How the Extractor Fan Calculator Works

Enter your room type and its dimensions. The calculator returns the Part F minimum extraction rate for that room, evaluates the rate needed to achieve recommended air changes per hour for the actual volume, and identifies which rate you should buy against.

The two unit systems cause confusion: 1 litre per second (l/s) equals 3.6 cubic metres per hour (m³/h). Building Regulations speak in l/s, while most retail fan boxes advertise in m³/h. A standard bathroom requirement of 15 l/s means buying a fan rated at least 54 m³/h. If you are fitting guttering to control external dampness, use our Guttering Calculator.

Why the Minimum Often Is Not Enough

Part F rates are minimum floors, not recommendations, and they take no account of the room size. The air change method does: multiply room volume by the recommended air changes per hour (6 to 10 for bathrooms, 10 to 15 for kitchens) and compare.

A large 4m × 2.5m × 2.5m bathroom holds 25 m³ and, at 8 air changes, needs 200 m³/h—nearly four times the Part F minimum. This is exactly why compliant fans still lose to black mould in larger or heavily used bathrooms. Buy against the higher of the two figures. If condensation is already a problem, go higher again or select a humidistat model that runs until moisture is gone.

Ducting: Where Good Fans Go to Die

A fan's rated extraction assumes a short, straight duct, and every metre of ducting and bend eats into real performance.

• Axial Fans: Standard wall fans handle short runs up to about 3 metres.
• Centrifugal/Inline Fans: Generating higher pressure, these are the right tools for longer duct runs.

Match the duct diameter to the fan: squeezing a 125mm fan onto 100mm ducting throttles it. Use rigid ducting over concertina flexible styles where possible, and always terminate outside through a wall, roof tile, or soffit vent. Venting into the loft simply relocates the condensation problem into your roof timbers, promoting rot.

Bathroom Zones and Controls

Electrics in bathrooms follow strict zone rules. A fan installed over or near the bath or shower in Zone 1 needs a suitable IP rating (IPX4 or better), or a low-voltage SELV (Safety Extra Low Voltage) model. Fan positioning belongs high on the wall or ceiling, opposite the main air inlet path (like the doorway) so fresh air sweeps the room.

Humidistat models suit rentals or anyone tired of reminding children to leave the fan on, while continuous dMEV (decentralised mechanical extract ventilation) systems suit new-build properties. Electrical installations in bathroom zones are notifiable works under Part P, so always use a qualified electrician.