How the paint calculator works
Paint is bought by volume but used by area, so the calculator works out the area you are painting and divides it by how far each gallon or litre goes.
Wall area = 2 × (length + width) × wall height
Paint needed = (wall area − doors − windows + ceiling) × coats ÷ coverage
Each door takes off about 21 ft² (a 3 × 7 ft door) and each window about 15 ft² (3 × 5 ft). In metric, that is about 1.95 m² and 1.4 m².
Worked example
You are painting a 12 × 14 ft bedroom with 8 ft walls, one door and two windows, using two coats.
- Wall area: 2 × (12 + 14) × 8 = 416 ft².
- Minus the door and windows: 416 − 21 − 30 = 365 ft².
- Two coats: 365 × 2 = 730 ft².
- Paint: 730 ÷ 350 = 2.09 gallons, so buy 2 gallons and a quart.
Tips for a better paint job
- Buy all the paint at once and mix the cans together in a bucket so the color is even across walls.
- Keep a little back for touch-ups. Label the leftover with the room and the date.
- Paint ceilings first, then walls, then trim.
- Use the right sheen. Matte hides flaws on ceilings; eggshell or satin cleans better in hallways, kitchens and bathrooms.
Frequently asked questions
How much paint do I need for a 12 × 12 room?
With 8 ft ceilings, the walls of a 12 × 12 ft room add up to 384 ft². Take off one door and two windows (about 51 ft²) and you have 333 ft² to paint. Two coats at 350 ft² per gallon need about 1.9 gallons, so buy 2 gallons.
How many square feet does a gallon of paint cover?
Most interior wall paint covers about 350 to 400 ft² per gallon on smooth, primed walls. Rough, textured or porous surfaces such as new drywall, brick or stucco can soak up much more. The coverage is printed on the can.
Do I need two coats of paint?
Usually, yes. Two coats give even color and better durability, especially when changing color or painting over a darker shade. Paint-and-primer products can sometimes cover in one coat on walls in good condition.
Should I prime first?
Prime new drywall, bare wood, patched areas and stained walls, and use a primer when going from a dark color to a light one. Primer covers a similar area per gallon as paint, so you can use this calculator with one coat to estimate it.
Is it cheaper to buy quarts or gallons?
Gallons are much cheaper per ounce than quarts, and 5-gallon buckets are cheaper again. The calculator suggests quarts only to top up a small amount and switches to a bucket once you need about 4 gallons or more.
Calculations are exact for the coverage you enter. Porous, textured or freshly patched surfaces can need more paint, so check the can's coverage and buy a little extra for touch-ups. Read the full disclaimer and how we check our formulas.