The folks at http://www.BestHeating.com have created a simple heat output (BTU) calculator. The BTU Radiator Heat Output Calculator adds up those “British Thermal Units” — the unit in which radiator efficiency is measured. Factors such as room dimensions, window size and wall material are used to calculate the required outputs of radiators, to adequately heat a room.
The best way to get an accurate calculation is to enlist the services of a plumber. However, Best Heating’s BTU Calculator is an accurate guide as to what level of BTU and Watt output will efficiently heat a room. Note: the calculated figure is for the combined heat output of all radiators and/or towel rails in the room (larger rooms may require more than one).
Click Here to access the calculator.