How much paint do I need?

1. The paint formula

Every paint estimate comes down to one line: paint = paintable area × coats ÷ coverage. The paintable area is the surface you will actually cover. The coats are how many times you will go over it — two is the standard recommendation, for reasons we come to below. The coverage, sometimes called the spread rate, is how far one unit of paint goes, and it is printed on the tin. Get those three numbers and the rest is division.

The reason paint is simpler than wallpaper is that paint genuinely behaves like an area: there are no full-height strips to cut and no unusable offcut at the end of a roll, so a straight area-÷-coverage sum is the right model. The only refinements are deducting the big openings, choosing the right number of coats, and rounding up to whole tins at the end.

2. Measuring the paintable area

Start with the walls. The wall area is the room's perimeter times its height. For a rectangular room the perimeter is twice the length plus twice the width, so a 12 × 12 ft room is 2 × (12 + 12) = 48 ft round, and at 8 ft high that is 48 × 8 = 384 ft² of wall. If you are painting the ceiling too, add its area — length × width, here 12 × 12 = 144 ft² — to the total. Measure the height in a couple of places, because older rooms are rarely square, and use a tape rather than guessing from the number of courses or boards.

Then take out the large openings. Unlike wallpaper, where the strips above and below a door are wasted anyway, paint genuinely skips the glass and the door leaf, so deducting them saves real paint. A standard interior door is about 21 ft² (1.95 m²) and an average window about 15 ft² (1.4 m²). Knocking one door and one window off our 384 ft² leaves about 348 ft² of wall to paint. You do not need to fuss over small windows — two coats and the final round-up absorb the slack — but do deduct a wide patio door or a picture window, which can be two or three times the size of a normal opening.

3. Coverage — how far a tin really goes

Coverage is the number that varies most, so it is worth getting right. A gallon of wall paint typically covers 350 to 400 ft² in one coat on a smooth, previously painted surface; in metric a litre covers roughly 10 to 12 m². The calculator defaults to 350 ft² per gallon and 11 m² per litre, which are sensible middle-of-the-road figures, but the tin always wins — read its stated spread rate and type that into the coverage field.

Three things push real coverage down. Bare or porous surfaces — new drywall, fresh plaster, bare wood, render — soak up the first coat, so prime them first or expect the first coat to go much further than the tin claims. Texture eats paint: a heavily textured or rough wall has more actual surface than its flat measurement suggests. And deep or saturated colours, and big light-to- dark or dark-to-light changes, need an extra coat to look even. When in doubt, prime, and treat the primer as its own coat with its own coverage.

4. Paint per room, at a glance

This chart shows the paint a square room needs for two coats at 350 ft² per gallon, walls only and before deducting doors and windows — so treat it as a slightly generous planning figure and round up to whole tins. Deducting a door and a window typically shaves the 12 × 12 ft room back under 2 gallons, which is the rounding you see in the calculator.

In metric the method is identical: a 3.6 × 3.6 m room with 2.4 m walls has 2 × (3.6 + 3.6) × 2.4 = 34.56 m² of wall, so two coats at 11 m² per litre is 34.56 × 2 ÷ 11 ≈ 6.3 litres — buy 7 litres, or a 5 L tin plus a 2.5 L tin. The paint calculator switches between feet and metres for you and does the deduction and round-up automatically.

5. Why two coats — and when one or three

Two coats is the standard recommendation, and the calculator defaults to it, because a single coat rarely gives an even film: you see brush and roller lines, patchy sheen and the old colour ghosting through. The second coat evens the build, deepens the colour to its true shade, and gives a more durable, washable surface. So order paint for two coats unless you have a specific reason not to.

One coat can be enough when you are repainting the very same colour over a sound, clean surface, or when using a thick one-coat paint exactly as the maker intends. Three coats come into play for the hardest jobs — a bold accent colour, bright reds and yellows that have weak hiding power, or covering a dark wall with a pale shade. Change the coats field in the calculator and the order scales straight away, so you can compare the paint for two coats against three before you buy.

6. Buying, leftovers and the wider job

Always round up to whole tins — you cannot buy a part-gallon, and a little left over is exactly what you want for touch-ups after the furniture goes back. Where you can, buy a single large tin rather than several small ones: it is usually cheaper per unit and guarantees the colour matches, since paint mixed in different batches can vary very slightly. If you do need more than one tin of a tinted colour, box them — pour them together into one larger container and stir — so any batch difference disappears across the wall rather than showing as a line.

Paint is usually the last step over fresh surfaces, so plan it alongside the rest of the job. If you are boarding out a wall first, the drywall calculator works out the sheets, and the same area maths underlies a wallpapered feature wall in the same room. For the order of work and the other estimating tools, browse the painting & decorating hub. Estimate each material once, with a sensible allowance, and you will order close to right the first time — which is the whole point of doing the sum before you open a tin.

Common questions

How can I calculate how much paint I need?

Total the paintable area, multiply by the number of coats, and divide by the coverage on the tin. The wall area is the perimeter (twice the length plus twice the width) times the wall height; add the ceiling if you are painting it and subtract the doors and windows. A 12 × 12 ft room with 8 ft walls, one door and one window is about 348 ft² of wall, so two coats at 350 ft² per gallon needs roughly 2 gallons.

How much will 1 gallon of paint cover?

About 350 to 400 square feet in one coat on a smooth, previously painted wall — roughly the four walls of a small bedroom. Bare drywall, new plaster, render or a textured surface covers less because it drinks the first coat, and a strong colour change usually needs an extra coat. Always use the spread rate printed on your tin as the real number.

How much paint is for 1 sq ft?

At the common 350 ft² per gallon, one square foot uses about 1 ÷ 350 of a gallon per coat — roughly 0.0029 gallons, or about a third of a fluid ounce. It is a tiny amount, which is why paint is estimated across the whole wall and then rounded up to whole tins, never bought by the square foot.

How to calculate the amount of paint to use?

Use paint = paintable area × coats ÷ coverage. Measure the room, add the wall (and ceiling) area, deduct the openings, choose the number of coats — two is standard for an even finish and true colour — and divide by the coverage from the tin. Round the result up to whole gallons or litres, because you cannot buy a part-tin and the leftover is your touch-up supply.