What to measure
How to use the paint calculator
Measure the floor’s length and width and the wall height. Count doors and windows — we subtract 20 sq ft per door and 15 sq ft per window. Keep two coats unless you’re refreshing the same color. If your can lists a different coverage, enter it.
The formula
- Wall area = 2 × (length + width) × height − openings
- Gallons = area × coats ÷ coverage per gallon
We then suggest cans: whole gallons, plus quarts for the remainder. Three quarts cost about the same as a gallon, so we round those up — and a remainder under 0.05 gallon is too small to buy a quart for.
Worked example: 12 × 12 ft bedroom, 8 ft walls
- Walls: 2 × (12 + 12) × 8 = 384 sq ft
- Minus 1 door (20) and 2 windows (30): 334 sq ft
- Two coats: 668 sq ft ÷ 350 = 1.91 gallons → buy 2 gallons
Tips that change the amount
- Dark over light or new drywall: prime first. Primer covers less (200–300 sq ft per gallon).
- Textured walls: lower coverage to about 300 sq ft.
- Keep a little extra for touch-ups — note the color code on the lid.
Paint by room size
With 8 ft walls, one door, two windows, two coats and 350 sq ft per gallon. Wall area is after the openings, for one coat.
| Room | Wall area | Walls only | Walls + ceiling |
|---|---|---|---|
| 10 × 10 ft | 270 sq ft | 2 gal | 2 gal + 1 qt |
| 10 × 12 ft | 302 sq ft | 2 gal | 2 gal + 2 qt |
| 12 × 12 ft | 334 sq ft | 2 gal | 3 gal |
| 12 × 14 ft | 366 sq ft | 2 gal + 1 qt | 3 gal + 1 qt |
| 14 × 16 ft | 430 sq ft | 2 gal + 2 qt | 4 gal |
| 16 × 20 ft | 526 sq ft | 3 gal | 5 gal |
| 20 × 20 ft | 590 sq ft | 3 gal + 2 qt | 6 gal |
Common mistakes
- Using 8 ft for every room. Measure the actual wall height; a 9 ft or vaulted wall adds a lot of area.
- Trusting the best-case coverage. Textured, porous or new drywall takes more paint — lower the coverage figure as above.
- Forgetting the second coat. Two coats double the paint, and a color change almost always needs them.
- Counting the ceiling with the walls in the wrong paint. If the ceiling gets flat ceiling paint, calculate it separately.
Frequently asked questions
How much paint do I need for a 12x12 room?
With 8 ft walls, one door, two windows and two coats, about 1.9 gallons — so buy 2 gallons. Add the ceiling and it rises to about 2.7 gallons.
How many square feet does a gallon of paint cover?
Most wall paint covers 350–400 sq ft per gallon per coat on smooth, primed walls. Textured, porous or new drywall soaks up more — check the can and lower the coverage figure.
Do I need two coats of paint?
Usually. Two coats give even color and hide roller marks, especially with a color change. One coat can work for a fresh coat of the same color.
Should I use the same paint on the ceiling?
Most people use a flat ceiling paint instead. If you do, calculate the ceiling separately by entering the room's length and width with a wall height of zero and the ceiling option on.
How much paint do I need for a 10x10 room?
With 8 ft walls, one door, two windows and two coats, the walls take about 1.54 gallons, so buy 2 gallons. With the ceiling in the same paint it's about 2.11 gallons, or 2 gallons and a quart.
How much paint for a 20x20 room?
About 3.37 gallons for the walls with 8 ft walls, one door, two windows and two coats, so 3 gallons and 2 quarts. Painting the ceiling too brings it to about 5.66 gallons, so buy 6.
How much primer do I need for a 12x12 room?
One coat over 334 sq ft of wall is about 1.34 gallons at 250 sq ft per gallon, or 1.11 gallons at 300. Set coats to 1 and enter the primer's coverage from the can.
Sources
Formulas and defaults last reviewed October 30, 2026. How we check our math.