Paint Calculator — Gallons per Room
Enter the room's perimeter and height, subtract the doors and windows, and get the number of gallons — including how much primer you need if the wall is bare.
About the Paint Calculator
Paint coverage is quoted at 350 square feet per gallon, which is accurate on smooth, previously painted, similarly coloured walls — and optimistic on anything else. This calculator uses the honest figure and subtracts your openings, because a room with four windows genuinely needs less paint than one with none.
How to use it
- Measure the perimeter — add up the length of all four walls.
- Enter the ceiling height, usually 8, 9 or 10 feet.
- Enter how many doors and standard windows the room has.
- Two coats is the default and almost always the right answer.
The formula
Wall area = perimeter × height
Paintable = wall area − (doors × 21) − (windows × 15)
Gallons = paintable × coats ÷ 350
A standard door is about 21 sq ft and an average window about 15. Coverage of 350 sq ft per gallon is the manufacturer figure for a smooth surface; textured walls can drop to 250.
Worked examples
| Room | Perimeter | Height | Openings | Paintable | Gallons (2 coats) |
|---|---|---|---|---|---|
| 12 × 12 bedroom | 48 ft | 8 ft | 1 door, 2 windows | 333 sq ft | 2 |
| 10 × 12 office | 44 ft | 8 ft | 1 door, 2 windows | 301 sq ft | 2 |
| 16 × 20 living room | 72 ft | 9 ft | 1 door, 2 windows | 597 sq ft | 4 |
| 5 × 8 bathroom | 26 ft | 8 ft | 1 door, 1 window | 172 sq ft | 1 |
Notice the first two rooms both land on 2 gallons despite different sizes. Paint is sold in whole gallons, so a wide range of rooms round to the same purchase — which is exactly why calculating in square feet alone misleads.
Common mistakes
- Measuring floor area instead of wall area. A 12 × 12 room has 144 sq ft of floor and 333 sq ft of wall. Paint goes on the walls.
- Forgetting the second coat. One coat over a colour change almost never covers. Buying for one coat and returning for more risks a different batch and a visible line.
- Ignoring the ceiling. Ceilings need their own paint and their own gallon count. This calculates walls only.
- Skipping primer over bare or patched drywall. New drywall drinks paint. Priming first is cheaper than a third coat of finish.
Terms explained
- Perimeter
- The total distance around the room. For a rectangle: two times length plus two times width.
- Spread rate
- Square feet one gallon covers in a single coat. 350 is standard, less on rough or porous surfaces.
- Cut in
- Painting edges and corners by brush where a roller cannot reach. Uses more paint than people expect.
- Sheen
- Flat hides imperfections, gloss survives cleaning. Higher sheen shows every wall flaw, so smooth walls first.
Common questions
- How many gallons of paint for a 12x12 room?
- Two gallons for two coats, allowing for one door and two windows. That covers about 333 square feet of wall.
- How much does a gallon of paint cover?
- About 350 square feet in one coat on a smooth, previously painted wall. Textured or porous surfaces can drop to 250.
- Do I need primer?
- Over bare drywall, patches, stains, or a dramatic colour change — yes. Over a similar existing colour in good condition, usually not.
- How many coats of paint do I need?
- Two is standard. One works only when refreshing the same colour on a sound surface. Dark-to-light changes sometimes need three.
- Should I subtract doors and windows?
- Yes, on anything but a very small room. Four windows and a door is over 80 square feet — a quarter of a gallon per coat.
- How do I measure the perimeter?
- Add the lengths of all the walls. For a rectangular room that is (length + width) × 2.
- Does ceiling height matter that much?
- Considerably. Going from an 8 ft to a 10 ft ceiling adds 25% to the wall area and can push you into an extra gallon.