Paint Calculator
Calculate how many gallons (or liters) of paint you need for a room, accounting for doors, windows, and coats.
How we calculate it
Net area = (Perimeter × Wall height) − Openings, then Paint = ceil(Net area × Coats / Coverage rate). Coverage rate defaults to 375 ft²/gallon (or 11 m²/liter), the middle of the typical 350–400 ft²/gallon (10–12 m²/liter) range. But check your paint can and adjust it if you know the real number.
Understanding your estimate
Buy too little paint and you're back at the store for a gallon that might not match the tint from last time, mixing on the fly and hoping nobody looks too closely at the wall in daylight. Buy too much and a mostly full can sits in the garage past its shelf life. This calculator works off perimeter and wall height, one number for the whole room, not a list of individual walls, and the two rounding decisions inside it are what keep you from a second trip.
A bedroom, worked out
Say you're painting a room with a 40 ft perimeter, 8 ft walls, one door, two windows, two coats, at the default coverage of 375 ft² per gallon.
- Gross wall area: 40 × 8 = 320 ft².
- Subtract openings: one door at 21 ft², two windows at 15 ft² each, 51 ft² total.
- Net area, what actually gets painted: 320 − 51 = 269 ft².
- Two coats: 269 × 2 = 538 ft² of paint applied.
- Divide by 375: 538 ÷ 375 = 1.43, rounded up to 2 gallons.
Why the coverage rate is two separate numbers, not one converted
375 ft² per gallon and 11 m² per liter look like they should be the same physical fact in two units. They aren't quite. Run 375 ft²/gallon through the conversion and you land around 9.2 m²/liter, not 11. The spec gives each range on its own, 350 to 400 ft² per gallon, 10 to 12 m² per liter, and the two don't reconcile because they come from different market references, US paint cans and metric paint cans, not one number translated into two unit systems. So the calculator treats them as two independent constants, each with its own default, each adjustable on its own. Switch units and you get the metric default back, not a converted version of whatever you typed in imperial.
Why a door and a window each get a flat number
A door subtracts 21 ft², a window subtracts 15 ft², no matter how big your actual door or window is. Those are estimates for a standard residential opening, not a measurement of yours specifically. We picked them because the alternative, asking for the width and height of every opening in the room, turns a thirty-second form into a five-minute one for a correction that mostly nets out anyway: a slightly bigger door in one room and a smaller window in the next average toward the same ballpark. If your project has an oversized picture window or a double door, the flat number will run a little light on how much you're actually saving, so round your gallon count up rather than down.
Why this shares its formula with the Wallpaper Calculator
Net wall area, perimeter times height minus openings, is the same calculation whether what's going on the wall is paint or wallpaper. When the Wallpaper Calculator needed it, we didn't write a second copy with paint's variable names swapped for wallpaper's. Both calculators call one shared function underneath, and each keeps its own units and labels on top, gallons and coats here, rolls and pattern repeat there. Fix a bug in how openings get subtracted and it's fixed in both places at once, not fixed in one while it's quietly still broken in the other.
Why gallons always round up, and coats can't be zero
1.43 gallons becomes 2, never 1. Rounding down a paint purchase means running out mid-wall with a color that may not match once remixed. And coats can't be set to zero: it isn't a way to skip waste, it's a way to skip painting, so the field starts at 1 and stays there.