Free Extractor Fan Size Calculator
Find the minimum extractor fan size for a kitchen, bathroom, utility or WC to meet UK Building Regulations Part F, in l/s and m³/hr.
UK Building Regulations Approved Document F sets the minimum extract ventilation rate for every wet room. Pick the room and whether the fan runs intermittently or continuously, and this calculator gives the required extract rate in litres per second and the fan size in cubic metres per hour, the figure fans are sold by, so your installation is Part F compliant.
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Get a free demoHow to use the Extractor Fan Size Calculator
- 1Select the room you're ventilating. Kitchens have two figures: a lower one if a cooker hood extracts to outside, a higher one if not.
- 2Choose intermittent for a normal switch-operated extractor fan, or continuous for a whole-house / MEV system running at a low background rate.
- 3Read the minimum extract rate in litres per second. This is the Part F requirement.
- 4Use the fan size in m³/hr to choose a fan: pick one rated at or above this figure at the duct length you're installing (ducting reduces real-world airflow).
- 5For a continuous system, the sum of all continuous extract rates must also meet the whole-dwelling supply rate in Table 1.3.
How to work it out yourself
Part F gives the minimum extract rate directly as a table lookup per room: there's no formula to derive it. The only maths is converting the fan's rating between the two units.
- Bathroom, intermittent → 15 l/s from ADF Table 1.1.
- Fan size = 15 × 3.6 = 54 m³/hr.
- So choose a bathroom fan rated at least 54 m³/hr: at the actual duct length, not the free-air figure on the box.
- Kitchen with no hood → 60 l/s → 216 m³/hr, a much larger fan.
Rates are from Approved Document F Volume 1 (2021 edition), which applies to dwellings in England. Manufacturers quote free-air performance; ducting, bends and grilles cut real airflow, so size up and keep duct runs short and straight. Wiring an extractor fan is notifiable electrical work under Part P.
Typical UK benchmarks
| Kitchen (cooker hood to outside) | 30 l/s intermittent: ADF Vol 1 2021, Table 1.1 |
| Kitchen (no hood) | 60 l/s intermittent: ADF Vol 1 2021, Table 1.1 |
| Utility room | 30 l/s intermittent: ADF Vol 1 2021, Table 1.1 |
| Bathroom / en-suite | 15 l/s intermittent: ADF Vol 1 2021, Table 1.1 |
| WC / toilet | 6 l/s intermittent: ADF Vol 1 2021, Table 1.1 |
Disclaimer
This tool gives an estimate for general guidance only, based on typical UK figures and the details you enter. It is not a formal quote, survey, or professional advice. Always confirm with a qualified tradesperson before relying on any figure, as regional prices, site conditions, and current standards vary.
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Get a free demoFrequently asked questions
What size extractor fan do I need for a bathroom?
UK Building Regulations Part F require a minimum extract of 15 litres per second for a bathroom or en-suite, which is about 54 m³/hr. Choose a fan rated at or above that at your actual duct length: most standard 100mm/4-inch bathroom fans meet it, but a long or bendy duct run reduces real airflow, so allow headroom.
What are the Part F extract rates for each room?
For intermittent fans (Approved Document F 2021): kitchen 30 l/s if a cooker hood extracts to outside or 60 l/s if not, utility room 30 l/s, bathroom or en-suite 15 l/s, and a WC 6 l/s. Continuous systems use lower background rates (kitchen 13, utility 8, bathroom 8, WC 6 l/s).
How do I convert litres per second to m³/hr?
Multiply by 3.6. Fans are usually sold by their extract rate in cubic metres per hour, while the regulations are written in litres per second, so a 15 l/s bathroom requirement is 54 m³/hr and a 30 l/s kitchen requirement is 108 m³/hr.
Why is a kitchen without a cooker hood 60 l/s?
Because with no hood capturing steam and grease at the hob, the whole-room fan has to do all the work, so Part F doubles the requirement from 30 to 60 l/s. Fitting a cooker hood that extracts to outside lets you use the lower 30 l/s rate and a smaller fan.
Does fitting an extractor fan need Building Regulations approval?
Ventilation must meet Part F, and the electrical connection is notifiable work under Part P since it's often in a bathroom or kitchen. A registered electrician can self-certify the electrical side; for a new fan meeting the Part F rate you generally don't need a separate application, but the installation must comply.
