How to accurately calculate BTU requirements?

[thelegohooverer]

I’m trying to work out how many/ type radiators I need and I’ve tried putting the room dimensions into 5 different online calculators with widely differing results. Can anyone recommend one that is accurate?

www.bestheating.com/btu-calculator?gclid=Cj0KCQiA0rSABhDlARIsAJtjfCdioMEtAhiTUnYNh8mG15BCc39j7sVj2LuUsiZ9c3bi7VEqmXxLq10aAj9LEALw_wcB&gclsrc=aw.ds

This one has never let me down.

Thank you