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.
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.
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.
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°
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.
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