Radiator Size and BTU Calculator

How the radiator size is worked out

Radiators are tested to BS EN 442 with water going in at 75 °C and coming out at 65 °C, in a room at 20 °C. The average water temperature is 70 °C, 50 degrees above the room, and the output measured at that point is the "ΔT50" figure in every catalogue. Output falls faster than the temperature difference does, so a radiator at half the ΔT gives well under half the heat.

The correction factor

n is the radiator exponent from the manufacturer's test. Panel radiators sit between about 1.29 and 1.36; 1.3 is the standard assumption when the model is unknown (BRE's low-temperature design tool uses it), and Stelrad and Purmo state that their conversion tables use it too. Our factors match Stelrad's published factor table to within 0.7% and Myson's to within 1.5% (both read September 2026). BTU/h is the output in watts multiplied by 3.412.

Estimating from the room's size

If you do not know the room's heat loss, the estimate panel works it out from the room's size, how many of its walls face outside, what is above and below it, the loft insulation, and the age and construction of the house. It uses the same method and U-values as the heat loss calculator, with RdSAP's default window area for the age of house scaled to the room, and a 2.4 m ceiling (2.7 m for a house built before 1930) unless you enter one. Tested on all eight rooms of a 1930s semi calculated in full, with the outside wall length and loft insulation entered, it lands within 10%, and within 2% for the six rooms without an outside door. It does not ask about outside doors or a wall to a garage, so rooms with them read low. If you leave the outside wall length blank it assumes the longer walls face outside, which reads on the high side. If you leave the loft as usual for its age, the estimate takes RdSAP's default and reads high if yours has been topped up, low if it has less. For the worked example's living room, with its 3.6 m outside wall entered, it gives 1,469 W against 1,379 W calculated wall by wall, because it assumes a larger window and a more exposed floor; left blank, it takes the 4.2 m wall and gives 1,541 W.

Example: the terrace living room on a heat pump

The living room from the heat loss calculator example loses 1,379 W at 21 °C. With a heat pump running 45 °C flow and 40 °C return:

• Mean water temperature = (45 + 40) ÷ 2 = 42.5 °C
• ΔT = 42.5 - 21 = 21.5 K
• Correction factor = (21.5 ÷ 50)^1.3 = 0.3338
• Output to buy = 1,379 ÷ 0.3338 = 4,131 W at ΔT50, or 14,100 BTU/h

That is three times the room's heat loss, for example a 1.8 m triple convector at 600 mm high or two 1.2 m double convectors. On a condensing boiler at 70/50 the same room needs 1,961 W. The 600 by 1,000 mm double convector a lot of these rooms already have (1,732 W at ΔT50) gives 578 W at 45/40, 42% of what is needed; it would need about 65 °C flow to heat the room on its own.

At 45/40 each watt of heat loss needs three watts of catalogue rating, so each watt saved by insulation takes three watts off the radiator you need. The heat pump size calculator takes the same room totals and sizes the unit.