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Paint Calculator: How Much Paint for a Room?
Enter your room measurements and this calculator works out the paintable wall area after doors and windows, then the gallons you need at standard coverage. It is built for the question most paint pages answer badly: exactly how much paint for this specific room, in gallons you can actually buy.
How This Calculator Works
One gallon of interior paint covers about 350 square feet per coat on smooth, previously painted walls. The calculator multiplies your wall perimeter by ceiling height, subtracts 21 square feet per door and 15 per window, multiplies by the number of coats, and divides by coverage. It then rounds up to whole gallons, because paint is sold that way and a part gallon is useless at the store.
Worked Example
A 12 by 12 foot room with 8 foot ceilings has 384 square feet of wall. Subtract one door (21) and two windows (30) for 333 square feet of paintable wall. Two coats is 666 square feet, divided by 350 gives 1.9 gallons, so buy 2 gallons.
Quick reference: gallons by room size (2 coats, 8 ft ceilings, 1 door, 2 windows)
| Item | Figure | Result |
|---|---|---|
| 10 x 10 ft | 270 sq ft paintable | 2 gallons |
| 10 x 12 ft | 302 sq ft paintable | 2 gallons |
| 12 x 12 ft | 333 sq ft paintable | 2 gallons |
| 12 x 14 ft | 366 sq ft paintable | 3 gallons |
| 14 x 16 ft | 430 sq ft paintable | 3 gallons |
| 16 x 20 ft | 526 sq ft paintable | 4 gallons |
Frequently Asked Questions
How many square feet does one gallon of paint cover?
About 350 square feet per coat on smooth painted walls, 250 to 300 on textured or bare drywall, which drinks more paint. Primer covers less, around 200 to 300 square feet per gallon.
How much paint do I need for a 12x12 room?
About 2 gallons for two coats of the walls in a standard 12 by 12 room with 8 foot ceilings, one door and two windows. Add roughly 1 gallon if you are also painting the ceiling with one coat.
Should I buy extra paint beyond the calculator?
A small extra is smart for touch-ups, and the calculator already rounds up to the next whole gallon, which usually covers it. Keep the leftover sealed and labelled with the room and color.
Does the calculator include the ceiling?
No, it calculates walls only, which is what most people mean. For a ceiling, multiply length by width for its area and divide by 350 per coat, then round up.