A quick and easy way to calculate the size of the radiator required for any room in your house is by measuring the room in cubic feet and then applying the following factors to the figure arrived at:

This will give you the output of any radiator in BTUs (British Thermal Units). Adding the total for all the rooms in your house or flat  will give you the demand in BTUs for the whole house. Add 20% to the total for a hot water circulating tank and 10% for general losses. This will give you the boiler size you need for your house.

For a precise calculation click here