Paint is always bought in whole cans, so the useful question is not just "how much area" but "how many gallons or litres, in what can sizes". This calculator measures your room, subtracts the doors and windows you will not paint, multiplies by the number of coats, and divides by the coverage of your paint.
The formula
Walls are a rectangle wrapped around the room: wall area = 2 × (length + width) × ceiling height. Subtract about 21 ft² (1.95 m²) per standard door and 15 ft² (1.4 m²) per average window, add the ceiling if you are painting it, then:
Coverage: the number that moves the answer
Most wall paints cover 300–400 ft² per gallon (roughly 8–12 m² per litre) on smooth, previously painted walls. Bare drywall, brick, and strong colour changes drink more — pick the lower coverage, and plan a primer coat. Two finish coats over primer is the combination that actually looks finished in daylight; one coat almost always shows the old colour at the edges.
Do I need primer?
Primer earns its cost on new drywall (it seals the paper so the finish coat goes on evenly), over stains, over glossy paint, and on dramatic colour changes. On clean, previously painted walls in a similar colour, most modern paint-and-primer products make a separate primer coat unnecessary.
Worked example
A 12 ft × 10 ft room with 8 ft ceilings: gross wall area = 2 × (12 + 10) × 8 = 352 ft². One door (21 ft²) and two windows (30 ft²) come off, leaving 301 ft². Two coats = 602 ft², and at 350 ft² per gallon that is 1.72 gallons — buy 1 gallon + 3 quarts (or 2 gallons and keep the spare for touch-ups).
How to use this tool
- Choose units, then measure the room's length, width and ceiling height.
- Pick the coverage that matches your walls — when in doubt, choose the lower number.
- Enter the doors and windows you will not paint, and the number of coats.
- Tick the ceiling and primer options if they apply.
- Buy using the rounded can list; keep the exact figure for comparing prices.
Small rooms, big colours
Deep reds, blues and greens often need an extra coat even over primer — budget three coats for saturated colours. And buy all the paint for a room in one trip: if you must go back, bring the can so the store can match the same tint batch, and stir the new can together with the leftover (“boxing”) for an exact match.