Radiator Sizing Calculator — BTU & Watt Calculator for Any Room

A common UK rule-of-thumb works in Watts per square metre of floor area: roughly 80 W/m² for a new build, 100 W/m² for an average double-glazed home, and 130 W/m² for an older, poorly-insulated property. Converted to BTU (1 W = 3.412 BTU), that is about 270 BTU/m² for a new build, 340 BTU/m² average, and 440 BTU/m² for poor insulation. With a standard 2.4 m ceiling these equate to roughly 33, 42 and 54 Watts per cubic metre respectively. Our calculator gives a more accurate figure by factoring in insulation, glazing, exposure and ceiling height. You can also use our Radiator BTU Calculator for a detailed room-by-room heat loss breakdown.

BTU (British Thermal Units) and Watts both measure heat output. 1 Watt = 3.412 BTU. UK radiator manufacturers typically list both values. BTU is the traditional UK measurement, while Watts (W or kW) aligns with European standards. When comparing radiators, ensure you are using the same unit.

Modern UK practice uses Delta T 50 (DT50) ratings, which assume a flow temperature of 75 °C, return of 65 °C, and room temperature of 20 °C. Older catalogues may use Delta T 60 which gives higher output figures. If your radiator is rated at DT60, you will need a larger radiator to achieve the same output at DT50. Heat pumps use even lower flow temperatures (DT30), requiring significantly larger radiators — see our Heat Pump Sizing Calculator for more guidance.

Yes. Bathrooms typically need 15-20% more heat output than other rooms because the desired temperature is higher (22°C vs 21°C for living rooms and 18°C for bedrooms) and there is additional heat loss from ventilation. Our calculator applies a bathroom uplift automatically.

Both approaches work. Two smaller radiators distribute heat more evenly across the room and are useful when wall space is limited. A single large radiator is simpler to install with less pipework. For rooms over 20m², two radiators on opposite walls often provide better comfort.