This wall painting calculator will help you to work out the minimum amount of paint required for your project, based on a coverage of 10m² per litre of paint.
Enter the dimensions of your walls and then decide the dimensions of the areas you don't want to paint. These could include windows, sockets or skirting boards, for example. List the amount of doors in your room and then decide how many coats of paint you want.
This is based on a coverage of 10m² per litre of paint. Always check the coverage on the side of the tin before buying.