Paint Calculator

How the math works

Paint coverage is an area problem. The calculator finds your paintable wall area — for a room, that's the perimeter times the ceiling height — subtracts the openings you won't paint (about 21 square feet per door, 15 per window), and optionally adds the ceiling.

Then it divides by the coverage rate for your surface and multiplies by the number of coats. Smooth, previously painted drywall covers at roughly 400 square feet per gallon per coat. Texture and masonry cover less because the rough surface has more actual area — heavy texture can drop to 250. Round up to whole gallons, because you can't buy 1.7 gallons, and you always want a little left for touch-ups.

Buying in the right cans

Above four gallons, a five-gallon pail is cheaper per gallon than five separate cans — and it keeps the color consistent (every can is a slightly different batch). The calculator suggests the cheapest pail-plus-can mix. It also keeps a minimum of one gallon: even a tiny accent wall needs a full can.

Common mistakes

• Buying for one coat. Almost every repaint needs two. Estimating one coat is the most common way to run short halfway through the second.
• Forgetting texture. The same room takes 60% more paint on heavy texture than on smooth drywall. Match the surface setting to your walls.
• Skipping primer on bare drywall. Unprimed drywall soaks up the first finish coat unevenly. A gallon of primer is cheaper than a third coat of finish.
• Not deducting big openings. A wall of windows or a pair of double doors is real square footage you're not painting.

When this calculator is the wrong tool

Use a different reference for: exterior siding (weather and substrate change coverage dramatically), cabinets and trim (sold and measured by the linear foot, with different products), and specialty finishes like Venetian plaster or epoxy floor coatings. This tool covers interior wall and ceiling paint.