It calculates the BTU required to heat the room, you then pick a radiator to match. There is more to it than the above system takes into account, such as construction/insulation method.
Your plumber / local heat merchants / DPL will be able to tell you as part of their quote for the material.
You will need to tell them detail like the room measurements, the number of windows, level of insulation and direction of room (north facing / south facing)