TradeCalculator.co.uk
Bathroom Extraction Rate Calculator — Fan Sizing Guide
Calculate the required ventilation extraction rate for your bathroom, shower room or kitchen. Based on UK Building Regulations Approved Document F (ventilation).
Longest dimension of the room
Shortest dimension of the room
Standard UK ceiling height is 2.4m
Next up: related pages
How We Calculate This
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.
Frequently Asked Questions
Under UK Building Regulations Approved Document F 2021 (Volume 1, Table 1.1), a bathroom, shower room or en-suite requires a minimum intermittent extraction rate of 15 litres per second (l/s) — equivalent to 54 m³/hr. A separate toilet/WC needs 6 l/s. Part F sizes dwelling extract by these fixed per-room minimum rates, not by a room-volume calculation, so 15 l/s applies whether the bathroom is small or large. An ordinary 100mm (4 inch) axial fan typically delivers 15–25 l/s, which meets the requirement for most bathrooms.
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 and Wet Room Tanking Calculator.
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.
It depends on whether the room has an openable window. Approved Document F requires a 15-minute over-run only for a room with no openable window — an internal bathroom or WC. There, the fan must keep running for at least 15 minutes after the light is switched off (or after the room is vacated with a PIR sensor) to clear residual moisture. For a bathroom with an openable window, an over-run timer is recommended best practice but is not a regulatory requirement. Humidistat fans that run automatically when humidity is high are also acceptable.
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.
Related Calculators
Was this calculator helpful?
Last updated: February 2026
Verified against UK standards · estimates only, confirm with your supplier.