How to Calculate BTU

By Trinity

17th Feb 2022

4 mins read

DIY & Technical

All radiators have a BTU measurement, but what does it mean? Follow our guide to find out how to calculate the correct BTU for any room in your house.

How to Calculate BTU

If you’ve ever gone to purchase a radiator there’s a chance that you’ve become overwhelmed with the task of trying to figure out what a BTU output is. Or perhaps you already know what a BTU is and need to know how to calculate the right BTU output required for a specific room in your home. That’s why we’ve put together a short and simple guide full of frequently asked questions so that you can get to grips with the jargon of radiators and heating. Finding the right radiator for you doesn’t have to be complicated!

What does BTU stand for?

To begin with, it’s probably best to break down what BTU actually stands for. BTU is an abbreviation for ’British Thermal Unit’. One BTU is equal to 1055 joules which is the amount of energy required to heat 1lb of water by 1°F.

So how does this relate to your radiator?

Urban Horizontal Radiator - White - Double Panel

What is BTU?

BTU output is essentially the measurement we use for how much heat is needed to keep a specific room in your house warm. But what does this mean in relation to your heating, the rooms in your house and what radiator you should purchase? Breaking it down to basics, the higher the BTU output, the higher the heat output of a radiator. Therefore, the higher the BTU calculation of your room, the higher the BTU you will need from a radiator to match this.

A great example is the BTU required for a bathroom vs a living room. A bathroom is generally smaller than your living room and so will require a lower BTU output. Because you don’t generally need your bathroom to be heated at a consistent rate, the BTU tends to be lower as the radiators in your bathroom are more for the purpose of warming towels than turning your bathroom into a sauna. That’s why the bathroom radiators we supply have a lower range in BTU, typically between 500 and 4000 for towel radiators and ladder radiators.

Milan Heated Towel Rail Chrome

For a living room, however, the BTU output required tends to be a lot higher because they are often larger rooms that you will want to keep at a consistent temperature for extended periods of time. Besides, who wants to be chilly whilst watching TV!? This is why radiators more suited to your living room or even a bedroom tend to have a higher range in BTU output, some reaching over 5,000 BTU for those with particularly large rooms.

When it comes to choosing a radiator it’s handy to know that one radiator does not have to be responsible for the entire BTU output of your room. You can combine the BTU output of radiators to meet the heat requirements of your home.

With BTU it’s important to remember that the calculation will be different for every room in your house and there are a variety of factors that will impact the necessary BTU output to heat your home.

How to calculate BTU for a room

Although the information required in order to calculate BTU might vary, there are some things that are absolutely necessary even for the simplest of BTU calculations.

1. Room Dimensions

The first is the area of your room. You will need to measure the length, height and width of the room you wish to heat.

2. Windows

Most BTU calculations take into consideration ways in which heat may be lost or increased by specific variables. The most common element taken into consideration is the heat lost by windows. Therefore, the dimensions of any of the windows in the room is also useful information to have.

3. Further factors that impact heat loss/retention/increase

Depending on who is calculating your BTU, and how accurate they want to be, other factors that impact the overall heat of a room will also be taken into consideration. This can be components such as whether your room is North or South facing, or what type of rooms are above and below the room you want to heat.

Through a combination of these pieces of information a rough outline for necessary BTU output can be created. Now all that’s left to do is a find a radiator to match it!

Chrome Vertical Radiator

As with all things technical, whilst you can calculate BTU yourself using various online BTU calculators it’s always best to have a professional do it. These BTU calculations are more like rough guidelines than completely accurate results and so if you’d like to get the most accurate BTU calculation, a professional is the best option to help you out.

We hope this has helped answer your questions in relation to how to calculate BTU ratings for your home. Now that you have a grasp on BTU, if you’d like some more guidance on heating, feel free to read our blog on choosing the best radiators for your home. Interested in smart heating too? Why not check out our smart heating range.



Trinity is one of our expert bloggers in bathroom design and DIYs. Read her blog posts for the latest coverage of style trends and easy-to-follow guides.

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