Two gallons and a quart
Four 12 ft walls at 8 ft high are 384 sq ft. Take off a door (21) and two windows (30) and you paint 333 sq ft. Two coats at 350 sq ft per gallon is 1.9 gallons; add a tenth for cutting in and you need 2.1.
Two gallons leaves you short on the last wall, three wastes most of a can. Two gallons and a quart is the buy.
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.