Quick answer
Add up the wall area (room perimeter × wall height, minus large doors and windows), multiply by the number of coats, and divide by the spread rate on the can. Sherwin-Williams lists 350 to 400 square feet per gallon for its Emerald interior paint, and other brands list 250 to 450. A 12 × 12 ft room with 8 ft walls has 384 square feet of wall, so two coats at 350 square feet per gallon take about 2.2 gallons before subtracting doors and windows.
How far a gallon goes
| Paint (data sheet) | Coverage per gallon |
|---|---|
| Sherwin-Williams Emerald Interior Latex Matte | 350 to 400 sq ft |
| Benjamin Moore Regal Select Matte | 400 to 450 sq ft |
| Behr Premium Plus Interior Eggshell | 250 to 400 sq ft |
The range matters. Benjamin Moore says coverage depends on "surface texture and porosity", and Behr says it depends on application method and porosity, with spraying losses not included. Textured or porous walls, such as bare drywall, land toward the low end, and spraying uses more than any of these figures, so plan on the lower number unless your walls are smooth and already painted.
How many coats
- Repainting a similar color: Benjamin Moore says previously painted surfaces can take 1 or 2 coats, but recommends two for full color and the best paint film performance.
- New or bare drywall: Sherwin-Williams and Behr both say to self-prime with 2 coats of the paint, or use 1 coat of primer and then 2 coats of paint. Benjamin Moore's Regal Select is self-priming, and its data sheet says two coats are required on bare substrates.
- A big color change: Sherwin-Williams and Behr both say a coat of primer improves how well the new color hides the old one.
The formula
Wall area = 2 × (room length + room width) × wall height. Subtract doors, windows and closet openings you will not paint, measured width × height. Then gallons = wall area × coats ÷ coverage per gallon. Do the ceiling separately: its area is length × width.
| Room (8 ft walls) | Wall area | 1 coat | 2 coats |
|---|---|---|---|
| 10 × 10 ft | 320 sq ft | 0.9 gal | 1.8 gal |
| 10 × 12 ft | 352 sq ft | 1.0 gal | 2.0 gal |
| 12 × 12 ft | 384 sq ft | 1.1 gal | 2.2 gal |
| 12 × 14 ft | 416 sq ft | 1.2 gal | 2.4 gal |
| 14 × 16 ft | 480 sq ft | 1.4 gal | 2.7 gal |
| 16 × 20 ft | 576 sq ft | 1.6 gal | 3.3 gal |
The table uses 350 square feet per gallon and does not subtract doors or windows, so most rooms will have a little left over for touch-ups. Paint comes in quarts, gallons and 5-gallon pails, so round up. The paint calculator subtracts doors and windows, adds the ceiling and primer if you want them, and rounds to cans.
Frequently asked questions
How many square feet does a gallon of paint cover?
Usually 250 to 450 square feet per coat, depending on the paint and the surface. Sherwin-Williams lists 350 to 400 for Emerald interior paint, Benjamin Moore 400 to 450 for Regal Select, and Behr 250 to 400 for Premium Plus eggshell.
How much paint do I need for a 12 × 12 room?
With 8 ft walls, the walls are 384 square feet. Two coats at 350 square feet per gallon take about 2.2 gallons, a little less after subtracting the door and windows. Two gallons and a quart (about 788 square feet at 350 per gallon) is enough for both coats, even before subtracting the door and windows. If your paint covers 400 square feet per gallon, 2 gallons will do.
Do I need two coats of paint?
Usually. Benjamin Moore recommends two coats even over previous paint, and bare drywall needs two coats of paint or a primer coat plus two coats, per Sherwin-Williams and Behr.
Do I need primer?
Not always. On new drywall, Sherwin-Williams and Behr allow either 2 coats of paint or 1 coat of primer plus 2 coats. For a big color change, both say a coat of primer improves how well the new color hides the old one.
Calculators for this
Paint
Find how much paint you need for a room in gallons and quarts, for walls and ceiling, with doors, windows, coats and wall texture included.
Drywall
Find how many 4x8, 4x10 or 4x12 sheets you need for walls and ceilings, plus joint compound, tape and screws for the job.
Wallpaper
Enter wall width, height and pattern repeat to see how many rolls of wallpaper you need and strips per roll, for US and European rolls.
Published 2026-11-10. Sources: Sherwin-Williams, Emerald Interior Latex Matte (K36) product data sheet; Benjamin Moore, Regal Select Matte 548 technical data sheet; Behr Premium Plus Interior Eggshell Enamel No. 2050 technical data sheet. See how we calculate.