Decorating calculators
Paint, wallpaper and the preparation that goes with them.
Paint calculator
Walls and ceiling for one room: litres, whole tins, what is left over and a printable list.
What these have in common
Decorating materials are bought by the tin or the roll and spread over an area, so all of them come down to the same question: how much area, how far does the material go, and what is the smallest amount you can buy. Each calculator shows that arithmetic rather than hiding it.
The coverage figure always comes from you, off the tin or the roll, because it varies more between products than any other number in the calculation.