There are loads of paint calculators on the web to help you work out how much paint you need, but before you use them bear the following in mind:

• each paint has a different coverage, so check that the coverage on the tin matches the coverage on the calculator;
• if you’re changing from a dark colour to a light colour, you will need more coats than if you’re just repainting with the same colour.  Moving from black to white would take a minimum of three coats, maybe more. Again, you’ll need to check the calculator to see how many coats it’s assuming;
• paint coverage can also be affected by the type of surface, whether or not it’s interior or exterior paint, and how well the surface has been prepared.

General guidelines are:

• for an emulsion paint (used on interior walls and ceilings), 1 litre of paint will cover approximately 12 square metres per coat;
• for a trim paint (undercoat and gloss for doors, windows etc.), 1 litre of paint will cover approximately 16 square metres per coat.
• 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.

To be on the safe side, you should check out your paint manufacturer’s website as they usually have calculators based on their own paint types.

And finally, remember to check the price of the cans: it’s often significantly cheaper to buy a large can, even if you think it’s going to be too much, than to buy the exact amount in smaller cans.