Calculating your radiator size

If you’ve found the right style of radiator for your home, all you need to do is calculate how much heat output is needed to heat the room properly and efficiently. This can be done by calculating your radiator size in three simple steps.

If you need a specific heat output then check with a local heating engineer to confirm the requirements.

Step 1
Calculate room volume (m3) Length x Width x Height

Step 2
Calculate required radiator heat output.

  • For British thermal unit (BTU): Room volume x 153
  • For Watts: BTU x 0.293

Step 3
Now add or subtract the relevant factors. Remember these figures are for guidance use only.

  • Solid floor (-10%)
  • Uninsulated cavity walls (+10%)
  • Foam filled cavity walls (-20%)
  • Upstairs bedroom (-25%)
  • Double glazing (-5%)
  • Two outside walls (+15%)
  • Three outside walls (+40%)
  • Northern aspects (+10%)
  • High ceiling – 3 metres (+20%)

Now you can choose the radiator which will best suit your estimated heat output for each room.

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