Enter the room, subtract the openings, pick your coats — get gallons for walls and ceiling
before you're standing in the paint aisle.
How to calculate paint
wall area = 2 × (length + width) × height
paintable = wall area − doors × 21 − windows × 15
gallons = paintable × coats ÷ coverage per gallon
Buy the rounded-up whole gallon when the exact figure is close to it — leftover paint is
touch-up insurance, and a second trip to match a custom tint costs more than the extra
quart. If the exact need is just over a whole gallon (like 1.1), a gallon plus a quart is
cheaper than two gallons.
Coverage by surface type
| Surface | Coverage per gallon |
|---|
| Smooth, previously painted | 350–400 sq ft |
| Light texture / orange peel | 300–350 sq ft |
| New drywall (unprimed) | 250–300 sq ft |
| Heavy texture / stucco / brick | 150–250 sq ft |
Frequently asked questions
How much does a gallon of paint cover?
One gallon covers about 350–400 sq ft with one coat on smooth, previously painted walls. Textured, porous or unprimed surfaces reduce coverage to 250–300 sq ft per gallon.
How much paint do I need for a 12×12 room?
A 12×12 room with 8 ft ceilings has about 384 sq ft of wall area. Subtracting one door and two windows leaves roughly 330 sq ft — about 2 gallons for two coats.
Do I really need two coats?
Almost always yes. Two coats give even color, full coverage and better washability. The exceptions are same-color refreshes with premium self-priming paint in excellent condition.
Should I subtract doors and windows?
Yes — a standard door is about 21 sq ft and a standard window about 15 sq ft. This calculator subtracts them automatically; it does not subtract small openings, which conveniently offsets the extra paint that cutting-in uses.
How much paint for the ceiling?
Ceiling area = length × width. A 12×12 ceiling is 144 sq ft — one gallon covers it twice. Use dedicated flat ceiling paint to hide roller marks.