Paint calculator: how much paint do I need?
Enter your room's size, doors and windows to get gallons for the walls (and ceiling, if you want).
Quick answer: a 12 × 12 ft room with 8 ft ceilings needs about 2 gallons of paint for two coats on the walls (about 333 sq ft after a door and two windows). A 10 × 10 ft bedroom needs about 2 gallons; a 12 × 15 ft living room about 2 gallons + 1 quart. One gallon covers roughly 350–400 sq ft per coat.
Room paint calculator
2 gallons
How this is calculated
Wall area = 2 × (length + width) × height. We subtract 21 sq ft for each door (3 × 7 ft) and 15 sq ft for each window (3 × 5 ft), multiply by the number of coats, and divide by the coverage on the can. The result is rounded up to whole gallons, or to a quart when you are only slightly over a gallon. Ceiling area is length × width. Trim, doors and cabinets need a separate, smaller amount of trim paint.
Coverage on real walls is often lower than the label promises, which is why we default to 350 sq ft per gallon.
When you need primer
- New drywall or patched areas: prime first, or they will show through as dull spots.
- Going from a dark color to a light one: a tinted primer saves a coat of paint.
- Glossy, stained or water-damaged walls: use a bonding or stain-blocking primer.
Frequently asked questions
How much paint do I need for a 12 × 12 room?
With 8-foot ceilings, one door and two windows, the walls are about 333 square feet. Two coats at 350 square feet per gallon need 1.9 gallons, so buy 2 gallons.
How many square feet does a gallon of paint cover?
Most interior paint labels say about 350 to 400 square feet per gallon for one coat on smooth, primed walls. Rough or porous surfaces, deep colors and fresh drywall use more, so this calculator defaults to the low end, 350.
Do I need two coats of paint?
Usually yes. Two coats give even color and better durability, especially when changing colors. One coat can be enough when repainting the same color over a clean wall, or with a paint-and-primer product over a similar shade.
Should I buy extra paint?
Keep a little for touch-ups. If the calculator says you are just over a whole gallon, a quart is often enough to finish. Buy all the paint for one room at once so the color matches, or mix (box) the cans together before you start.