SITE TOOLS // RADIATOR SIZING ESTIMATOR
Radiator BTU Calculator
Work out roughly what size radiator a room needs – enter your measurements below.
ABOUT THIS CALCULATOR
Extensions and loft conversions almost always need heating specified before the plasterer arrives, and a new room typically has more external wall than the rooms it joins onto – which changes the heat loss significantly.
This calculator estimates the output a room needs in BTU per hour and Watts, factoring in room type, insulation quality and how many walls face outside, with a 15% safety margin applied. It is a sizing guide for early conversations with a client or heating engineer, not a substitute for a full heat loss survey on the finished design.
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Recommended Output
0 BTU/h
0 Watts, including a 15% safety margin
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This is a simplified estimate for guidance only, based on typical UK room heat loss rates. Glazing type, floor level, radiator position and exact insulation values all affect the real figure – for anything beyond a rough guide, get a proper heat loss calculation from a qualified installer.