Decorating
Paint Calculator
Enter room dimensions, doors, and windows to get gallons of paint for walls, ceiling, and trim, plus cost.
Your answer
Rounded up to whole containers. Coat one is calculated at the full spread rate and each later coat at 80%, because a sealed surface absorbs less.
How this paint calculator works
The arithmetic behind a paint estimate is simple; the assumptions are where estimates go wrong. This tool measures the perimeter of the room, multiplies by wall height to get gross wall area, subtracts a standard allowance for each door and window, then divides by a spread rate chosen to match your surface. The result is converted into whole containers, because you cannot buy 1.4 gallons.
The one refinement worth knowing about is the coat factor. Manufacturers quote a spread rate for an ideal sealed surface, and a first coat on bare or chalky substrate never achieves it. Once that coat cures, the surface is sealed by the paint itself and later coats stretch further. We calculate coat one at the full rate and each subsequent coat at 80% of it, which lands much closer to real consumption than simply doubling.
Spread rates by surface
Texture adds surface area invisibly. A wall that measures 10 square metres flat may present 12 or more to the roller once you account for the peaks and hollows of render or a heavy textured finish. These are the rates this calculator uses.
| Surface | Coverage per litre | Coverage per gallon |
|---|---|---|
| Smooth — sealed plaster, previously painted | 12 m² | ~510 sq ft |
| Light texture — new plasterboard, woodchip | 10 m² | ~425 sq ft |
| Heavy texture — render, brick, artex | 8 m² | ~340 sq ft |
Measuring the room properly
- Measure the length and width of the floor at skirting level, not at eye level — walls are rarely parallel and the floor is the honest measurement.
- Measure wall height from the top of the skirting to the ceiling line, not from the floor, unless you are painting the skirting the same colour.
- Count openings larger than about 2 sq ft. Small windows and internal glazing panels are not worth subtracting individually.
- For a room with an alcove or chimney breast, treat the alcove sides as extra wall: measure them and add the area manually to the length figure.
- Sloped ceilings: treat the sloped section as a rectangle using the slope length rather than the vertical height.
How many coats do you actually need?
Two coats is the default for a reason: one coat of modern emulsion over a similar colour rarely gives an even sheen, and sheen unevenness is more visible than colour unevenness under raking light. But the honest answer depends on what you are covering.
- Same colour refresh over sound paint: one coat is usually enough.
- Any colour change: two coats, plus a tinted primer if you are going light over dark.
- Strong colours — deep red, saturated yellow, some blues: three coats is common because these pigments have poor natural opacity.
- New plasterboard or fresh plaster: a mist coat (thinned emulsion) first, then two full coats. The mist coat is absorbed almost entirely and should be counted separately.
- Bare wood or previously glossed trim: primer, then two coats of your finish.
Buying: round up, but round up deliberately
Larger containers are almost always cheaper per litre, and having a little left over is genuinely useful for touching up later. Buy the same batch number where you can — paint mixed at different times can differ subtly, and that difference shows on a large wall lit from the side.
Common mistakes this calculator prevents
- Estimating from floor area instead of wall area — the two are unrelated in a long narrow room.
- Using the label's spread rate on textured walls, which under-buys by 25-40%.
- Doubling the first-coat figure for two coats, which routinely over-buys.
- Forgetting that a mist coat on new plaster is an extra coat, not part of the two.
- Ignoring the ceiling until the walls are done and the shops have shut.
How much paint do I need for a 12x12 room?
A 12 ft x 12 ft room with 8 ft ceilings has about 384 sq ft of wall before openings, or roughly 340 sq ft after a door and two windows. At 350 sq ft per gallon that is one gallon per coat, so two gallons for two coats.
Does the second coat use the same amount of paint?
No. The first coat is partly absorbed by the substrate; the second sits on cured paint and typically covers 10-25% further. Estimating the second coat at about 80% of the first is more accurate than doubling.
Should I subtract doors and windows?
Yes, for anything larger than about 20 sq ft in total. A standard door is roughly 21 sq ft and an average window 15 sq ft. Subtracting them prevents over-buying on rooms with lots of glazing.
How much paint do I need for the ceiling?
Ceiling area equals floor area. Ceilings usually take one coat over an existing painted surface and two over new plasterboard, at roughly the same spread rate as walls.
Do I need primer?
Prime bare plasterboard, new filler, bare wood, glossy surfaces, and any dramatic colour change. Primer covers about the same area as paint, so budget one primer coat at the first-coat rate.
Last reviewed February 2026