The formula
Coverage is usually printed on the can. PaintCare’s Buy Right guide says most coverage rates fall between 300 and 400 square feet per gallon, and to use 350 for a rough estimate when you do not know.
Worked example
| Step | Math | Result |
|---|---|---|
| Gross walls | 2 × (12 + 12) × 8 | 384 sq ft |
| Door | 1 × 20 sq ft | − 20 |
| Windows | 2 × 15 sq ft | − 30 |
| Paintable area | 384 − 50 | 334 sq ft |
| Gallons for 2 coats | 334 × 2 ÷ 350 | 1.91 gal |
| Buy this | cheapest cans | 2 gallons, $90 |
Doors and windows
We use 20 sq ft for a standard door and 15 sq ft for a window. If you have big picture windows or sliding doors, measure them and subtract. Subtracting is optional: leaving them in simply buys a little extra paint.
How many coats?
Two coats is the usual plan for a fresh color. Similar colors may need only one; PaintCare notes a very dark to very light change (or the reverse) may take three or even four coats, and reds, oranges and purples can take more. Primer can reduce the number of top coats.
Textured walls and sheen
PaintCare says you may need up to 50 percent more paint on rough textures, and that flat paint covers more than semi-gloss or gloss (a job needing two flat coats may need three or more glossy ones). For rough walls, switch the coverage to 250 in the calculator.
Ceilings and trim
A 12 × 12 ft ceiling is 144 sq ft: 0.82 gallons for two coats, so 1 gallon. Trim is usually a quart or so per room. The paint calculator adds ceiling, primer and trim as separate lines, because they are different cans.
Which cans to buy
Paint comes in quarts (¼ gallon), gallons and 5-gallon buckets. The cheapest way to cover, say, 2.1 gallons is not always 3 gallons. At typical prices ($45 gallon, $20 quart, $200 bucket) it is 2 gallons + 1 quart. See gallons vs a 5-gallon bucket.
Estimates only. Check your paint can’s label for coverage. Prices are typical 2026 US figures; use your store’s.