Paint calculator
Wall area less the openings, times the coats, divided by what a gallon covers.
Paint
Wall area less the openings, times the coats, divided by what a gallon covers.
Doors deducted at 20 sq ft, windows at 15. Cheap paint needs a third coat more often than not.
$196
7.00 gallons of contractor grade at $28.00 / gal
Free. No signup. The quantities carry over as line items.
How this paint calculator works
Paint is sold by the gallon and spread by the square foot, so the whole job is one division problem — as long as you take the openings out first and count the coats honestly.
The formula
gallons = ⌈(wall area − doors×20 − windows×15) × coats ÷ coverage × (1 + waste)⌉
Assumptions and defaults
| Door deduction | 20 sq ft each |
| Window deduction | 15 sq ft each |
| Coverage | 350 sq ft per gallon |
| Default waste | 5% |
| Primer | one coat, ⌈net ÷ coverage⌉ gallons |
Pro tips
Coverage on the can is measured on sealed drywall in a lab. Bare drywall, rough plaster and dark-to-light changes all drink more — drop to 300 and you will not come up short.
Two coats of good paint beats three coats of cheap. The labour on the third coat costs more than the price difference on the paint.
Buy the whole job at once and check the batch numbers match. Two batches of the same colour do not always look the same on a long wall.
Ceilings drink more than walls because you are rolling overhead against gravity and the texture is usually rougher. If the job is ceiling-heavy, drop coverage to 300 and price the extra gallon rather than discovering it on day two.
Dark over light, or any strong colour change, is a three-coat job whatever the can promises. Price it as three, and if the second coat covers you have handed the client a small win instead of an argument.
Common questions
How much paint do I need for a 12 × 12 room?
A 12 × 12 room with 8 ft ceilings is about 384 sq ft of wall. Two coats at 350 sq ft per gallon is 2.2 gallons before openings — so three gallons buys the walls with a little left for touch-ups.
Should I deduct doors and windows?
Yes, but only the big ones. This calculator takes 20 sq ft per door and 15 per window, which is close enough on a normal house. If you have a wall of glass, measure it and enter the real number.
Does primer count as a coat?
No. Primer seals and blocks — it is not a finish coat and it covers differently. Toggle it on and the calculator adds one primer coat over the net area at its own price.
How much waste should I add for paint?
Five per cent is enough on a normal repaint. Go to ten if you are cutting a lot of small rooms, spraying, or working from more than one batch.