What extraction rate does my bathroom fan need?

Under UK Building Regulations Approved Document F, a bathroom, shower room or en-suite requires a minimum extraction rate of 15 litres per second (l/s). A separate toilet/WC needs 6 l/s. These are minimum values — larger rooms may need a higher rate based on room volume and air changes per hour. Our calculator gives you the correct figure.

Do I need a fan in every bathroom?

Yes. Under Building Regulations Approved Document F, all bathrooms and shower rooms require mechanical ventilation if there is no openable window providing adequate background ventilation. Even with a window, a fan is strongly recommended and often required by building control. An intermittent fan with a 15-minute overrun timer is the most common solution. For wet room installations, also see our shower tray fall calculator (/tiling/shower-tray-fall-calculator/) and wet room tanking calculator (/tiling/wet-room-tanking-calculator/).

What is the difference between an axial and centrifugal fan?

Axial fans are the most common type — they are compact, fit in a wall or ceiling, and are suitable for short, straight duct runs up to about 1.5m. Centrifugal (or inline) fans are more powerful and can push air through longer duct runs with bends. If your duct is longer than 1.5m or has more than one bend, use a centrifugal fan. Check the fan's specific fan power (SFP) rating for energy efficiency.

Does my bathroom fan need a timer?

Yes. Building Regulations require bathroom fans to have a 15-minute overrun timer. The fan must continue running for at least 15 minutes after the light is switched off (or after the room is vacated if using a PIR sensor). This ensures adequate moisture removal after bathing. Humidistat fans that run automatically when moisture levels are high are also acceptable.

What size duct do I need for a bathroom fan?

Most bathroom fans use 100mm (4 inch) diameter ducting, which is sufficient for extraction rates up to about 30 l/s. For higher rates or very long runs, 150mm (6 inch) ducting reduces resistance and noise. Always use rigid or semi-rigid ducting rather than flexible foil duct, which creates more resistance. Keep duct runs as short and straight as possible.

Bathroom Extraction Rate Calculator — Fan Sizing Guide UK

How We Calculate This

Our bathroom extraction rate calculator determines the required ventilation rate using two methods, then recommends the higher value to ensure compliance with UK Building Regulations Approved Document F.

Method 1: Volume-based calculation

Extraction rate (l/s) = Room volume (m³) × Air changes per hour ÷ 3.6

For example, a bathroom measuring 2.5m × 2.0m × 2.4m = 12 m³. At 15 air changes per hour: 12 × 15 = 180 m³/hr ÷ 3.6 = 50 l/s.

Method 2: Part F minimum rates

Building Regulations Approved Document F specifies minimum intermittent extraction rates:

Bathroom / Shower room / En-suite: 15 l/s minimum
Toilet / WC: 6 l/s minimum
Kitchen: 13 l/s minimum (above hob) or 30 l/s (if cooker hood)
Utility room: 8 l/s minimum

Which value to use

Our calculator takes the higher of the volume-based rate and the Part F minimum. This ensures your fan is powerful enough for both the room size and regulatory compliance. The fan recommendation considers typical products available from UK suppliers such as Manrose, Xpelair, Vent-Axia and Envirovent.

Air changes per hour

Bathrooms, shower rooms, en-suites: 15 ACH
Toilets / WCs: 6 ACH
Kitchens: 10-15 ACH
Utility rooms: 10 ACH

Frequently Asked Questions

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Last updated: February 2026

All calculations are estimates. Verify with your supplier.