When choosing an inline fan it is important to know what size fan you will need in order to provide adequate ventilation for the room it is to be installed in.

The size of the fan is important for a few reasons. Firstly, if the fan is too small and not powerful enough for the environment in which it is installed then the risk of mould and damp remains. Secondly, extractor fan installations must satisfy Building Regulations F(1) which exists with the aim "to protect the health of occupants of the building by providing adequate ventilation".

To work out what size fan you need follow the steps below


1. Work out the cubic meterage (M³) of the room

This can be worked out by multiplying the height, width and depth of the room together in meters


Height x Width x Depth = M³


2. Find out the Maximum Extraction Rate of the Fan in M³/hr

The maximum extraction rate is the amount of air that the fan is able to extract within 1 hour. When looking for the maximum extraction rate, make sure that you find out what it is in m³/hr and not litres per second (l/s) for the purposes of these calculations.


3. Divide the Max. Extraction Rate by the Room Size

Let's use our best selling 4" Manrose MF100T Inline Fan as an example in the following calculation. It has a maximum extraction rate of 245m³/hr. Lets say our bathroom is 11m³

245m³ / 11m³ = 22


This means that the fan is capable of extracting the air in the room 22 times in an hour.


4. Find out the Room Rate Per Hour as Stated in Building Regulations


The room rate per hour indicates how many times the air within the room must be extracted within an hour in order to effectively protect against damp and mould and satisfy building regulations.

The table below gives a breakdown for the rooms where extractor fans are most commonly installed.

As you can see, a bathroom has a minimum room rate of 15 per hour and the MF100T can extract the air 22 times in 1 hour, therefore it will be suitable for installation


RoomRoom Rate per hourMinimum Litres per second
Bathroom / Shower Room 15-20 15 L/s
Kitchen 10-20 60 L/s
Kitchen (If Installed within Cooker Hood or 300mm of Hob) 10-20 30 L/s
Utility Room 15-20 30 L/s
Download the Ventilation Building Regulations Information here


We hope that this guide has helped you to work out what size inline fan you need, however if you need any further help, please contact us on 0203 994 5470, email us at sales@electricpoint.com or use our Contact Form.