Give it the room. It takes the doors and windows off the walls, derates the can's coverage for the surface you are actually painting, and tells you what to buy — in whole cans and buckets, the cheapest way.
Sample prices — put in your store's. The cheapest mix depends on them: when a bucket costs less than four cans, one bucket is the right buy even for four gallons.
Worked through the calculator above with one door, one window, an 8 ft ceiling, two coats on smooth painted walls, and a paint that claims 350 sq ft per gallon. Walls only — the ceiling column is the extra paint if you do that too.
| Room, ft | Wall area to paint | Paint for walls | Buy | Ceiling adds |
|---|---|---|---|---|
| 10 × 10 | 288 sq ft | 1.65 gal | 2 × 1 gal | 0.57 gal |
| 10 × 12 | 320 sq ft | 1.83 gal | 2 × 1 gal | 0.69 gal |
| 12 × 12 | 352 sq ft | 2.01 gal | 3 × 1 gal | 0.82 gal |
| 12 × 14 | 384 sq ft | 2.19 gal | 3 × 1 gal | 0.96 gal |
| 14 × 16 | 448 sq ft | 2.56 gal | 3 × 1 gal | 1.28 gal |
| 16 × 20 | 544 sq ft | 3.11 gal | 4 × 1 gal | 1.83 gal |
Look at the 12 × 12 row. Two coats need 2.01 gallons — a hair over two — and that hair is a third can. A plain "two gallons for a bedroom" rule of thumb runs you out on the last wall.
Written out: gallons = (perimeter × height − openings) × coats ÷ (coverage × surface factor).
The coverage printed on a can — usually 350 to 400 sq ft per gallon — is measured on a smooth, sealed wall. Texture has more surface than it looks, and bare drywall soaks the first coat up. This is how much of the can's number you really get:
| Surface | Counts as | A 350 sq ft/gal can covers |
|---|---|---|
| Smooth, painted before | 100% | 350 sq ft |
| Light texture, orange peel | 85% | 297.5 sq ft |
| Heavy texture, stucco, masonry | 70% | 245 sq ft |
| New drywall | 70% | 245 sq ft |
New drywall also wants a coat of primer first. The calculator adds it when you pick that surface, at a primer's own spread rate of 250 sq ft per gallon rather than the topcoat's — primers are thinner and porous board takes more of them.
Paint is bought in containers, not gallons, and the cheapest way to get seven gallons is rarely seven cans. With cans at $40 and five-gallon buckets at $170, seven gallons is one bucket and two cans for $250 — not seven cans for $280, and not two buckets for $340. And when cans cost $45, four gallons is cheaper as one $170 bucket than as four cans, even though the bucket leaves a gallon over.
That choice is worked out exactly, not guessed: every combination is priced in quarter-gallon steps and the cheapest one that covers the job wins. Quarts are handled the same way, which matters for trim, where a quart is often all you need.
For 1,000 sq ft of wall at two coats and 350 sq ft per gallon, 5.7 gallons. For 2,000 sq ft of wall, 11.4.
But if the 2,000 sq ft is the size of a house, it is floor area, and floor area does not tell you the wall area — ceiling height, the number of rooms and how open the plan is all change it. Measure the rooms and add them up; that is the only number worth buying paint against.
This calculator is the quantity half of PaintQuote, an iPhone app for painting contractors that scans the room with LiDAR and turns it into a priced quote. The coverage factors, the opening sizes and the cheapest-fill rule on this page are ported from the app's own estimating engine and checked against the same test numbers.
PaintQuote is not in the App Store yet. Until it is, the painting estimate calculator prices a whole job — paint, primer, labour and tax — and prints it as a quote.
Last reviewed 22 September 2026