BTUCalculator - UK To determine an accurate number of BTUs for your radiator. Our BTU Calculator is the best BTU calculator which is leading on the net. Also, we have Delta T50-T60 Conversion & Watts - BTU Conversion Tool. Your radiator should have an accurate number of BTUs to efficiently and effectively heat your room. BTU stands for 'British Thermal Units' , British standard unit of energy. The unit in which radiator efficiency is measured. 1 British Thermal Unit is similar to the amount of heat needed to increase the temperature of one pound of liquid water by 1 degree Fahrenheit at its peak density, Which Comes at a temperature of 39.1 degrees Fahrenheit. Factor such as the type of room, window size, room usage criteria, room dimensions, the presence of windows, other heat draining openings, the insulation scope, and wall material are needed to calculate the outputs required by radiators in BTUs or Watts our Watts to BTU Calculator or BTU to Watts Conversion