How it is worked out
Wall area is the distance around the room (two lengths plus two widths) × the ceiling height, less doors and windows. Ceiling area is length × width.
Litres are area × coats ÷ coverage. The page opens at 16 m² per litre, the figure Dulux gives for its wall, trim and ceiling paints. Use the number on your own tin.
Round up to the tin sizes your shop sells. Having some left over for touch-ups beats trying to match a colour later.
Worked example
The numbers this page opens with: room length 4 m, room width 3.5 m, ceiling height 2.4 m, doors and windows to take out 3.5 m², coats 2 coats, coverage on the tin 16 m² per litre, Paint the ceiling too: yes.
The answer: wall area after openings 32.5 m²; wall paint, 2 coats 4.1 litres; ceiling area 14.0 m²; ceiling paint, 2 coats 1.8 litres.
Questions
Do I need an undercoat or primer as well?
That depends on the surface and the paint, and it is separate from this figure. If you do, run the calculator again with one coat and the coverage printed on the primer tin.
Why is my wall paint figure different from the shop's calculator?
Usually the coverage figure or the openings. Check that both calculators are using the same coverage and the same number of coats.