BTU Calculator
Calculate the heat loss and recommended radiator size for any room. Enter dimensions, window type, insulation level, and room type to get an accurate BTU requirement.
Frequently asked questions
What is a BTU and how is it used for radiators?
BTU stands for British Thermal Unit — the amount of energy needed to raise 1 pound of water by 1°F. For heating, BTU/hr measures a radiator's heat output. A typical UK living room needs 5,000–10,000 BTU/hr depending on size, insulation, and windows.
How do I calculate the radiator size I need?
Measure your room dimensions (length × width × height), count external walls and windows, and assess insulation quality. Multiply the room volume by a base factor (~153 BTU per m³) and adjust for heat loss factors. This calculator does all of that automatically.
Should I oversize or undersize my radiators?
It's better to slightly oversize radiators (10–15%) to ensure the room heats quickly and the boiler cycles efficiently. Undersized radiators will struggle to reach the desired temperature, running constantly and increasing energy bills.
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