Bathroom Ventilation Calculator — Part F Extract Rates & Duct Sizing

Under Part F of the Building Regulations (2021 edition), a bathroom needs 15 L/s (litres per second) for intermittent extract or 8 L/s for continuous extract. Intermittent fans run when the room is in use (usually switched with the light plus overrun timer). Continuous systems run 24/7 at a lower rate, boosting during use. An en-suite has the same requirements as a bathroom.

Purge ventilation is rapid ventilation at 4 air changes per hour (ACH) to quickly clear pollutants — for example, after painting or if there is a spillage. In bathrooms it is usually provided by an openable window. The opening area required (AD F 2021 Vol 1 Table 1.4) depends on the window type: for sliding/sash windows, external doors, and hinged or pivot windows that open 30° or more, at least 1/20th of the floor area; for hinged or pivot windows that open 15–30°, at least 1/10th of the floor area; windows opening less than 15° are not suitable for purge. If the bathroom has no openable window, the window can't provide purge, so an intermittent extract fan must instead extract at the 4-ACH purge rate directly to outside (AD F 2021 Vol 1 paras 1.26–1.31; para 1.48 states this explicitly where a wet room has no external walls). This calculator raises the required fan rate accordingly when the room has no openable window. Note that para 1.51 also requires the fan to keep running for at least 15 minutes after the room is vacated where there is no openable window.

As an installer rule of thumb (not a Part F figure), each 90-degree bend in the ductwork adds resistance roughly equivalent to about 1 metre of straight duct. On that basis a fan connected to 2m of duct with 2 bends has an effective duct length of around 4m. The added resistance reduces the fan's effective flow rate, so you need a fan with higher capacity to compensate. Use flexible ducting sparingly as it adds more resistance than rigid duct. These figures are approximate first estimates — for the final selection, check the fan against its manufacturer airflow-vs-static-pressure curve for your actual duct run.

100mm (4 inch) duct is the standard for most bathroom extract fans delivering up to 15-20 L/s. For longer duct runs (over 3m) or where there are multiple bends, step up to 125mm duct to reduce resistance. 150mm duct is used for high-capacity fans or very long runs. Always match the duct size to the fan spigot — do not reduce duct size downstream.

Yes. AD F 2021 Vol 1 para 1.52 states that all rooms with external walls should have background ventilators (trickle vents), and Table 1.7 sets the minimum equivalent area (EA): a bathroom needs at least 4000 mm²; habitable rooms and the kitchen need 8000 mm² (10,000 mm² in single-storey dwellings); a utility room and WC have no minimum. An openable window does not remove this requirement — the document is explicit that a window left in a night-latch position is not adequate for background ventilation (risk of draughts, security, and you cannot measure its equivalent area). If a bathroom has no external façade to fit a ventilator (Table 1.7 note 4), its 4000 mm² is added to the ventilators in other rooms instead. Trickle vents in windows or wall-mounted air bricks provide this. For windowless bathrooms, mechanical extract is mandatory and must be ducted to outside.