How do I calculate what size radiator I need?

Work out the room's heat loss in watts, then multiply by (50 / ΔT)^1.3, ΔT being the mean water temperature minus the room's. For 1,200 W with a boiler at 75/65 °C that is 1,232 W of radiator at ΔT50.

What size radiator do I need for a heat pump?

About 3.0 times the room's heat loss at 45/40 °C flow and return with the room at 21 °C: (50 / 21.5)^1.3 = 3.00. A room losing 1,200 W needs 3,595 W at ΔT50.

How much heat does a radiator give at a lower flow temperature?

Its ΔT50 rating × (ΔT / 50)^1.3. A 1,500 W radiator gives 501 W at 45/40 °C with the room at 21 °C, 33 % of its rating.

How many BTU is a watt?

1 W = 3.412 BTU per hour, so a 1,000 W radiator is 3,412 BTU/h.

What does ΔT50 mean on a radiator?

That the output is measured with the mean water temperature 50 K above the room, for example 75 °C flow, 65 °C return and a 20 °C room (EN 442). It is the figure radiators are sold by.

Building envelope · Heating

Radiator size calculator

A radiator rated at ΔT50 gives less with cooler water: output = rating × (ΔT / 50)^1.3. For a room that loses 1,200 W, a heat pump at 45/40 °C needs 3,595 W (12,265 BTU/h) of radiator at ΔT50; a boiler at 75/65 °C, 1,232 W. Radiators are sold by their ΔT50 output, so that is the figure to compare in a catalogue.

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Radiator to buy, ΔT50

— W

Mean water to room

— K

Existing radiators give

— W

Room heat loss

— BTU/h

How radiator output changes with water temperature

ΔT = (flow + return) / 2 − room output = ΔT50 rating × (ΔT / 50)^n n ≈ 1.3 (EN 442) rating needed = room heat loss × (50 / ΔT)^n

A heat pump at 45/40 °C with the room at 21 °C has a mean water temperature of 42.5 °C, so ΔT = 21.5 K, and (50 / 21.5)^1.3 = 3.00: the radiator has to be rated about 3.0 times the room’s heat loss. At 75/65 °C, ΔT = 49 K and the factor is 1.03.

Output of a radiator by flow and return temperature

Flow / return	ΔT (K), room 21 °C	Share of ΔT50 output	ΔT50 rating for 1,000 W
75 / 65 °C	49.0	0.97	1,027
70 / 60 °C	44.0	0.85	1,181
60 / 50 °C	34.0	0.61	1,651
55 / 45 °C	29.0	0.49	2,030
50 / 45 °C	26.5	0.44	2,283
45 / 40 °C	21.5	0.33	2,996
40 / 35 °C	16.5	0.24	4,226
35 / 30 °C	11.5	0.15	6,757

Arithmetic mean ΔT, as radiator makers’ tables use, and n = 1.3. With a very small temperature difference the logarithmic mean of EN 442 gives a little less.

What this page does not do

It does not work out the heat loss. That comes from the walls, windows, floor, roof and air change of each room: the heat loss calculator does it, and the average heat loss of a house page gives the heat pump size for the whole house.
It does not design the system. Pipe sizes, flow rates and the heat pump’s own curve are for the installer; a heat pump survey (MCS in the UK) sizes them room by room.

Questions people ask

How do I calculate what size radiator I need?: Work out the room's heat loss in watts, then multiply by (50 / ΔT)^1.3, ΔT being the mean water temperature minus the room's. For 1,200 W with a boiler at 75/65 °C that is 1,232 W of radiator at ΔT50.
What size radiator do I need for a heat pump?: About 3.0 times the room's heat loss at 45/40 °C flow and return with the room at 21 °C: (50 / 21.5)^1.3 = 3.00. A room losing 1,200 W needs 3,595 W at ΔT50.
How much heat does a radiator give at a lower flow temperature?: Its ΔT50 rating × (ΔT / 50)^1.3. A 1,500 W radiator gives 501 W at 45/40 °C with the room at 21 °C, 33 % of its rating.
How many BTU is a watt?: 1 W = 3.412 BTU per hour, so a 1,000 W radiator is 3,412 BTU/h.
What does ΔT50 mean on a radiator?: That the output is measured with the mean water temperature 50 K above the room, for example 75 °C flow, 65 °C return and a 20 °C room (EN 442). It is the figure radiators are sold by.