The heat output of a radiator needs to be sufficient enough to heat your room, this measurement is called 'BTU' (British Thermal Unit). The calculator here is a great way to work out how much radiator power you need. There are several factors which can affect this, such as room dimensions, heating levels and types of window, which are all covered in our radiator BTU calculator. Double glazing, French Windows, North Facing wall will affect your BTU requirements.