Radiator Size Calculator
Works out the radiator size to specify for a room. Catalogue outputs assume ΔT50; this corrects for the flow temperature you will actually run.
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The formula
ΔT = (flow + return) ÷ 2 − room correction = (ΔT ÷ 50)^1.3 rated output = heat required ÷ correction
Radiators are catalogued at ΔT50 — mean water temperature 50 K above the room — so a radiator run cooler than that gives less than its headline figure. The 1.3 exponent is from EN 442 and reflects that a radiator moves heat mostly by convection, which does not scale linearly with temperature difference.
What to watch out for
The correction is the entire point of this page.
- Catalogue figures are at ΔT50. That means 75 °C flow, 65 °C return, 20 °C room. Almost nobody runs those temperatures any more.
- Lower flow temperatures need bigger radiators. At ΔT50 a radiator gives 100%. At ΔT30 it gives about 51%, so you need nearly twice the rated output for the same heat.
- This is why heat pump retrofits change the radiators. A heat pump is efficient because it runs cool, and running cool is exactly what halves a radiator's output. The emitters have to grow to compensate.
Frequently Asked Questions
What size radiator do I need for a room?
Work out the room’s heat loss first, then divide by the correction factor for your flow temperature. A room needing 1,500 W at a ΔT of 40 K needs a radiator rated about 1,960 W at ΔT50.
What is ΔT50 on a radiator?
The standard test condition radiators are rated at: the mean water temperature is 50 K above the room. With a 20 °C room that is 75 °C flow and 65 °C return. Every catalogue output you see assumes it unless stated otherwise.
Do I need bigger radiators for a heat pump?
Usually yes. A heat pump runs at 35–50 °C rather than 70–75 °C, which puts you around ΔT25–ΔT30 instead of ΔT50 — roughly 40–50% of the catalogue output. Radiators typically need to be about twice the size, or replaced with fan-assisted or underfloor emitters.