BTU Calculator

Find your perfect radiator

Correctly calculate your BTU requirements to -

Create warm & inviting rooms
Increase the efficiency of your heating
Take control of your fuel bills

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What size radiator do I need?

We recommend choosing a radiator size that is within a 10% range of your calculated BTU requirement. If it’s a choice between buying a radiator that is too small, versus buying one that is too big, it’s always best to buy one that is too large and oversize the radiator. You can always turn a bigger radiator down, but you can’t make a smaller radiator give out more heat!

How do I choose the right radiator?

BTUs are ‘British Thermal Units’, the unit in which radiator efficiency is measured. Factors such as room dimensions, window size and wall material are used to calculate the required outputs of radiators, in order to adequately heat a room.

Use our BTU Calculator to determine the heat requirements of your room. Simply enter the details in the corresponding fields and we’ll show you how many BTUs you need to ensure your space is always warm, cosy and inviting.

How do I calculate the BTUs for my room?

The best way to get an accurate calculation is to enlist the services of a plumber. However, BestHeating's BTU calculator is a good guide as to what level of BTU and Watt output will efficiently heat your room. Please note: the calculated figure is for the combined heat output of all radiators and/or towel rails in your room (larger rooms may require more than one).

Calculate the required BTU output of your room

Room Dimensions

Metric (metres) Imperial (feet)

Width (m) *

Height (m) *

Length (m) *

Window Area X² (Calculate this by multiplying window height by window length): *

If your room does not have a window, please enter '0' in the field below & select 'no window' in the relevant field below

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: *

Reset

Your Requirements

BTU's Required

Watts Required

View suitable products >>

>> What is a BTU?

A BTU – or British Thermal Unit – is an approximation of the amount of energy required to heat 1lb (one pound) of water from 39°F to 40°F, and is roughly equal to 1.055 KJoules. Simplified, a single BTU is more or less equal to the amount of heat and/or energy produced by burning a single wooden match from end-to-end.

>> How is your room’s BTU requirement calculated?

A room’s BTU requirement is based upon the cubic volume of the space – the height, length and the width of the room multiplied by four (done for you by our calculator) – and what is above, below and besides the room.

With this information – and a bit of mathematical prowess - we can then determine the expected heat loss for your room and provide you with a BTU output to heat the space efficiently – by which we mean to a comfortable temperature and at the lowest possible fuel costs to you.

>> Why is it important to calculate the correct BTU output?

Being sure to make the correct BTU calculation is very important. Selecting a radiator with an insufficient heat output will mean that your room never reaches its optimum temperature. Conversely, choosing a radiator with too high a heat output could mean that you spend more than you need to on your heating bills – but one of the great things about radiators is that you can always turn them down if you need to.

Please remember, it is also possible to combine radiators to achieve the required BTU level. Many rooms have more than one radiator in them for this very reason.

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