How Much Paint Do I Need?

Paint is sold by the gallon but priced by the job, so the difference between a good estimate and a rough guess is usually one or two gallons — and one wasted trip back to the store. The underlying math is a single area calculation adjusted for openings and coverage rate.

Wall area: perimeter times height, minus openings

Start with the room’s perimeter — the sum of all four wall lengths — and multiply by the wall height (typically 8 feet for a standard room, higher for vaulted ceilings). A 12-by-10-foot room has a perimeter of 44 feet; at 8 feet high, that’s 352 square feet of gross wall area. Then subtract doors and windows, since paint doesn’t go on glass or on a door that’s already finished: a standard door is about 20 square feet and a typical window is about 15 square feet. Two windows and a door bring that 352 down to roughly 297 square feet of paintable wall. The wall paint calculator and room paint calculator run this subtraction automatically across multiple walls and openings.

Roughly 350 square feet per gallon, per coat

A gallon of paint covers about 350–400 square feet in a single coat on a smooth, primed, similarly colored surface — use 350 as a conservative planning number. Divide your paintable area by 350 to get gallons needed for one coat: 297 ÷ 350 ≈ 0.85 gallons. That number only covers one coat, which is rarely the whole job.

Why two coats is standard

Almost every interior paint job needs two coats, not one. A single coat frequently shows roller marks, streaks, and patches where the old wall color or primer shows through — especially under direct light. Two coats is the point at which color depth and film build become consistent across the whole wall. Double the one-coat gallon figure for a realistic total: the 297-square-foot room needs about 1.7 gallons for two coats, which in practice means buying two gallons since paint isn’t sold in fractional amounts.

Primer: when it’s a separate line item

Primer is worth budgeting separately, not folded into the topcoat count, whenever you’re painting bare drywall or wood, covering a significantly darker or more saturated existing color, patching drywall repairs, or switching between finish types (e.g., oil-based to latex). A primed wall lets the topcoat behave like the “350 sq ft per coat” figure above; an unprimed wall usually needs an extra coat of topcoat to compensate, which costs more paint than a dedicated primer coat would have.

Rough and dark surfaces cut coverage

Textured surfaces — stucco, heavy orange-peel drywall, brick — soak up more paint per square foot because of surface area hidden in the texture, cutting effective coverage by 20% or more below the 350 sq ft/gallon baseline. Very dark existing colors under a much lighter new color have the same effect for a different reason: full opacity just takes more paint. For exterior work, add porous siding materials like unfinished wood or stucco to that same list — the exterior paint calculator accounts for these surface-type adjustments alongside standard elevation measurements.