Interior Paint Calculator
Buy once, not twice. Enter the room, how many coats, and your paint's coverage, and get the gallons to pick up — with doors and windows taken out so you're not over-buying, and a cost estimate if you want it.
Coverage is on the can — often 350–400 ft²/gal. Porous or dark-to-light walls cover less.
Advanced: openings & price
Formula & how it works
Wall area = 2 × (length + width) × height. Subtract door and window area, add the ceiling (length × width) if you're painting it.
Gallons = paintable area × coats ÷ coverage, rounded up to what you can buy.
Worked example
A 12 × 12 room with 8-ft walls has 384 ft² of wall. Take out a door and two windows (50 ft²) for 334 ft². Two coats is 668 ft²; at 350 ft²/gal that's 1.9 gallons — buy 2 gallons.
Estimating paint
Coverage is not a fixed number
The 350–400 ft² per gallon on the label assumes a smooth, primed, similar-colored wall. Textured, porous, or bare drywall drinks more, and a big color change may need an extra coat, so trust your can and round up.
Buy in the right containers
If the estimate lands just over a whole gallon, a gallon plus a quart is often cheaper than two gallons and leaves less waste. Keep a sealed remainder for touch-ups — matching later is never perfect.