Scroll to the bottom of the page for the use of our Free Heat Loss Calculator for Radiators. This heat loss calculator should be used as a guide only. Always confirm with your plumber or architect before order.

A radiator is sized to provide the heat required for your room. This heat is usually referred to using the terms Watt or BTUs. Note: 1 Watt = 3.412 BTU. E.g. I need 1000 watts/3412 BTU


The Watt/BTUs (i.e Heat) you require is dependent on two main factors:

  • The temperature you wish the room to heat to e.g. 21ºC
  • How much heat the room loses (heat loss) through walls, windows etc. You are trying to calculate how much energy (Watts/BTU) you need to put into a room to reach a certain temperature given that X amount of this energy or heat will be lost through walls, windows etc.


Temperature Required for Room

The following are recommended temperatures required for rooms depending on their type:

Kitchen/Hall/Toilet/Bedrooms 18ºC
Living Areas, playrooms, games rooms etc. 20-21ºC
Bathrooms (with shower) 22 ºC


Heat Loss of the Room

  • To get a definitive calculation on this you need to take into account the area of all floors, walls, and windows. The U Values of all construction materials and other relevant information such as whether the building is on a sheltered or exposed site.
  • There is an older more general method which just multiplies the volume of the room by a factor of 40 to give you the number of watts required. There are similar calculations for BTUs. We recommend that you do not use such a general system. The old method of over-sizing your radiator “just to be sure” is an inefficient way of heating. Yes you can use a thermostatic valve to turn it down, but you will be paying for a larger radiator that you don’t need to begin with.


The Heat Requirement Calculator or Heat Loss Calculator on this website was designed by an Irish energy expert and takes into account room dimensions, year of build, glazing area, type of glazing, type of room etc. It is an accurate system if the data is inputted correctly. In general this calculator will return a much lower heat requirement for new builds than the traditional methods of calculation as mentioned above. This is mainly because our calculator takes into account the low U valves of materials in buildings constructed to 2008 building regs which are:

  • Roof 0.20
  • External Wall 0.27
  • Floor 0.25