Work out radiator btu instantly with clear inputs, formula shown and shareable results.
Radiator sizing uses watts per cubic metre by room type: about 45 for a living room, 40 for a bedroom, 35 for a kitchen where appliances add heat, and 55 for a bathroom where higher temperatures are wanted. Each large window adds roughly 300 W of loss.
Radiator output
watts = volume x room type factor + 300 W per large window; BTU/h = watts x 3.412
Cooking, the fridge and other appliances contribute significant heat, so the radiator only has to make up the difference.
Only at the same delta T. Outputs are usually quoted at delta T 50, and a modern condensing boiler running at lower flow temperatures delivers considerably less.