How to calculate the correct BTU's for your room

How to calculate the correct BTU's for your room



When updating your home, it’s not always straightforward calculating your radiator output. 

Shopping for a radiator or heated towel rail that fits the theme of your home is severe enough, but knowing whether it meets the heating requirements of your room is another matter entirely.

The British Thermal Unit (BTU) is the amount of energy needed to heat a single pound of water by 1°F. 1 BTU is equivalent to 1055 joules. Put, the higher the BTU output, the hotter the radiator or heated towel rail is going to be.

Different radiators provide different BTU outputs; for example, vertical column radiators provide a BTU output of 7913 BTUs whereas slim panel radiators provide 1120 BTUs.

To avoid spending an unnecessarily large sum on energy bills, it’s essential to find a radiator with a BTU that fits your needs. Therefore, you need to know how to calculate your required BTU output.

Naturally, every room has a different heating requirement so your chosen radiator for one room may not necessarily be suitable for another place. Some typical values for how warm each room should be:

·       Living area: 21-22°C/70-72°F

·       Kitchen: 20°C/68°F

·       Bedroom: 18°C/65°F

·       Bathroom: 21°C/70°F


Of course, BTU is not just calculated by the desired temperature, but also takes into account room size. Things you need to consider before calculating the BTU output for your room:

Ø  Measure the height, width and length of your interior space. If your rooms are unusually shaped, separate them into rectangles and then add them together to get the values.

Ø  Calculate the amount of heat that is lost from your room. To get this value, consider the number of doors and windows in your room. What rooms or spaces are above and below the room and what insulation does the room have?  

Calculating the BTU for your room

For precise readings, you’ll probably need to hire a qualified heating engineer to calculate the BTU output for you. However, for an estimated reading, there are plenty of tools online that can provide a guide. However, calculating BTU is not an impossible task, and you can even do it all by yourself.

If you are calculating by yourself you need to:

·       Calculate the cubic volume of your room in feet or meters. This is calculated by multiplying the height, width and length of your room.

·       After that, multiply the figure by the following values for a rough estimate of your suggested BTU.


                  Multiplication Factor                                        

Lounges and Dining Rooms

                  Multiply Cubic Feet by 5


                  Multiply Cubic Feet by 4

Common Areas and Kitchens

                  Multiply Cubic Feet by 3

For Rooms Facing North

                  Add 15%

For French Windows

                  Add 20%

For Double Glazing        

                 Deduct 10%


To gain an exact figure, you’ll want to perform calculations including heat loss, number and size of windows and room insulation, but the above will give you a rough guide.

If the radiator you want doesn’t fall within the BTU value, there is some leeway, generally, so long as the value isn’t lower than 10% of your estimate, you should be okay. Naturally, there isn’t an upper limit as you can turn your radiators off! 

Please also find a link to a handy BTU calculator that will make things a lot easier for you!

BTU Calculator

For more information on BTU, talk to our team today on 01257 758247 or email us on


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