For an emulsion paint (used on interior walls and ceilings), 1 litre of paint will cover approximately 12 square metres per coat. However, it depends on the type of paint being used, the condition of the surface and how it has been prepared.
For a trim paint (undercoat and gloss for doors, windows etc.), 1 litre of paint will cover approximately 16 square metres per coat. Again, it depends on the type of paint being used, the condition of the surface and how it has been prepared.
Paints for exterior wooden surfaces, e.g. sheds, fences, decking etc., will have lower coverage and 1 litre will cover between 3 to 6 square metres per coat depending on type of paint and condition of the wood.
These are general guidelines and therefore when looking to find out how much paint to buy, it is important that you think about these factors as well as how many coats you will be applying.
Decorative paint manufacturers often provide their own guidance for how much of each type of paint is required. See the following member websites for examples: