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. 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
- Big color change 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, and 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, so 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, so 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 use one, 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.