British Thermal Unit (BTU) measures the output of heat from a radiator. The higher the heating requirement, the more heating power you’ll need. BTU is impacted by what the room is used for, the amount of windows, type of windows and room dimension. Our calculator considers these factors to figure out how much power you will need from a radiator to heat the room. If you have a large room, you might benefit from more than one radiator for sufficient heating.