What extract rate does a bathroom need?

Under Part F of the Building Regulations (2021 edition), a bathroom needs 15 L/s (litres per second) for intermittent extract or 8 L/s for continuous extract. Intermittent fans run when the room is in use (usually switched with the light plus overrun timer). Continuous systems run 24/7 at a lower rate, boosting during use. An en-suite has the same requirements as a bathroom.

What is purge ventilation and do I need it?

Purge ventilation is rapid ventilation at 4 air changes per hour (ACH) to quickly clear pollutants — for example, after painting or if there is a gas leak. In bathrooms, this is typically achieved by an openable window with an area of at least 1/20th of the floor area. If there is no window, the extract fan must be capable of achieving the purge rate.

How do bends in ductwork affect fan performance?

Each 90-degree bend in the ductwork is equivalent to approximately 1 metre of straight duct in terms of resistance. This means a fan connected to 2m of duct with 2 bends has an effective duct length of 4m. The added resistance reduces the fan's effective flow rate, so you need to select a fan with higher capacity to compensate. Use flexible ducting sparingly as it adds more resistance than rigid duct.

What duct size should I use for a bathroom fan?

100mm (4 inch) duct is the standard for most bathroom extract fans delivering up to 15-20 L/s. For longer duct runs (over 3m) or where there are multiple bends, step up to 125mm duct to reduce resistance. 150mm duct is used for high-capacity fans or very long runs. Always match the duct size to the fan spigot — do not reduce duct size downstream.

Do I need background ventilators in a bathroom?

Yes. Part F requires background ventilators (trickle vents) in all habitable rooms and wet rooms. For a bathroom with an intermittent extract fan, you need at least 2500 mm² equivalent area (EA). Trickle vents in windows or wall-mounted air bricks provide this. For windowless bathrooms, 4000 mm² EA is recommended to ensure adequate make-up air for the extract fan.

Bathroom Ventilation Calculator — Part F Extract Rates & Duct Sizing

Calculate the correct extract fan capacity, duct sizing and background ventilation for your bathroom to comply with Part F of the Building Regulations.

Room Length

Internal room length

Room Width

Internal room width

Ceiling Height

Standard 2.4m

Ventilation System

Intermittent is most common for bathrooms

Duct Size

100mm standard, 125mm for long runs

Duct Run Length

Straight-line length to external wall/roof

Number of 90° Bends

Each bend = ~1m extra effective duct length

Room has openable window

Affects background vent requirements

Fan Price (optional)

Extract fan unit price

Duct Price per Metre (optional)

Including connectors

How We Calculate This

This calculator determines the extract fan capacity, duct configuration and background ventilation needed for your bathroom in compliance with Approved Document F (2021).

The formula

Fan capacity = Required rate ÷ ((1 − Duct loss) × Efficiency)

Purge rate = Room volume × 4 ACH ÷ 3600 (in L/s)

For a 6 m³ bathroom with 15 L/s required, 3m duct + 2 bends (5m effective), 15% duct loss, 80% fan efficiency: 15 ÷ (0.85 × 0.8) = 23 L/s fan needed.

Part F extract rates (2021)

Bathroom (intermittent): 15 L/s
Bathroom (continuous): 8 L/s
Kitchen (intermittent): 30 L/s adjacent to hob, 60 L/s elsewhere
Utility room: 30 L/s intermittent
Purge: 4 air changes per hour via openable window or fan

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.