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Heater Size For Room Calculator

Work out heater size for room instantly with clear inputs, formula shown and shareable results.

About Heater Size For Room Calculator

Heating requirement is volume times a heat loss factor times the temperature rise needed. The factor ranges from about 1.2 W per cubic metre per degree in a well-insulated modern room to 2.4 in a solid-wall Victorian room, and extra external walls add roughly 10% each.

Formula

Heating load

watts = volume x heat loss factor x temperature rise

Worked example

With the default values below, Heater Size For Room Calculator returns Heater output required of 1.95. Change any input above and every figure here is recalculated in your browser.

Inputs used

Room volume
58 m³
Insulation qualityone of Well insulated / modern, Average, Poor / solid wall
Average
External walls
2 walls
Temperature rise required
18 °C

Results

Heater output required
1.95 kW
Heater output
6,661 BTU/h
Heater output
1,952 W
Heat loss factor
1.870 W/m³/°C

Step by step

  1. 1Values used
  2. 2Heating load
  3. 3Heater output required= 1.95 kW
  4. 4Heater output= 6,661 BTU/h
  5. 5Heater output= 1,952 W
  6. 6Heat loss factor= 1.870 W/m³/°C

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 3 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

Why does insulation change the answer so much?

It halves the heat loss factor between poor and good. Insulating first is always cheaper than sizing a bigger heater and running it for years.

Should I size for the coldest day?

Yes, using the local design external temperature. Undersizing means the room never reaches temperature on the coldest days.

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