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BTU Calculator

Our BTU Calculator is the best on the web as it accurately determines the number of BTUs your radiator should have in order to effectively and efficiently heat your room. Unlike all other BTU calculators however, the BTU calculator goes one big step further and then shows you all the radiator on the website that can produce that level of heat output. You can then filter further and narrow the results down to exactly what you are looking for! We have taken all the confusion and stress out of finding the ideal radiator by clearly listing the number of BTUs related to each size radiator on the product pages.

Learn the optimum number of BTUs required for heating your room and the radiators you need to create a warm and cosy room by completing the fields below.

What does BTU mean?

BTU stands for British Thermal Units and is a unit of measurement for energy. Example: 150 British Thermal Units is equal to the amount of energy used to raise the temperature of 150 pounds of water 1-degree Fahrenheit. You will see BTU used when referring to the amount of heat output a radiator emits to increase the heat in a room. The higher the BTU number associated with a radiator, the more heat that radiator can output to warm the room.

You can calculate the BTU heat output needed for all rooms including lounge, hallways, bathroom, bedroom and kitchen with a BTU Calculator. You can get a rough idea of heating needs based on just the dimensions of the room and walls but for more accurate results what you need to know is;

  • Room Dimensions Including ceiling height in metres or feet
  • Number of windows in the room
  • Window Size
  • Type of glazing (single or double usually)
  • Number of walls on the outside of the building
  • Outside wall material
  • What is above and below the room

Why do you need a BTU radiator calculator before buying a radiator?

Everyone at some point in their lives will have experienced being in rooms which always seem too cold despite the radiator heating always being on. It is likely unless there were always open doors that this was because the wrong radiators were installed for the room dimensions. Namely, the heat output of the radiator wasn't sufficient to heat the room. And you may have also experienced rooms being swelteringly hot when the heating was turned on. This again is a heating calculation failure, namely radiators may give out too much heat for the size of the room. To prevent our customers getting the wrong size radiators for their heating needs, we recommend them using our BTU calculator to get their perfect radiator. 

Get the perfect radiator with our BTU Radiator Calculator

Use the BTU calculator to find the BTU output requirements of each room you are adding or replacing radiators in. Remember the number that the calculator returns is the total number of BTUs required to heat a room, so you can use as many radiators as you would like to reach this number.

If you are concerned at all, we always recommend going above the BTU number. Some houses or buildings are just naturally colder than others and you are the best judge of that. Going over your total BTU number does no harm except you could make it too hot if not using a thermostat or your bills could be unnecessarily high, again, unless you use a thermostat. Not having enough however could result in your heating being on full power and still not achieving the cosy heat feeling you are striving for.

We always recommend working with a plumber to get an expert’s opinion on site but our detailed BTU calculator works brilliantly and ensures you achieve an accurate guide as to how many BTUs and Watts you need to heat your room effectively and efficiently.

How to Use Our BTU Heating Calculator

  • The first step is you need to measure in metres or feet your room dimensions and write them down. So that's the width of the room in metres or feet, the height of the room walls in metres or feet and length of the room in metres feet. The length tends be longer than the width. 
  • Once you have those measurements,  second step is to fill out all of the fields below and click “calculate”.  Our smart radiator calculator will do the hard work for you and display the radiators that are suitable for your needs.

From there you can use the filter on the left to narrow your search down to the perfect size, colour and style of radiator to heat your rooms.

Select number of outside walls (Optional) :

Room Dimensions

Width* Height* Length*
Window Area X² (Calculate this by multiplying window height by window length)

Room Information

Type of room:


Where are you heating?

What is below the room?

Think about the flooring of your room, and whether there is another room below

What is above the room?

Where applicable, choose the amount of insulation

Select type of outside wall:

This helps us calculate how much heat the room might lose

Select type of window:

This also helps us determine the expected heat loss in the space

Select number of outside walls (Optional) :

Your Requirements
BTU's Required
Watts Required