What decides how much paint a room takes
Four things, and only one of them is the room's size. The paintable area is the wall surface left once doors and windows are taken out. That area is then multiplied by the number of coats, because two coats over 400 square feet is 800 square feet of painting. The result is divided by the coverage rate — how far one gallon actually spreads. And finally it is rounded up to whole cans, because paint is not sold by the fluid ounce.
Floor area is conspicuously not on that list. A 12 × 12 room and a 16 × 9 room have the same 144 square feet of floor but different wall perimeters, so they need different amounts of paint. Perimeter and height are what matter, which is why this calculator asks for length, width and wall height separately rather than for a room size.
Using this paint calculator
Enter the room's length, width and wall height, then the number of doors and windows. Each door deducts 20 sq ft and each window 15 sq ft — conventional 3 ft × 6 ft 8 in and 3 ft × 5 ft openings. The results show the paint needed, the cans to buy rounded up, the paintable area, the total area across all coats, what was deducted for openings, and the coverage rate being assumed.
The surface selector is the one to set honestly. Smooth, sealed, previously painted drywall gets 400 sq ft to the gallon; anything textured, porous or freshly patched gets 350. Manufacturers publish both figures for the same product — Sherwin-Williams quotes “350–400 sq ft per gallon” on SuperPaint — and the difference across a large room is close to a full can.
Painting the ceiling too? That is in the customize panel, along with the unit system, since it changes what is being estimated rather than a dimension of the room. It adds the floor area to the paintable surface.
The formula, worked through
Wall area is the perimeter times the height. Subtract the openings, multiply by coats, divide by coverage.
Take a 12 ft × 12 ft room with 8 ft walls, one door and two windows, two coats, on a textured surface. The perimeter is (12 + 12) × 2 = 48 ft, so the walls are 48 × 8 = 384 sq ft. One door and two windows deduct 20 + 30 = 50, leaving 334 sq ft to paint. Two coats make 668 sq ft. At 350 sq ft per gallon that is 1.91 gallons — so two gallons, with a little to spare.
The mistake this arithmetic exposes is the one most people make by eye: they estimate for one coat. Two coats is the standard for any real colour change, and a genuinely dramatic one over a dark base can need three. Buying for a single coat and going back mid-job risks a batch with a slightly different tint, which shows.
Where the estimate stops and judgement starts
A few things this calculator deliberately leaves to you. Primer is a separate product and a separate quantity — run the numbers again with one coat if you need it. Trim, doors and window frames are usually a different paint in a different finish, and their area is small enough that a quart typically covers a room. And bare drywall, raw plaster or new patches drink far more than the data sheet suggests; treat the 350 figure as a floor there, not an average.
It is also worth buying whole cans rather than exactly enough. Leftover paint from the same batch is the only reliable touch-up you will get, and a wall repaired a year later with a fresh mix rarely matches. The “buy” figure above already rounds up for that reason.
FAQ
How much paint do I need for a 12 × 12 room?
Just under two gallons for two coats, which means buying two. The arithmetic, with 8 ft ceilings, one door, and two windows:
Buy the second full gallon rather than a quart. Touch-ups a year later want paint from the same batch, and the leftover is the only way to get it.
Does a gallon of paint really cover 400 square feet?
On the right wall, yes. Interior latex is rated at 350–400 sq ft per gallon at 4 mils wet, and every major brand lands in much the same band. The difference between the two ends of it is the surface, not the paint:
| Surface | Realistic coverage |
|---|---|
| Smooth, previously painted drywall | ≈ 400 sq ft |
| Lightly textured or fresh primer | ≈ 350 sq ft |
| Knockdown texture, raw drywall | 250 – 300 sq ft |
| Bare masonry, stucco, rough wood | As low as 200 sq ft |
Rollers with a deep nap hold more paint and spread it thicker, which lowers coverage further. This calculator defaults to the 350 figure because it is the one that leaves you with paint rather than a second trip to the store.
How many coats of paint do I need?
Two, almost always. One coat over an existing similar colour will look fine while it's wet and patchy once it dries, because the film is thinner than the manufacturer's spread rate assumes and the sheen goes uneven.
Use three when you're going dramatically lighter over a dark wall, or when a strong colour — reds and deep yellows especially, whose pigments are weak hiders — has to cover anything at all. In those cases a tinted primer does more good than a third finish coat, and costs less.
Should I subtract doors and windows?
Yes, once there's more than one or two. This calculator deducts 20 sq ft for each door and 15 sq ft for each window — a standard 3 ft × 6 ft 8 in door and a 3 ft × 5 ft window. A room with a door and two windows loses 50 sq ft, which is a sixth of a small room's wall area and a meaningful fraction of a gallon.
What it does not deduct is baseboards, and it should not: you cut in above them, and the paint you save is the paint you waste on the cut. If a wall is mostly glass — a sunroom, a slider — measure the glazing and subtract it as extra windows.
Do I need separate paint for the ceiling and the trim?
Different products, so estimate them separately. Open the customize panel (the sliders icon at the top right of the calculator) and switch Ceiling to Also paint the ceiling, and its area is added to the walls — but ceiling paint is a distinct flat formulation, so don't buy one gallon and split it.
Trim is small enough that area math misleads. A quart of enamel does the doors, baseboards, and casings of an ordinary room; a gallon does a small house.
Buying separately also lets you buy properly: flat on the ceiling to hide roller lap, and a scrubbable satin or semi-gloss on the trim.
What else am I going to need to estimate?
If you're painting because you just built something, the rest of the job has its own arithmetic: the Concrete Calculator for a slab or footing, the Sand Calculator for a paver base, and the Topsoil Calculator for what goes back in the beds afterwards.