Area divided by coverage, times coats
Take the wall area, divide by the paint\'s coverage (about 350 sq ft per gallon), then multiply by the number of coats and round up. A 400 sq ft room at two coats is 400 ÷ 350 × 2 = 2.3, so buy 3 gallons. Always round up — running short mid-wall means a return trip and a possible color-batch mismatch.
Coverage by product
| Product | Coverage (sq ft/gal) | Coats |
|---|---|---|
| Interior latex paint | 350–400 | 1–2 |
| Exterior acrylic paint | 300–350 | 2–3 |
| Primer | 200–300 | 1 |
| Wood stain | 200–400 | 1–2 |
| Floor epoxy | 250–350 | 2 |
Manufacturers rate coverage on a smooth lab panel. In a real room it runs 15 to 25% lower, so lean toward the low end of each range and add a 10% margin.
What eats into coverage
- Surface texture. Smooth walls give the full spread; rough or new drywall drops it to 70 to 85%, so the same paint covers less.
- Color change. Going lighter is one coat; going darker or into a saturated red or blue usually takes two to three.
- Openings. Subtract windows and doors from the wall area — each large opening is roughly a gallon of paint you do not need.
Common questions
How many square feet does a gallon of paint cover?
About 350 to 400 square feet for one coat of interior wall paint on a smooth surface. Exterior and primer coats spread less, around 200 to 350, because those surfaces are more porous.
Do I need to subtract windows and doors?
For a close estimate, yes. Measure the total wall area, then take off the openings. A rough rule is that one standard door or large window (about 20 square feet) trims roughly a gallon off a large job.
How many coats should I plan for?
Two is normal for any color change. One coat can work for a like-for-like refresh over a similar shade, but going darker, or over bare or patched surfaces, often needs a primer plus two coats.
