This is calculated by the BTU (British Thermal Units) needed to heat the interior space. Factors such as the room size, ceiling height, double glazing and position of the radiators can affect what's needed. Please contact us with your room size and we can advise which sizes and styles of radiator would work for the spaceThe required BTUs can be calculated by the following formula using dimensions from the room: (L X W X H) X 5 (formula uses inches).More information can be found in our Radiator Buying Guide