If the radiator is the only heat source in a space, then it will need to supply enough heat to compensate for heat loss in the room. Heat losses are dependent on a number of factors, like insulation, new builds, etc.
An exact calculation of this is rather extensive and very technical.
It is also possible to work with a coefficient per m³. We do point out that this is an indicative calculation.
For a more exact calculation we strongly advise that you consult the person responsible for Central Heating.