Paint Calculator — How Much Paint You Need
Estimate how many gallons or litres of paint a room needs from wall size, doors, windows and coats, metric or imperial.
Perimeter is every wall length added together. For a rectangular room that is 2 × (length + width): a 12 × 10 ft room has a 44 ft perimeter.
Blank takes off the usual size shown; a patio door or a bay window is bigger.
A drastic colour change usually needs a second coat; the same colour over sound paint often needs only one. Either button just fills in Coats.
A smaller tin is used only while those tins come to no more than half the next size up — past that, the bigger tin is the better buy.
🔒 Calculated in your browser. Coverage is an editable default — real coverage varies with surface, colour and primer. Round up when buying.
How much paint to buy
Enter your wall perimeter and height, the number of doors and windows to subtract, and the number of coats. The tool works out the paintable area and how much paint that needs at the coverage rate (default ~350 sq ft per gallon or ~10 m² per litre), then rounds up to whole tins.
Coverage is an editable default
Real coverage varies with the surface, colour change and primer, so the coverage is an editable default, not a fixed truth. The “Surface” select allows for the common cases by lowering it — new or bare, rough or porous, or exterior walls, each by the share printed in the option — and the buttons under “Changing the colour?” fill in Coats for you, fewer for the same colour than for a new one. Doors and windows come off at 21 ft² and 15 ft² each, or 1.9 m² and 1.4 m² in metric, unless you type their real size in “Door size, each” and “Window size, each”; the size in use is printed on their labels. When in doubt, buy a little extra for touch-ups. Painting several rooms? “Add this room to the total” keeps a list and adds the paint up, rounding the tins once for the whole job. Everything is calculated in your browser.
Coverage is the number that decides everything
The figure on the tin is measured on a smooth, sealed, previously painted wall. Bare plaster, new plasterboard, render, textured paper and anything porous will drink noticeably more — a first coat on new plaster can take 30% more than the tin claims. Colour matters too: a strong pigment over a pale base covers in two coats, a pale one over a strong base can need three or four. The coverage field is editable because your surface, not the tin, sets the real number, and the tin is where to start rather than where to finish.
Openings, coats and the tin you should buy anyway
Subtracting doors and windows is right for large openings and pointless for small ones — the paint you save on a window is roughly the paint you lose cutting in around it. Two coats is the normal answer; one coat only works when you are repainting the same colour on a sound surface. Priming a bare or heavily patched wall costs a coat but saves more than a coat of finish. Whatever the arithmetic says, buy in whole tins with a margin: touching up later with a second batch of a mixed colour rarely matches, and paint keeps for years.
Frequently asked questions
How much does a gallon of paint cover?
Roughly 350 square feet (about 32 m²) for one coat on a smooth, primed wall. Rough or unprimed surfaces need more, so adjust the coverage field.
How many coats should I plan for?
Two, in almost every case. One is enough only when you are going over the same colour on a sound, sealed surface. Going pale over dark, or covering a patched wall, can take three — prime it instead and you will usually get away with two. The tool multiplies by whatever you enter.
Why did I use more paint than the tool said?
Almost always the coverage figure. Tin coverage assumes a smooth, sealed surface; bare plaster, new board and textured finishes absorb far more. Adjust the coverage field down and recalculate.
Should I subtract windows and doors?
Subtract large openings — patio doors, big windows. Leave small ones in: the paint you save is about the paint you spend cutting in around the frame.
Can I measure the room instead of the wall perimeter, and see how many tins to buy?
Yes. Measure the walls by the room's length and width if that is what you have, and the page works out how many tins to buy — gallons and quarts, or in metric a mix of the tin sizes you list.
Can it include the ceiling and a primer coat, and tell me what the paint costs?
Yes. Tick “Paint the ceiling too” and the ceiling is added — the room's length × width when you measure by the room, or a “Ceiling area” you type when you measure by the perimeter — with no doors or windows taken off it. “Prime first” adds one coat of primer at its own editable coverage and shows “Primer needed” beside the paint. An optional paint price gives “Paint cost, for the tins above”: the tins the page tells you to buy, not the exact amount.
Can it allow for a new or rough surface, or add up several rooms?
Yes. “Surface” lowers the coverage for a new or bare surface, a rough or porous one, or exterior walls, by the percentage shown in each option, so the tins and the price follow. And “Add this room to the total” keeps a list of rooms, each with its own Remove button, and totals the paint, rounding the tins once for the whole job rather than room by room.