Radiator Size Calculator (BTU & kW)
Calculate the exact BTU and kW output needed to heat any room. Accounts for insulation, windows, external walls, and room use.
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Radiator Sizing Guide
How BTU and kW Relate?
BTU (British Thermal Unit) is the traditional unit for heating output; kW is the metric equivalent. 1 kW = 3,412 BTU/hr. Radiator catalogues often list both. A typical 60×60cm single-panel radiator (Type 11) outputs approximately 700–900 BTU/hr. A double-panel with convector fins (Type 22) of the same size outputs approximately 1,500–2,000 BTU/hr. The Type designation (11, 21, 22, 33) indicates panel count and convector fin rows — Type 22 is the most common for living rooms.
What's the key thing to understand about Rule of Thumb Calculation?
A simple starting point: multiply room volume (m³) by a factor depending on insulation — well-insulated: 25 BTU/m³; average: 35 BTU/m³; poor: 45 BTU/m³. Add 10% for each external wall, 20% for north-facing rooms in colder climates, 15% for bathrooms (higher target temperature). The result is total BTU needed — split between multiple radiators if required. Online BS EN 442 heat loss calculators (used by heating engineers) are more accurate for large installations.
What should I know about Heat Pumps Require Larger Radiators?
Air source heat pumps operate at lower flow temperatures (40–55°C) than gas boilers (65–80°C). At lower temperatures, the same radiator outputs less heat — approximately 30–50% less at 45°C vs 70°C flow temperature. This means a house switching from gas to a heat pump typically needs significantly larger radiators (or underfloor heating, which is ideal for heat pumps as it operates at 30–40°C). When specifying radiators for a heat pump system, size for 45°C flow temperature, not the standard 70°
What do I need to know about Common Radiator Sizes?
Standard heights: 300mm, 450mm, 600mm, 900mm. Standard lengths: from 400mm to 2000mm+. A 600×1200mm Type 22 is one of the most common sizes — outputs approximately 3,200–4,500 BTU depending on manufacturer. For a 15–20m² living room with average insulation, you typically need 3,500–5,000 BTU, which a single 600×1200 Type 22 can cover. Bathrooms often use towel radiators which output less heat for their size — always check the BTU rating, not just physical dimensions.