Paint Calculator
Find out how many gallons of paint to buy for a room's walls and ceiling, based on doors, windows, number of coats and the coverage of your paint.
Formulas shown on this page · Last reviewed October 2026
How the paint calculator works
wall area = 2 × (length + width) × height
paintable area = wall area + ceiling (length × width, optional) − doors × 20 − windows × 15
gallons = paintable area × coats ÷ coverage per gallon, rounded up
The result rounds up to a whole gallon because paint is sold that way. If the exact amount is small, the "smaller-can option" shows how many gallons and quarts would cover it, so you do not buy a gallon for a closet.
Gallons for common rooms (walls only, 8 ft ceiling, 1 door, 2 windows, 2 coats)
| Room size | Paintable wall area | Exact gallons | Buy |
|---|---|---|---|
| 10 × 10 ft | 270 sq ft | 1.54 | 2 gal |
| 12 × 12 ft | 334 sq ft | 1.91 | 2 gal |
| 12 × 15 ft | 382 sq ft | 2.18 | 3 gal |
| 15 × 20 ft | 510 sq ft | 2.91 | 3 gal |
| 20 × 20 ft | 590 sq ft | 3.37 | 4 gal |
Practical tips
- Check the coverage printed on the can; premium paints often list 400 sq ft per gallon but real-world results are closer to 350.
- Buy all the paint at once, or have the store box-mix cans together, so the color is consistent across the room.
- Trim, baseboards and doors are not included. Budget a separate quart or two for those.
- Keep a labeled leftover quart for touch-ups.
Paint by room size
8x10 room · 10x10 room · 10x12 room · 10x14 room · 11x12 room · 12x12 room · 12x14 room · 12x15 room · 12x16 room · 13x13 room · 14x14 room · 14x16 room · 15x15 room · 15x20 room · 16x16 room · 16x20 room · 18x18 room · 20x20 room
Frequently asked questions
How many gallons of paint do I need for a 12×12 room?
With 8 ft ceilings the walls are 2 × 24 × 8 = 384 sq ft. Subtracting one door (20 sq ft) and two windows (30 sq ft) leaves 334 sq ft. Two coats at 350 sq ft per gallon is 668 ÷ 350 = 1.91, so buy 2 gallons. Painting the 144 sq ft ceiling too brings it to 2.73, or 3 gallons.
How much does one gallon of paint cover?
About 350–400 square feet on smooth, primed walls. Rough or textured surfaces, bare drywall and deep color changes reduce coverage to roughly 250–300 sq ft per gallon. Primer usually covers 200–300 sq ft per gallon.
Do I need two coats of paint?
Plan for two coats in most cases. One coat is enough only when repainting a similar color with premium paint-and-primer. Going from dark to light, or covering new drywall, can take a primer coat plus two finish coats.
How much paint is in a quart?
A quart is one quarter of a gallon and covers about 85–100 sq ft per coat. Quarts are handy for trim, small accent walls and closets, but a gallon is cheaper per ounce.