Paint calculator
Paint covers walls,
not floor.
So floor area is the wrong input. This works from the perimeter of the space you measured, times your ceiling height, minus the doors and windows — and it tells you when two quarts beat a gallon.
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Three surfaces, one measurement
Measure the room once in the ledger above; each surface reads what it needs from it.
Interior walls
Perimeter × wall height, minus openings. A 3'0" × 6'8" door is 20 ft² and a 3' × 5' window is 15 ft² — both appear as a visible line in the result so you can check them rather than trust them. Ceilings are optional and add your floor area.
Exterior
Footprint perimeter × wall height, plus gable ends worked out from the building width and the roof pitch. Openings are not deducted by default: cutting in around every window uses more paint than the opening saves, which is how painters bill it.
Trim & doors
Baseboard follows your perimeter minus the doorways. Casing follows the door and window counts you already entered. You get a paint quantity and a linear-foot buy list, because trim is bought by the foot and painted by the square foot.
Coverage is a label claim, not a fact
The single biggest error in paint estimating is treating 350 ft² per gallon as a constant. It is a mid-range figure for a previously-painted wall, and the substrate moves it enormously.
Take a 40 × 30 house, nine-foot walls, two gable ends, two coats. On previously-painted siding at 350 ft² per gallon that is about 8.5 gallons. The same house in stucco at 200 ft² per gallon is 14.85 gallons — three-quarters again as much paint, for identical dimensions.
That is why every substrate here seeds an editable coverage field rather than hiding a constant. Pick the closest surface, then overtype it with whatever the can in your hand actually claims.
Primer, sheen and the second coat
Primer is calculated separately because it covers differently — a porous surface drinks the first coat, which is why primer coverage on new drywall runs lower than finish coverage on the same wall.
Two coats is the honest default for most colours. One coat genuinely works when you are going over the same colour in the same sheen, and genuinely does not when you are covering a dark colour, a strong red or yellow, or any bare substrate.
If you are buying two or more gallons of a custom colour, ask the desk to box them — intermix the cans into one container and redistribute. Tint dispensing has a tolerance, and boxing removes the risk of a visible shift halfway along a wall.
Common questions
How much paint do I need for a room?
Wall area is the room's perimeter times the ceiling height, minus the doors and windows. Divide by the coverage on the can and multiply by the number of coats. A typical gallon covers about 350 ft² per coat on a previously-painted wall, and most colours need two coats.
Does the type of wall change how much paint I need?
Substantially. Previously-painted drywall runs about 350 ft² per gallon; stucco and unpainted brick can be 200 or less. On a whole exterior that difference can be several gallons, so check the substrate before you buy.
How much paint do I need for exterior walls with gables?
Footprint perimeter times wall height gives the rectangular walls. Each gable end is a triangle: the building width squared, times the pitch rise over twelve, divided by four. A 30 ft wide gable at 6/12 pitch adds about 112.5 ft² per end.
How do I calculate paint for baseboards and trim?
Multiply the linear feet of trim by its paintable width in inches, then divide by twelve to get square feet. Paintable width is the developed face of the profile — usually half an inch to an inch more than the nominal height, because of the top edge and the moulding detail.
Should I buy a gallon or quarts?
Whichever covers the requirement with least waste. Three quarts almost always costs more than one gallon of the same product, but a single quart is far cheaper than rounding a 3.2 gallon requirement up to four.
The rest of the toolbox
One measurement, then whichever material you are buying. Every tool reads the same ledger.