Two coats, plus a little
Measure round the room, multiply by the height, take off about 21 sq ft (1.9 m²) for each door and 15 sq ft (1.5 m²) for each window. That's your wall area. A gallon covers about 350 sq ft per coat; a litre about 12 m².
Most rooms need two coats, so double it, and add a tenth for cutting in round edges. Coat does that sum and then does the part the other calculators skip: it picks the mix of tins that leaves the least paint in the shed.
Questions
How much paint do I need for a room?
Wall area (perimeter × height, minus doors and windows) × coats ÷ coverage. A gallon covers about 350 sq ft per coat, a litre about 12 m². A 12 × 12 ft room with 8 ft ceilings needs about 2 gallons for two coats.
How much does a gallon of paint cover?
About 350 to 400 sq ft per coat on smooth, painted walls. Textured walls, bare plaster and cheap paint cover less, often 250 sq ft.
How many square metres does a litre of paint cover?
About 10 to 14 m² per coat for most emulsions. The tin says the exact figure; type it into Coat under coverage.
Do I need two coats of paint?
Usually yes. One coat is enough only when you repaint the same colour and sheen. New plaster needs a mist coat plus two, and big colour changes often need three.
Should I buy extra paint?
Add about 10% for cutting in and spills, and keep a little for touch-ups. Coat adds the 10% and shows what you'll have left in the last tin.
How much paint for a ceiling?
Length × width, two coats. A 12 × 12 ft ceiling is 144 sq ft: under a gallon of ceiling white.