Most paint calculators give you a litre figure you cannot buy. This one subtracts your doors and windows, allows for coats and primer, and tells you how many tins to actually put in the trolley.
Notes
Why coverage varies so much
A smooth sealed wall takes far less than bare plaster, new render or a porous masonry block. The tin quotes the best case; the first coat on new work can drink half again as much.
Do I subtract openings?
Below about 10% of the wall area it makes little difference and the extra is useful. Above that, on a room with patio doors, ignoring it means buying a tin you do not need.
Two coats is not optional
Any colour change needs two, and one coat over primer will look patchy in raking light however good the paint is.
What this does not do
It does not judge whether you need primer. Bare plaster, new timber, stains and strong colour changes do; sound painted walls in a similar shade usually do not.