Radiator BTU calculator

Estimate the BTU per hour a room needs and the radiator output to buy for your system.

LHW
Letters on the drawing match the letters next to the inputs.
ft
ft
ft
Heat needed5,565 Btu/h

Shopping list

  • Radiator output from catalog (ΔT 50 K / 90 °F)5,565 Btu/h

How we got there

Room volume
1,440 ft³
Temperature correction
1 ×

Results are estimates. Always check the coverage printed on the product and your local building code.

How it is calculated

Heat demand is estimated from room volume: about 2.9 Btu/h per cu ft for a well insulated home, 3.9 for an average one and 4.8 for a poorly insulated one. Corner rooms get 20 % more. Radiator output is rated at a set temperature difference, so it is converted to your water temperature with the EN 442 formula. For whole-house sizing, a Manual J calculation is the standard.

FormulaHeat, Btu/h = length × width × height × (2.9, 3.9 or 4.8)Correction = (system ΔT ÷ rated ΔT)^1.3Catalog output = heat ÷ correction

Example

A 15 × 12 ft room with 8 ft ceilings in an average home needs about 5,600 Btu/h (1,630 W). With a boiler at 167/149 °F, a panel radiator rated at ΔT 50 K works at its catalog output, so choose one rated at 5,600 Btu/h or more.

Tips from practice

  • Place radiators under windows to block cold drafts.
  • With a heat pump, radiators need 2–3 times the catalog output you would need with a boiler.
  • Bleed radiators each fall so air does not block the flow.

Questions and answers

How many BTU per square foot?

Roughly 25–40 Btu/h per sq ft with 8 ft ceilings, depending on insulation and climate.

What does ΔT 50 mean?

The difference between the radiator’s average temperature and the room air at which output is measured: 50 K, or 90 °F.

Why does a heat pump need bigger radiators?

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

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