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Radiator Sizing Calculator

Work out radiator sizing instantly with clear inputs, formula shown and shareable results.

About Radiator Sizing Calculator

Radiator duty is commonly set from room volume using a watts-per-cubic-metre figure that reflects insulation standard. Because manufacturers publish output at a 50 K mean water-to-air difference while modern condensing systems run nearer ΔT30, the required duty is divided by the ΔT30 correction factor of 0.789 to give the catalogue rating to order.

Formulas

Radiator duty

Watts = length x width x height x W/m³ factor

Catalogue correction

Catalogue W at ΔT50 = required W / 0.789 (ΔT30 correction)

Worked example

With the default values below, Radiator Sizing Calculator returns Radiator output required of 1,680. Change any input above and every figure here is recalculated in your browser.

Inputs used

Room lengthminimum 0.5 m
4 m
Room widthminimum 0.5 m
3.5 m
Room heightminimum 1.8 m
2.4 m
Insulation standardone of Well insulated — 40 W/m³, Average — 50 W/m³, Poorly insulated — 60 W/m³
Average — 50 W/m³

Results

Radiator output required
1,680 W
Room volume
33.60 m³
Output in BTU/h
5,732 BTU/h
Catalogue rating needed at ΔT50
2,129 W

Step by step

  1. 1Values used
  2. 2Radiator duty
  3. 3Catalogue correction
  4. 4Radiator output required= 1,680 W
  5. 5Room volume= 33.60 m³
  6. 6Output in BTU/h= 5,732 BTU/h
  7. 7Catalogue rating needed at ΔT50= 2,129 W

Assumptions and limits

  • The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
  • Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

How is the Radiator Sizing figure worked out?

Watts = length x width x height x W/m³ factor. Radiator duty is commonly set from room volume using a watts-per-cubic-metre figure that reflects insulation standard.

Why is the ΔT correction needed?

A radiator emits less heat when the water is cooler. Sizing on the ΔT50 catalogue figure while running at ΔT30 leaves the room roughly 20 percent short of its design output.

What values do I need to enter?

This calculator takes 4 inputs: Room length, Room width, Room height, Insulation standard. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.

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