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).
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.