Calculating the heat output of a central heating radiator

Have a look on one of the radiator suppliers' websites? They list output against the various sizes.

Measure it, and look up the specs of a s similar sized one. ;-)

Indeed, but make sure that the one you look up is the same type of construction - of which there are many[1]. Also be aware that the heat output depends not only on dimensions and construction but also on delta-T - the difference between room temperature and the mean temperature (average of flow and return) of the radiator. If you run a radiator "cool" in order to maximise the efficiency of a condensing boiler, it will output a lot less heat that it would if run "hot".

Indeed. I was pleasantly surprised having looked at a number recently (just about to set about the heating and water system in a big way), that most of the sites are quoting Delta Ts of 50 degrees now, and assuming condensing boiler friendly flow rates.

Some makers are even doing triples now... looked a tad fugly though and not in that old school "old school" rad kind of way ;-).