How much paint do I need for a room?
By AreaLab · Published June 10, 2026 · Updated June 10, 2026
Multiply the room's perimeter by its ceiling height to get wall area, multiply by the number of coats, divide by coverage of about 350 square feet per gallon, and round up to whole cans — two coats in a 12 ft × 10 ft room with 8-ft ceilings comes to 3 gallons, or 2 once a door and two windows are subtracted.
Wall area: perimeter × ceiling height
Paint covers walls, so the starting number is wall area, not floor area. Add up the wall lengths around the room to get the perimeter, then multiply by the ceiling height: a 12 × 10 ft room has 2 × (12 + 10) = 44 ft of perimeter, which under 8-ft ceilings is 352 square feet of wall. Two coats means the paint must cover that area twice — 704 square feet of total coverage — and at 350 square feet per gallon this site's engine returns 3 gallons, because paint is sold by the can, not the ounce.
That whole-can rounding is the defining feature of paint math. The 704-square-foot job actually needs 2.01 gallons of liquid, but no store sells 0.01 of a gallon, so the answer is 3. Small changes to the inputs can therefore swing the purchase by a full can in either direction — which is exactly why the two refinements below are worth thirty seconds each.
Subtract doors and windows before you buy
Doors and windows do not get painted, and the standard allowance is about 20 square feet per door and about 15 square feet per average window. In the 12 × 10 ft room, one door and two windows remove 50 square feet, taking the wall area from 352 to 302. Two coats on 302 square feet is 604 square feet of coverage — and the engine's answer drops from 3 gallons to 2. Thirty seconds of subtraction saved a whole can.
The saving is not guaranteed — it only appears when the subtraction crosses a can boundary — but on small and mid-sized rooms it frequently does, because a gallon's 350 square feet is large relative to one room's walls. Run the estimate both ways; if the gallon count does not move, the openings were already absorbed by the round-up.
Coats and coverage move the answer more than room size
The 350-square-feet-per-gallon figure is the long-standing planning convention for interior latex on smooth, primed walls; Sherwin-Williams' paint-calculator guidance quotes 350–400 square feet per gallon (checked 2026-06-10), and the low end is the safer planning number. Texture changes it fast: rough, porous, or previously unpainted surfaces cover meaningfully less. Drop the coverage input to 300 for the same 302-square-foot, two-coat room and the engine's answer climbs back from 2 gallons to 3.
Coat count is just as powerful. Two coats is the standard for color changes and a uniform, washable finish; a same-color refresh on sound walls may get away with one. But note the cliff even there: a single coat on the full 352 square feet still needs 2 gallons, because 352 already exceeds what one 350-square-foot gallon covers. Coats double the demand; coverage divides it — room dimensions only set the starting point.
Bigger rooms, and why the ceiling is a separate run
The same arithmetic scales up without changing shape. A 14 × 12 ft room with 9-ft ceilings has 52 ft of perimeter and 468 square feet of wall; subtract two doors and two windows (70 square feet) to get 398, and two coats — 796 square feet of coverage — comes to 3 gallons by the engine. Without the subtractions the same room would round up to 936 square feet of coverage, still 3 gallons: on larger rooms the openings matter less often, because each gallon boundary is a smaller fraction of the total.
The ceiling is its own estimate, not an add-on to the walls. It is the floor area — 120 square feet in a 12 × 10 ft room — and it is usually painted with a dedicated flat ceiling paint, a different product with its own coverage. Two coats on that 120-square-foot ceiling is 240 square feet of coverage, which fits in 1 gallon. Running it separately also keeps the wall color purchase clean if the ceiling stays white.
Primer, quarts, and a sensible buying strategy
Primer is a separate product with its own coverage rate, needed over bare drywall, patched areas, stains, or when going light over dark. Estimate it as its own one-coat run of the calculator rather than folding it into the paint count. Skipping needed primer does not save paint — it usually adds a third finish coat, which costs more than the primer did.
For overruns, think in quarters: a quart is a quarter gallon, so at the 350 convention it covers roughly 87 square feet per coat. If the estimate lands just past a can boundary — say 2 gallons plus a few dozen square feet — a gallon and a quart of the same color can beat buying a third gallon. Keep the leftover labeled for touch-ups, and keep the coverage input honest about your wall texture; the cheap insurance against a second store trip is a conservative coverage number, not an extra can bought on faith.
Questions
- How many gallons of paint do I need for a 12 × 10 room?
- With 8-ft ceilings the walls total 352 square feet, so two coats at 350 square feet per gallon round up to 3 gallons. Subtract one door and two windows (about 50 square feet) and two coats fit in 2 gallons.
- How much area does one gallon of paint cover?
- About 350–400 square feet per coat for typical interior latex on smooth, primed walls, per Sherwin-Williams' paint-calculator guidance. This calculator defaults to the conservative 350 figure; rough or porous surfaces cover less.
- Do I really need two coats?
- For a color change or a uniform, washable finish, yes — two coats is the standard. A same-color refresh on walls in good condition may need only one, but check the result in daylight before returning the leftover can.
- How much paint does a ceiling need?
- The ceiling area equals the floor area — 120 square feet in a 12 × 10 ft room — and two coats of dedicated ceiling paint at 350 square feet per gallon fit in 1 gallon. Estimate it separately from the walls.