Paint Calculator
Figure out how many gallons a room needs — wall area, coats, and door-and-window deductions all baked in.
How it works
The wall area is the room's perimeter times the ceiling height. A 15-by-12 room has a 54-foot perimeter, and at 8 feet tall that's 432 square feet of wall before any deductions.
Every door and window you enter knocks about 15 square feet off, since you're not painting glass or a door slab. Then we multiply by the number of coats — two is standard, especially over a color change or fresh drywall.
A gallon covers roughly 350 square feet on a smooth wall, so we divide by that and round up. Textured walls and deep colors drink more, so if the wall's rough or you're going bold, round up a touch further.
Frequently asked questions
How much does a gallon of paint cover?
About 350 square feet per coat on a smooth, previously painted wall. Fresh drywall, rough texture, or a dramatic color change soaks up more, so treat 350 as the friendly ceiling, not a promise.
Do I really need two coats?
Usually. One coat rarely hides the old color evenly, and two gives a solid, uniform finish. Going from dark to light — or bare drywall to any color — sometimes wants a primer coat on top of that.
Why deduct for doors and windows?
You don't paint the glass or the door itself, so leaving that area in would have you buying paint you'll never use. Each opening trims roughly 15 square feet, a reasonable stand-in for a standard door or window.
Does this cover the ceiling too?
No, it's just the walls. For a ceiling, measure its length and width in the Square Footage Calculator and figure that as a separate area — ceilings often want their own can anyway.