Extract Fan Sizing Calculator — Kitchens, Bathrooms & WCs

Per Approved Document F (2021) Tables 1.1 and 1.2, a bathroom requires a minimum intermittent extract rate of 15 l/s (or 8 l/s if using continuous mechanical extract). A standard 100mm axial fan rated at 15+ l/s will meet the intermittent requirement for most domestic bathrooms. Where the room has no openable window, the fan must also have a 15-minute overrun timer (ADF 2021 para 1.51); with an openable window the overrun is good practice but not mandatory. For larger bathrooms or those with long duct runs, a centrifugal/in-line fan may be needed to overcome duct resistance.

Under current Building Regulations (Approved Document F 2021), bathrooms (with a bath or shower) are wet rooms that require dedicated extract ventilation. For new builds and major renovations, Part F is met by natural ventilation with background ventilators plus an intermittent extract fan, continuous mechanical extract ventilation, or mechanical ventilation with heat recovery (the three system types in ADF 2021 Table 1.6). An openable window provides purge ventilation but does not on its own replace the required wet-room extract fan.

Approved Document F 2021 Table 1.1 specifies 30 l/s minimum intermittent extract where a cooker hood extracts to the outside, or 60 l/s where there is no cooker hood or the hood does not extract to the outside (e.g. a recirculating hood). If using continuous mechanical extract (MEV/dMEV), Table 1.2 sets a 13 l/s high rate for kitchens. The intermittent rate applies when the fan is switched on during cooking, while continuous runs at a low background rate at all times.

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.

Approved Document F 2021 (para 1.51) requires a 15-minute overrun specifically where a room has no openable window: the intermittent extract fan must keep running for at least 15 minutes after the room is vacated, to clear residual moisture. A bathroom, shower room or WC that does have an openable window is not obliged to have an overrun timer, though fitting one is good practice. Kitchen extract fans do not require an overrun timer — they are typically manually controlled or linked to a humidistat. Note also para 1.48: a wet room with no external walls must extract at four air changes per hour to meet the purge standard.