Paint quantity
Wall area = 2 × (length + width) × height, minus doors and windows. Multiply by coats. Litres = area ÷ coverage. Buy a little extra for touch-ups. Ceilings are not included unless you add them as extra area in the doors box as a negative number (or raise length/width).
Worked example
A 4 m × 3 m room, 2.4 m ceiling, four walls, no subtraction for windows: about 33.6 m². At 10 m²/L and two coats you need about 6.7 L, so two 3.7 L cans with a little waste, or a 3.7 L plus a 1 L depending on the brand.
Textured walls, dark colours, and new drywall (primer) use more. Ceilings and trim are extra. Buy the same batch if the colour is mixed in-store.
Common questions
Should I paint the ceiling too?
Add length × width to the job. Put 0 in “subtract” extra, or add that area into coats on a second run.
Dark colours?
They often need a primer plus extra coats. Bump coats to 3.