How to use the paint calculator
- Measure the room. Enter the floor length and width and the wall height in feet. The calculator works out the area of all four walls.
- Count doors and windows. They are subtracted from the wall area. Standard sizes are used (21 sq ft per door, 15 sq ft per window), and you can change them under Coverage and price.
- Choose the number of coats. Two coats is standard for even color and durability.
- Decide on the ceiling. Include it with the walls if you are using the same paint, or calculate it separately for ceiling white.
Paint formula
Paintable area = wall area − (doors × 21) − (windows × 15)
Gallons = paintable area × coats ÷ coverage per gallon
Most interior paints cover 350 to 400 square feet per gallon on smooth, primed walls. Check the can for your product's figure.
Worked example
A 14 × 12 ft bedroom with 8 ft walls has 2 × (14 + 12) × 8 = 416 square feet of wall. Subtract one door (21) and two windows (30) to get 365 square feet. Two coats need 730 square feet of coverage. At 350 square feet per gallon, that is 2.09 gallons, so buy 2 gallons and 1 quart. The 168 square foot ceiling needs about 1 gallon of ceiling paint for two coats.
Quick reference: paint for common rooms
| Room (8 ft walls) | Walls, 2 coats |
|---|---|
| Small bathroom, 5 × 8 ft | About 1 gallon |
| Bedroom, 12 × 12 ft | About 2 gallons |
| Master bedroom, 14 × 16 ft | About 2.5 to 3 gallons |
| Living room, 16 × 20 ft | About 3 to 3.5 gallons |
These assume one door, two windows and 350 square feet per gallon.
Tips to get the amount right
- Changing color dramatically? Going from dark to light usually needs a primer coat plus two coats of color, or a high-hide paint.
- New drywall soaks up paint. Prime it first, then plan for two coats.
- Textured walls such as orange peel or knockdown can use 10% to 25% more paint.
- Buy it all at once. Colors mixed on different days can vary slightly. Box (mix together) multiple gallons for one consistent color.
- Keep a little extra for touch-ups. A labeled quart is enough for years of small repairs.
Painting a floor or need the floor area for other materials? Use the square footage calculator.
Frequently asked questions
How many square feet does a gallon of paint cover?
Most interior paint covers 350 to 400 square feet per gallon for one coat on smooth, primed walls. Rough, porous or unprimed surfaces get less coverage.
How much paint do I need for a 12x12 room?
A 12 by 12 ft room with 8 ft walls has 384 square feet of wall. After subtracting a door and two windows, two coats need about 1.9 gallons, so buy 2 gallons.
Do I need two coats of paint?
Usually yes. Two coats give even color, hide the old color and last longer. One coat can be enough when repainting the same color with a high-quality paint.
How much paint do I need for a ceiling?
Multiply the room length by the width to get the ceiling area. A 12 by 14 ft ceiling is 168 square feet, so two coats need about 1 gallon.
How many quarts are in a gallon of paint?
There are 4 quarts in a gallon. If you need a little more than a whole number of gallons, a quart or two is often cheaper than another full gallon.
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Last updated October 2026.