What Size Extractor Fan do I need for my Bathroom?

With all the different bathroom extractor fans out there it can be difficult to know which size extractor fan you need for your bathroom. With so many brands, shapes, and types, how will you know which to pick? That is where we come in.

For most domestic bathrooms 4-inch (100mm) extractor fans are more than sufficient, it all depends on your requirements and also preference for things like noise levels and extraction rate.

This post will explore the different sizes of bathroom extractor fans as well as how to know which size to go for and which bathroom fan to choose.

Let’s get into it!

What does size refer to with bathroom extractor fans?

For a bathroom extractor fan, size typically refers to the diameter of the fan’s blade and the diameter of the spigot (the thing that houses the motor and blade). The size is important as this will determine what kind of diameter for the duct pipe you will need, as well as all the other related grilles and accessories necessary to install your bathroom fan.

When thinking of size, things like extraction rate and noise level go hand in hand and should be considered when working out what size extractor fan you’d like for your bathroom.

The different sizes of bathroom extractor fans?

While it seems like there are lots of sizes of bathroom extractor fans – there actually are not that many!

For domestic fans, there are usually three sizes: 4-inch (100mm), 5-inch (125mm) and 6-inch (150mm) extractor fans. The ones you will commonly see on the market will be 4-inch fans as they are more often than not perfectly suitable for the average domestic bathroom.

Anything above that is too large and often unsuitable, mainly due to the higher extraction rates, which in turn could affect the temperature of the room.

You will find that 5-inch and 6-inch extractor fans are for very large bathrooms or kitchens that get very humid and so need a higher extraction rate to sufficiently expel the heat and stale air in the room.

There are also extractor fans that are 200mm and beyond but these are typically for commercial applications.

Extraction Rate

Although the large majority of bathroom extractor fans are 4-inches in size, they often vary in extraction rates. The Manrose MF100T, for example, is a great inline fan with one of the top extraction rates at 245 m³/hr (68 litres per second).

The Xpelair C4TS, on the other hand, has two modes with one of them extracting just 54 m³/hr (15 litres per second). Both of these examples are 4-inch fans but largely differ, depending on what you are looking for in an extractor fan and your desired application.

Noise Level

As well as size and extraction rate, noise levels are a very important thing to consider when buying an extractor fan. Noise levels are measured in decibels (db) and for extractor fans, they are often written as db(A) or db(A) at 3m (distance). The db(A) measurement is one that has been adjusted to keep in mind the sensitivity of the human ear, to the different frequencies of sound.

Naturally, most of us will prefer to have a lower noise level, but this is tied to the extraction rate and therefore will depend on the extractor fan you need. A quiet extractor fan is typically one that is under 30 db(A).

While this is a good indicator to look for, usually the lower the decibels, the lower the extraction rate – but not always. Inline fans, for example, are known to have lower extraction rates because they are usually kept ducted further away from the bathroom (in the loft e.g.) unlike other bathroom extractor fans.

If we take the Vent Axia VASF100t, for example, it is one of the quietest bathroom fans on the market at just 12db, but it also has one of the lowest extraction rates of 73 cubic meters per hour (15 litres per second) as well.

Are there any Legal Requirements for the Size of Bathroom Extractor Fans?

Even though there are no legal requirements that outline a specific size of extractor fan that must be installed, in the Building Regulations – Ventilation: Approved Document F, they specify the minimum requirement of extraction rate, for different applications – as seen in the table below:

As seen in the table, for a bathroom and toilet you will need an extractor fan that extracts 15 l/s and 6 l/s, respectively. Most, if not all, of the bathroom exhaust fans on the market, will already be suitable for this and usually have 2 speed settings anyway, so its fine if one of them happens to be below these measurements.

How do I know which Size Bathroom Extractor Fan I need?

Now that we have covered the above, the way to calculate which bathroom fan you will need is fairly straightforward. In order to work out which size fan you’ll need, a good rule of thumb is to refer to the cubic metres squared (cm3) of the fan you are considering.

Meaning, if your bathroom is 50 square feet or less, you will need an extraction rate of at least 50m3/hr fan. With that said, you need to consider how big your bathroom is, how many people use the bathroom and how often, etc to really know the right size bathroom fan.

Otherwise, you can use our extraction rate calculator to quickly calculate it for you, along with recommendations of top models.


As mentioned earlier, most 4-inch fans are more than sufficient for most domestic bathrooms, it all depends on your preference for things like noise levels and extraction rate.

Typically, the higher the extraction rate the more powerful the fan. This also means that it will also be noisier, more expensive and also more expensive to run, in terms of wattage. Despite the fact that size is important, be sure to consider these things when looking for the best extractor fan for your bathroom.

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