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How to calculate the BTU for your room

What we’ll cover…

  • What are BTUs (British Thermal Units)?
  • The importance of performing proper BTU calculations to ensure heating efficiency
  • Which factors can affect BTU requirements
    • Room dimensions (width, height, length)
    • Window glazing and insulation levels
    • Amount of usage and different room types
    • Number of doors, windows and any other openings
  • The step-by-step process for calculating BTU requirements
    • How to measure the room
    • Consideration of glazing and insulation factors
    • Utilising BTU calculators online
    • Accounting for safety margins
  • Typical BTU requirement figures for various room types
  • Tips to choose the right radiator size in accordance with BTU output
  • Supplementing BTU output levels with additional heating sources
  • Cost factors for different radiator types and BTU outputs

A guide to calculating BTUs

Buying a new radiator or heated towel rail is a tough enough job to get right as it is, but knowing whether or not the radiator you want and like is powerful enough to heat your space can confuse things even further.

To ensure that a new radiator is going to get hot enough, you must work out exactly (or as close as possible) how much heat you require to make your space warm and inviting.

Thankfully, at BestHeating, our experts love to help you make the most of your home’s heating and in this guide, we’ll show you how to calculate the correct BTU output for your space, to make sure you never feel the cold again – well, at least whilst your heating’s on.

What is a BTU?

We’ve mentioned this before, but if this is your first time coming across the term BTU (British Thermal Unit), don’t worry, it’s really simple.

Cast your mind back to school and you might recall that 1 BTU is equal to 1055 joules – or is the amount of energy needed to heat a single pound of water by 1°F.

Put simply – the higher its BTU output is, the hotter the radiator or heated towel rail will be.

For example…

This traditionally styled Milano Windsor vertical column radiator has a BTU output of 7,913 BTUs – making it the perfect heating solution for very large rooms.

Milano Windsor Traditionally styled vertical radiator on an industrial style wall

Whereas this Slim Panel Milano Alpha radiator would be better suited to heating a small bathroom or ensuite space, offering a much lower output of 1,120 BTUs.

mirrored vertical radiator in a bathroom next to a sink

It’s important to work out exactly how much heat you need for a room to ensure you are getting the best value for money.

If you buy a towel rail or a radiator with a BTU output that is too high for your room, you will quite literally be burning money on paying for unnecessary energy bills.

Alternatively, buying a new radiator with an output that is too low and there just won’t be enough heat coming from it to keep the space warm. In this scenario, you’ll need to have your heating turned up to eleven – a’ la Spinal Tap.

In order for you to get the most out of your home’s heating, it’s very important that you know how to calculate your required BTU output.

How do I calculate the correct BTU?

How you heat each room will depend on a number of different factors and where you are trying to heat.

Each room has a different heating requirement and a different ‘ideal’ temperature, so a radiator that is perfect for one room, may not be ideal for another.

Here are what we would call ‘typical’ examples of how warm each room should be.

  • Sitting Room/Dining Room: 21-22°C/70-72°F
  • Kitchen: 20°C/68°F
  • Bedroom/Hallway: 18°C/65°F
  • Bathroom: 21°C/70°F

You might prefer to have your rooms warmer or colder (it really is up to you), these are just simple guidelines that most people tend to follow.

Measure the height, width and length of your space in metres (or imperial measurements in you prefer) and keep these close to hand.

If your room is an unusual shape, divide it into separate rectangles and add them together to get the calculation.

an architectural floor plan with a pencil

When you have these measurements you’ll need to work out how much heat is lost from your room.

To do this, you’ll need to answer a couple of simple questions:

Are there any windows or doors?

Are they single, double or perhaps even triple glazed?

What is above and below the room you intend to heat?

Do you have cavity wall insulation or not?

Once you have this basic information, you are primed and ready to perform your BTU calculation.

For a quick and easy solution, enter this information into the BestHeating BTU Calculator to get your required output and a selection of suitable radiators.

Below are a few examples of heat requirements for particular rooms.

What BTU output do I need for my bathroom?

Obviously, bathrooms come in all sorts of shapes and sizes, so we’re just going to use a basic example to show you the kind of requirements you might come across.

Let’s imagine you have a family bathroom measuring 3m (W) x 2.5m (H) x 2.6m (L) –

  • it has double glazed windows measuring 2m²
  • an outside wall and loft space with adequate insulation
  • a single exterior wall
  • and a heated room beneath it

Your required BTU output will be 2,050, with a wattage of 600 – making this Milano Nero Black Heated Towel Rail the ideal solution to your bathroom heating requirements.

Milano Nero black heated towel rail in a bathroom

What BTU output do I need for my living room?

When it comes to your living room the look of your radiator is probably as (if not more) important as the heat output, but still, you want to get it right so you have a warm and inviting space in which to relax and entertain.

So let’s say your living room has double glazed windows measuring 4m², wooden flooring, a heated room above it, two exterior walls with an insulated brick cavity and the room dimensions are 5m (W) x 3m (H) x 5m (L).

A room with these attributes and dimensions will require a BTU output of 7,200 – or 2,110 Watts – making the Milano Riso flat panel central inlet radiator a prime candidate for consideration.

grey flat panel radiator on a white background

Or, as an alternative, you could split that calculation in half and opt for two separate radiators that – when their outputs are combined – will provide the same heat output as the single radiator.

So if a horizontal radiator style is more your cup of tea (or perhaps you’re replacing two old convector radiators), two of these Milano Aruba will do the job just as well as a single radiator; its BTU output of 3,836 (1,124 Watts) easily matching the required 7,200 BTUs when you install two of them together.

White designer radiator on a grey wall with a blind

What BTU output do I need for my bedroom?

You may not spend as much of your day in the bedroom (unless you’re really lucky) but you still want warmth and comfort to be the order of the day in there, regardless of whether you’re a bed-dwelling home-body or not.

A big double bedroom (4m x 5m x 3m) with a large 3m² double glazed window, a heated room beneath it, loft insulation above it and with cavity wall installed on its two external walls will require a radiator with a BTU output of 3,157 or 925 Watts.

That would make a Milano Windsor traditional low profile column radiator an ideal installation for a bedroom of that size and matching it with some period-style thermostatic radiator valves really finishes the look off.

low profile horizntal traditional column radiator

It’s time to calculate your BTUs

As I mentioned above, these are just a selection of examples to give you a rough idea of how to correctly calculate the BTU output you require to heat your space and the type of products available to complete the job.

Though many other sites would ‘thoroughly recommend’ hiring a qualified heating engineer to fully calculate your required BTU output – and to be fair that won’t do you any harm – it is without a doubt something that you can do for yourself, particularly with the help of our BTU calculator.

So save yourself some time and money, input your details and find a selection of suitable radiators that will leave your space warm, inviting and looking the part in no time!

Good luck finding the best heating solution for your home.

Stay Safe & Happy Heating!

Frequently Asked Questions

Several factors will determine if cheap designer radiators deliver an adequate BTU output, such as the size of the space they’ll be installed in, as well as the actual radiator size.

Furthermore, what you define as a cheap designer radiator will be influenced by a number of reasons, like what your radiator budget is, and how much you prioritise the look of your designer radiator in relation to performance quality and durability.

Our BTU Calculator can be used to show whether your chosen cheap designer radiator provides a high enough BTU output to adequately heat the room it will be fitted in. And for more information on the subject, you can take a look at our dedicated blog – Cheap Designer Radiators: Too Good To Be True?

British Thermal Units (BTUs) are specifically a unit of energy. Contrastingly, Watts (W) represent a measurement of the rate that energy is transferred.

When a conversion occurs from BTUs to Watts, it is actually essentially a conversion from BTUs per hour (BTU/h) into the equivalent amount of Watts.

One Watt equates to 3.41 BTUs per hour, so you divide the total number by 3.41 to convert from BTUs to Watts.

You can use our BTU Calculator to find the ideal heat output you may need to heat your chosen space.

However, when calculating wattage outputs, it is important to take into account the heat lost through transfer from the heating element, to the heating solution, and then from the heating solution to the metal of the radiator itself.

And while our BTU Calculator will give you a conversion based on some clever mathematics, if you are unsure of what this heat loss calculation may be, and want to be as accurate as possible, it may be advisable to consult a plumber or electrician for advice before purchase.

A single Watt (W) is equivalent to 3.41 British Thermal Units (BTUs) per hour (h).

As such, to convert BTUs per hour into Watts, you have to divide by 3.41.

In order to convert Watts into BTUs per hour, you should multiply by 3.41.

The BTU requirement per radiator will be determined by the amount of radiators featured in a specific room.

So, use a BTU Calculator to work out the total BTU requirement of the space, and divide this total by the number of radiators you are planning to install.

The result will be the average BTU level needed per radiator.

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