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BTUs - What Are They For?

What are BTUs

‘BTU’ is a common acronym in the bathroom industry, usually seen in the specifications for towel rails and radiators. However, not many people outside of the industry actually know what it means/is. We’re all about making buying bathrooms, including everything in it as easy as 1-2-3 so we’re going to help you understand BTUs and what it means!

‘BTU’ stands for British Thermal Unit.

It is the standard unit measurement which is used in the plumbing and heating industry.

1 BTU is equal to about 1055 joules – the energy required to raise 1 pound of water by 1°F. To help remember it; 1 BTU is essentially the heat output of 1 Match Stick.

Why Is This Important?

Radiators serve a very important functional purpose in our bathrooms – heating the room. So it’s important to ensure that the size and power of the radiator are suitable and adequate enough to heat the room to a satisfactory level.

Buying a radiator with an insufficient BTU rating will not heat the room well whilst one with a much higher BTU rating will overheat the room and therefore waste money. However, there is some leeway.

Simply put, the higher the BTU output the warmer a radiator can heat a room.

The simplest and safest way to know which radiator would be suitable for your bathroom would be to consult your plumber. They’ll be able to give you an estimate based on the size of your room and estimated heat-loss through doors, insulation and windows.

However, in general the formula to calculate required BTUs for your bathroom (in metres) is below;

(Length x Width x Height) x 90

For example, if your bathroom is 3m x 3m x 4m then you will need a radiator with a heat output of around 3240 BTUs.


Our Norden 600 x 1260mm White Single Oval Tube Horizontal Radiator, £139.97, would be the perfect radiator for this bathroom as its heat output is 3194 BTU. Whilst this is lower than the calculated value, it is well within the recommended 10% threshold – so heating your bathroom will not be a problem!

By Peter Wootton 3 months ago 262 Views