Extractor fan size calculator (Part F)
The calculator needs JavaScript. The worked example, formula and figures below show exactly how it works, so you can do the sum yourself. A guide, not advice. Check your supplier's or maker's figures before ordering.
Short answer
Worked example
| You enter | Value |
|---|---|
| Size the fan by | Part F minimum |
| Room | Bathroom or shower room |
| How the fan runs | Intermittent (switched) |
| Room length | 2.5 m |
| Room width | 2 m |
| Ceiling height | 2.4 m |
| Your fan's extract rate | 20 l/s |
Fan: at least 15 l/s (54 m³/h)
- Within the guidance Fan extract rate vs the Part F minimum: 20 l/s England: Approved Document F 2021 Vol 1, extract ventilation rates (Researcher to cite the table number). Check with the source
| Result | Value |
|---|---|
| Minimum extract rate | 15 l/s |
| Minimum extract rate | 54 m³/h |
| Air changes per hour with your fan | 6 /h |
| Room volume | 12 m³ |
Show the working
- Room volume: 2.5 × 2 × 2.4 m = 12 m³
- Part F minimum (intermittent) for this room: 15 l/s = 15 × 3.6 = 54 m³/h
- Your fan: 20 l/s × 3.6 = 72 m³/h
- Air changes: 72 m³/h ÷ 12 m³ = 6 /h
How it works
- Part F mode: the minimum intermittent extract rate for the room type from Approved Document F (England)
- Air change mode: m³/h = room volume × air changes per hour
- l/s = m³/h ÷ 3.6; air changes a fan gives = fan m³/h ÷ room volume
Box ratings are at free air: long or narrow ducts and bends cut the real flow, so keep ducts short and choose a fan with some margin.
In a bathroom the fan's IP rating and position depend on the zones in BS 7671 Section 701. Wiring in a bathroom is notifiable work in England and Wales.
Our default (adjustable)
These are typical planning figures and our own allowances, not facts about your product. Change any of them in the calculator to match your bag, pack or tin.
This calculator has no built-in defaults: every figure comes from you or your product's data.
Regulations and standards
- Approved Document F 2021 Vol 1 (minimum extract ventilation rates), England
- BS 7671:2018+A4:2026 Section 701 (bathroom zones)
Explained: Approved Document F (ventilation), BS 7671 (the IET Wiring Regulations).
Our extract rates are England's Approved Document F. Wales, Scotland and Northern Ireland have their own guidance, which we're still adding.
| Table | Clause | Nations | Status |
|---|---|---|---|
| Minimum extract ventilation rates (Part F) | Approved Document F 2021 Vol 1, extract ventilation rates (Researcher to cite the table number) | England | 5/10 values entered Not yet checked against the source |