Calculates radiator sizes in kW and BTU

Radiator Sizing Calculator

This radiator size calculator will help calculate the correct radiator size heat output (in BTU and KW)) when designing a new central heating system or adding rooms to an existing system. This radiator calculator tool will help you in getting the correct heat output for your radiators

Room Length (metres)
Room Width (metres)
Room Height (metres)
Use Of Room
North Facing?
Room has French Doors?
Double/Single Glazing?


You'll find a few tools to work out radiator sizes online. Most of these calculators make various simplifications and assumptions in working out the radiator sizes so it's probably an idea to check out a few of them and compare the results you get.

What is the Perfect Temp for a Room

A common preferred temperature is below but obviously this varies depending on personal preference

  • Lounge - 21 °C (70 °F)
  • Bedroom - 18 °C (64.4 °F)

Limitations of Online Radiator Sizing Calculators

Most of the online radiator calculators, including this one, use various factors to estimate the required radiator size heat output (KW and BTU). They provide a good estimate of required BTU output and a useful check on figures your heating engineer may be specifying in a quote.

They are limited by the factors they take into account