9, just buy three; the leftover becomes your touch-up supply, and matching a custom tint two years later is harder than storing a quart now.

The coverage field is the input most people should actually edit. The 350 square feet per gallon default assumes smooth, primed drywall. Knock it down to something like 250 to 300 for heavy texture, bare drywall, or masonry, since porous surfaces drink paint, and the difference between 350 and 275 on a big room is often a full extra gallon. Your paint can lists its own coverage range; use the low end of it.

The tool also handles partial jobs cleanly. Painting a single accent wall? Enter just that wall's length instead of the room perimeter, keep the same height, and zero out doors and windows that are not on it.

A bedroom, worked out

A 14 by 16 foot room has a 60 foot perimeter. Say the ceilings are 9 feet, you want the standard two coats, and the room has two doors and three windows. Wall area is 60 x 9 = 540 square feet, minus 40 for the doors and 45 for the windows, leaving 455 paintable square feet. Two coats makes 910 square feet of coverage needed. At 350 per gallon with the built-in 10% margin, the exact requirement is 910 / 350 x 1.10 = 2.86 gallons, and the buy recommendation rounds up to 3 gallons. Skip the margin and buy exactly 2.6 gallons' worth and you would likely finish the second coat scraping the can.

The arithmetic, step by step

Wall area is total length times height, minus 20 square feet per door and 15 per window, floored at zero so an over-counted room cannot go negative. That area is multiplied by coats, divided by the coverage figure, and inflated by 10% for waste, roller absorption, and the cutting-in you will redo. The result is then rounded up to the next quarter gallon and expressed as gallons plus quarts, because a quart is the smallest can worth buying. The exact pre-rounding figure is shown beside it so you can make the two-gallons-and-a-quart versus three-gallons call yourself.

What is not in the estimate

Ceilings, trim, baseboards, and the doors themselves are separate jobs with separate paint, usually in a different sheen, and none of them are counted here. Primer is also excluded: a drastic color change or bare surface needs a priming pass that this calculator treats as somebody else's problem. The door and window deductions use fixed standard sizes, so a wall of glass or a garage door will throw the area off; measure odd openings and subtract them from your wall length instead. And no formula can see your technique. First-time rollers apply thicker coats, and the 10% margin is sized for average hands, not learning curves.