Extract Fan Sizing Calculator — Kitchens, Bathrooms & WCs

This calculator sizes extract fans for domestic rooms using the minimum extract rates specified in Approved Document F (2021 edition) Tables 1.1 and 1.2. 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 and then recommends an appropriate fan size and duct diameter. The regulations set two separate, independent conditions for wet rooms. Approved Document F 2021 para 1.48 applies where a wet room has no external walls (an internal room): the intermittent fan must extract at least four air changes per hour to meet the purge ventilation standard, so the calculator uses the greater of this and the Table 1.1 minimum. Separately, para 1.51 applies where a room has no openable window: the fan must keep running for at least 15 minutes after the room is vacated (overrun timer). A room can meet one condition without the other — an internal room with no external walls triggers the 4 ACH purge rule whether or not it has a window, while a room with an external wall but no openable window needs only the overrun. There is no across-the-board percentage uplift for internal rooms in the regulations.

Duct pressure estimation (indicative)

The static pressure the fan must overcome is estimated from the duct length, number of bends, and grille resistance, to indicate whether an axial fan (low pressure) or centrifugal/in-line fan (higher pressure) is needed. The figure is an indicative ballpark, not a regulatory value: it assumes roughly 1–1.5 Pa/m for smooth duct, about 2.5 Pa per 90° bend and around 10 Pa for grilles. Flexible duct can be three to four times more resistive (~3–4 Pa/m), so always size the fan against the manufacturer’s resistance chart for your actual duct.

ADF 2021 Tables 1.1 and 1.2 rates

• Kitchen: 30 l/s intermittent where a cooker hood extracts to outside, 60 l/s where there is no hood or it does not extract outside (e.g. recirculating), 13 l/s continuous high rate
• Bathroom / Shower: 15 l/s intermittent (8 l/s continuous high rate); 15-min overrun required if no openable window (para 1.51)
• WC (sanitary accommodation): 6 l/s intermittent and continuous; 15-min overrun required if no openable window (para 1.51)
• Utility: 30 l/s intermittent (8 l/s continuous high rate)