Radiator calculator

Find out how much heat a room needs and how many sections or what radiator output to buy.

LHW
Letters on the drawing match the letters next to the inputs.
m
m
m
Heat needed2,160 W

Shopping list

  • Radiator output from catalogue (ΔT 50)2,160 W

How we got there

Room volume
54 m³
Temperature correction
1 ×

Results are estimates. Always check the consumption printed on the product you buy.

How it is calculated

Heat demand comes from the room volume: 30 W per m³ for a well insulated house, 40 for an average one and 50 for a poorly insulated one. A corner room gets 20 % more. A radiator’s rated output is measured at a set temperature difference, so it is converted to your system using the EN 442 formula.

FormulaHeat, W = length × width × height × (30, 40 or 50 W/m³)Correction = (system ΔT ÷ rated ΔT)^1.3Sections = heat ÷ correction ÷ output per section

Example

A 5 × 4 m room with a 2.7 m ceiling in an average house needs 2,160 W. With a 75/65 °C boiler a steel panel radiator rated at ΔT 50 works at full output, so pick one with at least 2,160 W in the catalogue.

Tips from practice

  • Place the radiator under the window and at least 70 % as wide: the warm air stops cold draughts from the glass.
  • Connect long radiators diagonally: flow at the top on one side, return at the bottom on the other.
  • With a heat pump, radiators need 2–3 times the catalogue output you would need with a boiler.

Questions and answers

What size radiator for a 10 m² room?

With a 2.7 m ceiling and average insulation, about 1,080 W at your system temperature.

What do ΔT 50 and ΔT 70 mean?

The difference between the radiator’s mean temperature and the room air at which output is measured. European catalogues use ΔT 50; some others use ΔT 70.

Why does a heat pump need bigger radiators?

The water is cooler, so each square metre of radiator gives off much less heat. The calculator corrects for this.

Found a mistake in a formula? Write to [email protected]