Homeowners Professionals

What is BTU?

BTU is a way to measure thermal energy. It describes the amount of energy that it takes to heat one pound of water by one degree fahrenheit. To put this into perspective, this would be the equivalent of heating around one pint of water. BTU is used in all sorts of industries and applications and it’s a great way of working out how much heat a radiator will emit and whether it will heat the room efficiently. It is how you can calculate whether a radiator is the right size for a room, though this will depend on lots of other factors too, including the height of the ceilings, the number of windows and whether you have single, double or triple glazing.

What does BTU stand for?

BTU stands for British Thermal Units. Despite its name, this measurement is also very popular in the United States.


How to calculate BTU

Calculating BTU to find the right size radiator isn’t too difficult. All you need is a tape measure so that you can work out the area of the room and a calculator to get the final figure.

First, use your tape measure to find out the area of the room you are calculating the BTU of. This can be done by measuring the height, width and length of the room and multiplying these three figures together. Make a note of this total number as you’ll need it later.

It’s likely that each room in your home is used for a different purpose, and the number and size of windows in your rooms may differ too. This needs to be taken into account when you’re calculating BTU. It’s recommended that your main living areas, such as your kitchen, bathroom and living room, are heated to a comfortable 20 to 21℃, whereas your bedroom’s optimum temperature is slightly lower than this at around 18℃. To account for this, the calculation will be slightly different for each room.

We’ve created the handy table below that you can use to work out BTU. Take the area of the room, which you worked out before, and multiply it by the following, depending on the kind of glazing you have:

For example, if your living room has double glazing and is 4.8 metres in width and length and 2.4 metres in height, the calculation would look like:

4.8 x 4.8 x 2.4 = 55.3 metres cubed 55.3 x 135 = 7,465.5 BTU

This is an estimate that will give you some insight into the size of radiator you will need in order to maintain the  temperature  you want.  

How many BTU in a kW?

It can be handy to know how to convert BTU into kilowatts (kW) and vice versa, as some appliances will tell you one figure and not the other.

To convert BTU to kWs, multiply the BTU by 0.000293. For example:

7,465.5 x 0.000293 = 2.19 kWs (rounded up to the nearest decimal place)

To convert kWs into BTU, multiply the kW by 3412.14. For example:

1.5 x 3,412.14 = 5,118.21 BTU

Below, we’ve done some of the calculations for you, so use our handy charts to see the conversions more easily:  

Does a higher BTU use more electricity?

The higher the BTU of an appliance, the more energy it requires to run properly. For example, a large air conditioning unit will use more power than a small one because it requires more energy to cool a room down.

This is why it’s important to size the appliances correctly. For instance, a radiator that is too big for a room will require more hot water from the boiler, therefore increasing the amount of  gas that is used.  This is a common cause of high utility bills.

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