BTU - 4600 to 5500 (depending on which online calculation tool you use)
While you're choosing your radiator, it might be worth checking how hot the existing radiator gets to aid you with your heat load calculations.
If memory serves, radiators heat output is calculated on T +50ºC, so if the room is at 20ºC the radiator will need to be at 70ºC to provide its full output.
I've got a cheap £20 infra red heat detector gun from Maplin, it looks very similar to this one:
http://www.maplin.co.uk/p/infrared-ther ... 7Qod7QQKoAIt is telling me that the radiator next to the desk is ticking over at 40ºC - Even with the heating on full bore, the radiators in this part of the house (and a long way from the boiler) don't get much over 55ºC, which means my heating calculations were a long way off when I specced the rads - Thankfully, I had a large fudge factor and specced for twice as much heat as the calculations suggested which still results in a toasty warm room, though it can take a considerable time to warm up, especially if we've been away for a few days in winter.
You might consider seeing how much heat you really need by turning off the radiator and using a fan heater for a few days, while it's not a precise number 3600BTU is close enough to 1kW to be usable for rough calculations. If a fan heater on the 1kW setting runs all the time, you'll need a bigger radiator, if the 2kW setting runs all the time, you're going to have to scale up again.
Regards,
B.
Incidentally, those thermometer guns are really useful in the kitchen as you can check oil temperatures for deep frying as well as the temperature of a cast iron pan when cooking steak, we've found that just getting it as hot as possible is not the best solution and you get better results come from a pan heated to about 300ºC