Bathroom Extraction Rate Calculator — Fan Sizing Guide

Our bathroom extraction rate calculator returns the rate required by UK Building Regulations Approved Document F 2021 (Volume 1, dwellings). For dwellings, Part F sizes intermittent extract by a fixed minimum litres-per-second rate per room — it does notuse a room-volume × air-changes calculation. The fixed Part F figure is the governing result; we also show an optional, non-statutory design-headroom figure for reference.

Method 1: Part F minimum rates (governing)

Approved Document F Table 1.1 specifies these minimum intermittent extract rates for dwellings:

• Bathroom / Shower room / En-suite: 15 l/s (54 m³/hr)
• Toilet / WC: 6 l/s
• Kitchen: 30 l/s (ducted cooker hood adjacent to the hob) or 60 l/s (no hood / recirculating / extract elsewhere); 13 l/s if continuous
• Utility room: 30 l/s intermittent (8 l/s continuous)

So a bathroom measuring 2.5m × 2.0m × 2.4m (12 m³) needs 15 l/s — the same as any other bathroom, because the figure is fixed by room type, not room size. That maps to an ordinary 100mm (4 inch) axial fan rated 15–25 l/s (Manrose, Xpelair, Vent-Axia, Envirovent), not a commercial unit.

Method 2: Volume-based design headroom (optional, not Part F)

Some designers add headroom by applying a typical air-changes-per-hour (ACH) figure to the room volume. This is a design rule of thumb, not a Part F requirement, and we show it only as supplementary information:

Design rate (l/s) = Room volume (m³) × Design ACH ÷ 3.6

Using a defensible bathroom design figure of about 8 ACH (the Home Ventilating Institute recommends 8 ACH for intermittent spot ventilation; Vent-Axia design guidance gives 6–10 ACH), a 12 m³ bathroom works out at 12 × 8 ÷ 3.6 ≈ 27 l/s of headroom — above the 15 l/s minimum, but still well within a domestic fan. Where the two differ, install to whichever is higher for your situation, but the 15 l/s Part F minimum is the figure you must meet.

Typical design ACH ranges (industry rules of thumb)

• Bathrooms, shower rooms, en-suites: 6–8 ACH
• Toilets / WCs: 6–10 ACH
• Kitchens: 10–15 ACH
• Utility rooms: 6–10 ACH

These ACH values are conventional engineering design figures, not statutory rates. For dwellings the Part F l/s minima above always govern.