Find out how many gallons of paint you need for a room. Enter the room size, doors, windows and coats, and get gallons, quarts and exactly what to buy.
Running out halfway through a wall means a second trip to the store and a risk of a slightly different batch. Buying too much wastes money. How much you need depends on the wall area, the number of coats and how far your paint spreads, which is printed on the can. This calculator handles all three, takes off doors and windows, and tells you how many gallons and quarts to buy.
How much paint do I need for a room?
Multiply the room’s perimeter by the wall height, subtract about 20 sq ft per door and 15 sq ft per window, multiply by the number of coats and divide by the coverage on the can. One gallon covers about 350 to 400 sq ft per coat. A 12 × 14 ft bedroom with 10 ft walls, one door and two windows has about 470 sq ft of wall, so two coats need roughly 2.7 gallons. Buy 3 gallons.
The formula
Gallons = wall area × coats ÷ coverage per gallon
The calculator uses 20 sq ft for a standard interior door and 15 sq ft for a standard window. If you have a sliding patio door or a large picture window, add one or two extra “windows” to match.
How much paint for common room sizes
Two coats, one door, one window, walls only, at 350 sq ft per gallon:
| Room | Wall area | Paint needed |
|---|---|---|
| 10 × 10 ft, 8 ft walls | 285 sq ft | 1.6 gal |
| 12 × 12 ft, 8 ft walls | 349 sq ft | 2.0 gal |
| 12 × 14 ft, 8 ft walls | 381 sq ft | 2.2 gal |
| 12 × 14 ft, 10 ft walls | 485 sq ft | 2.8 gal |
| 15 × 20 ft, 9 ft walls | 595 sq ft | 3.4 gal |
| 20 × 20 ft, 10 ft walls | 765 sq ft | 4.4 gal |
Typical coverage per gallon
| Surface | Coverage per gallon |
|---|---|
| Interior latex on smooth, painted drywall | 350–400 sq ft |
| Primer | 200–300 sq ft |
| New, unprimed drywall | Less, so prime first |
| Textured or rough walls | Noticeably less, check the label |
Figures for latex paint and primer are the ranges Sherwin-Williams gives for its paint calculator. Porous or textured surfaces soak up more, and a big color change, such as dark to light, often needs an extra coat or a tinted primer.
Gallon, quart or 5-gallon bucket?
A gallon is 4 quarts (about 3.8 liters). Quarts cost more per ounce, so once you need more than half a gallon extra, a full gallon is usually the better buy. For a whole house, or one color across several rooms, 5-gallon buckets are cheaper per gallon and keep the color consistent.
How to use the Paint Calculator
Enter the room size
Pick feet or meters, then enter the room’s length, width and wall height.
Add doors and windows
Enter how many doors and windows the room has so their area is taken off.
Set coats and coverage
Two coats is standard. Check the can for its coverage in square feet per gallon and enter it.
See what to buy
The gallons needed appear in large text, with the cans to buy, the paintable area, liters and paint per coat.
Frequently asked questions
How many square feet does a gallon of paint cover?
About 350 to 400 sq ft per coat on smooth, previously painted walls. Primer covers less, about 200 to 300 sq ft per gallon.
How much paint do I need for a 12 × 12 room?
With 8 ft walls, one door and one window, the walls are about 350 sq ft. Two coats need about 2 gallons.
Do I need two coats of paint?
Usually yes. Two coats give even color and better durability, especially when you’re changing color.
Should I buy extra paint?
Keep a little for touch-ups. The calculator already rounds up to whole gallons or quarts, which usually leaves enough.
How much paint do I need for a ceiling?
Multiply length by width for the ceiling area, then divide by the coverage. Choose “Paint the ceiling” and the calculator adds it for you.
Sources and further reading
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Last reviewed: October 2026