What size extract fan do I need for a UK bathroom?

Per Approved Document F (2021) Table 5.1b, a bathroom requires a minimum extract rate of 8 l/s when operated intermittently. A standard 100mm axial fan rated at 8-15 l/s will meet this requirement for most domestic bathrooms. The fan must have an overrun timer of at least 15 minutes. For larger bathrooms or those with long duct runs, a centrifugal/in-line fan may be needed to overcome duct resistance.

Do I need an extract fan if my bathroom has a window?

Under current Building Regulations (2021 onwards), mechanical extract ventilation is strongly recommended for all bathrooms, even those with openable windows. For new builds and major renovations, Part F requires either System 1 (background ventilators plus intermittent extract) or System 3 (continuous mechanical extract). Opening a window alone is no longer considered sufficient to meet current requirements.

What is the minimum kitchen extract rate in the UK?

Approved Document F specifies 13 l/s minimum extract for a domestic kitchen using a general extract fan, or 30 l/s for a recirculating cooker hood, or 60 l/s for a ducted cooker hood. These are intermittent rates activated during cooking. If using continuous extract (System 3), the minimum background rate is 13 l/s with no boost needed.

Should I choose an axial or centrifugal fan?

Axial fans are cheaper and simpler but cannot overcome much duct resistance — they are suitable for wall-mounted installations with short, straight duct runs (under 1.5m). Centrifugal or in-line fans generate much higher static pressure and should be used for longer duct runs, multiple bends, or roof-mounted terminals. As a rule: if the total duct run exceeds 3m or has more than 2 bends, use a centrifugal fan.

What overrun timer is required by Building Regulations?

Approved Document F requires that intermittent extract fans in bathrooms, shower rooms, and WCs have an overrun timer of at least 15 minutes after the room is vacated or the light is switched off. This ensures residual moisture is removed. Kitchen extract fans do not require an overrun timer — they are typically manually controlled or linked to a humidistat.

Extract Fan Sizing Calculator — Kitchens, Bathrooms & WCs

How We Calculate This

This calculator sizes extract fans for domestic rooms using the minimum extract rates specified in Approved Document F (2021 edition) Table 5.1b. These rates are the legal minimum for new dwellings and major renovations in England and Wales.

How it works

The calculator looks up the minimum intermittent and continuous extract rates for your room type, applies a 20% uplift for windowless (internal) rooms where natural background ventilation is absent, and then recommends an appropriate fan size and duct diameter.

Duct pressure estimation

The static pressure the fan must overcome is estimated from the duct length, number of bends, and grille resistance. This determines whether an axial fan (low pressure) or centrifugal fan (high pressure) is needed. Each 90-degree bend adds approximately 2.5 Pa of resistance, and intake/exhaust grilles typically add 10 Pa.

ADF Table 5.1b rates

Kitchen: 13 l/s (or 30-60 l/s cooker hood)
Bathroom / Shower: 8 l/s with 15-min overrun
WC: 6 l/s with 15-min overrun
Utility: 8 l/s

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.