Paint Calculator
Work out how many gallons of paint a room needs.
Paint needed
2.5 gal
rounded up to half-gallons
Wall area
356 sq ft
Coverage assumed
350 sq ft/gal
Quarts equivalent
9
The coverage math
Wall area is the perimeter times the ceiling height, minus about 20 sq ft for every door or average window. A gallon covers roughly 350 sq ft on smooth, primed drywall, so gallons = wall area × coats ÷ 350. That 350 is the industry mid-point; cans claim anywhere from 250 to 400 depending on how thickly the paint is formulated to go on.
Worked example: a 14 × 12 bedroom
The perimeter is 2 × (14 + 12) = 52 ft. At an 8 ft ceiling that is 416 sq ft of wall, and subtracting a door and two windows (3 openings × 20 sq ft) leaves 356 sq ft. Two coats means 712 sq ft of coverage, or 2.03 gallons — which is why the calculator lands on 2.5 gallons once it rounds to a buyable size. Two gallons would leave you short by about a third of a wall.
When 350 sq ft per gallon is wrong
Bare drywall, new plaster, and raw wood drink the first coat: expect 25–30% less coverage, which is exactly what primer is for. Knockdown, orange-peel, and stucco textures add surface area you can't see — budget 20% more on light texture and up to 50% on heavy. Deep reds and yellows have weak pigment coverage and often need three coats over a light wall, while flat sheens spread further than semi-gloss or high-gloss.
Ceilings, trim, and the second-coat rule
One gallon of ceiling paint covers a typical 12 × 14 room with a little to spare, and trim usually needs only a quart, since a 6-inch casing adds up slowly. Skip the second coat only when you are repainting the same color in the same sheen — any color change, any sheen change, and any patched wall will telegraph through one coat as soon as light hits it sideways.
Buy a little smart
Round up to the next half-gallon and keep the surplus sealed for touch-ups; re-matching a custom tint two years later is a gamble even with the formula on the lid. If the job runs past about four gallons, have the store mix them in one batch, or box the cans together into a single bucket before starting — small tint differences between mixes show up across a long wall.
Frequently asked questions
- How much paint do I need for a 12 × 12 room?
- With 8 ft ceilings, a 12 × 12 room has 384 sq ft of wall before openings, or about 344 sq ft after a door and two windows. Two coats needs roughly 2 gallons on smooth, previously painted walls. Add a gallon of ceiling paint and a quart of trim paint for the whole room.
- Should I subtract doors and windows from the wall area?
- Yes, but only at roughly 20 sq ft each — a standard door is about 21 sq ft and an average window 15–20. Don't deduct small windows at their true area or you'll underbuy, because you still cut in around every opening and lose paint in the tray and on the roller.
- Do I need primer, and does it count as a coat?
- Primer is needed on bare drywall, raw wood, patched areas, and when covering a dark color with a light one. It's a separate purchase and a separate coat — don't count it toward your two finish coats. Paint-and-primer-in-one products only stand in for primer over sound, previously painted surfaces.
- How much does one gallon of paint actually cover?
- About 350 sq ft on smooth, primed, previously painted drywall. Expect closer to 250 sq ft on textured or porous surfaces, and less again in high-gloss finishes. The figure on the can is a best case measured on a sealed test panel, not a real room.
Related calculators
Age Calculator
Calculate exact age in years, months, and days between two dates.
Date Calculator
Add or subtract days from a date, or count days between dates.
Hours Calculator
Count hours between two times — great for timesheets.
GPA Calculator
Calculate your GPA from course grades and credit hours.