How this calculator works
Approved Document F gives the least rate an extract fan must achieve in each wet room. An intermittent fan (switched on, or on a humidity sensor) needs 15 l/s in a bathroom, 30 l/s in a utility room, 6 l/s in a separate WC and 30 l/s in a kitchen where the cooker hood extracts to the outside, or 60 l/s if it does not. Fans that run all the time have lower boost rates: 13 l/s in a kitchen, 8 in a utility room and bathroom, and 6 in a WC.
Worked example: a bathroom fan needs 15 l/s, which is 54 m³/h. A kitchen fan needs 30 l/s (108 m³/h) with a hood that extracts outside, and 60 l/s (216 m³/h) without. A 3-bedroom, 90 m² home needs a whole-dwelling rate of 31 l/s (the bedroom figure beats 0.3 × 90 = 27); a 4-bedroom, 140 m² home needs 42 l/s.
Assumptions and sources
- These are the minimum rates in Approved Document F Volume 1 (2021 edition, dwellings). A fan must achieve its rate in the installed system, so allow for the duct length and bends: the rating on the box is measured with no duct. Fans should be fitted as high as practical in the room and no more than 400 mm below the ceiling. For a kitchen the cooker hood must extract outside to use the 30 l/s rate, and it should span the full width of the hob. Intermittent fans give the room its rate when they run, and the home still needs background or whole-house ventilation. A competent installer should size and commission the system, and electrical work in a bathroom must meet the wiring regulations (Part P).
The formula and a worked example are shown above, so you can check any result by hand. Read how we calculate and check our numbers. Results are estimates for planning: supplier products vary, so check the size printed on the label.
Minimum extract rates, Approved Document F
Intermittent extract rates (Table 1.1) with the equivalent in m³/h.
| Room | l/s | m³/h |
|---|---|---|
| Kitchen (hood extracting outside) | 30 | 108.0 |
| Kitchen (no hood or recirculating) | 60 | 216.0 |
| Utility room | 30 | 108.0 |
| Bathroom | 15 | 54.0 |
| Sanitary accommodation (WC) | 6 | 21.6 |
Tips before you buy
- The 30 l/s kitchen rate needs a hood that extracts outside, wide enough for the whole hob. A recirculating hood does not count, so you would need 60 l/s from a fan in the wall or ceiling.
- Fans are marked in l/s or m³/h. To convert, multiply litres per second by 3.6. Check the rate quoted is for the installed fan with its ducting, not the free-air figure.
- Long or narrow ducts and sharp bends cut the airflow. Use the shortest, straightest duct, in smooth rigid pipe, and insulate it where it runs through a cold loft to stop condensation running back.
- In a bathroom with a shower, a humidity-sensing fan with overrun is sensible. Bathroom fans have electrical zone rules: check the IP rating.
- A very airtight home needs a planned whole-house system, with background ventilators or mechanical supply to replace the air the fans take out.
Tools & materials for this project
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Frequently asked questions
What extract fan rate does a bathroom need in the UK?
15 l/s (54 m³/h) for an intermittent fan, from Table 1.1 of Approved Document F. A continuous fan needs a high rate of 8 l/s.
How many l/s for a kitchen extractor fan?
30 l/s if the cooker hood extracts outside, and 60 l/s if there is no hood or it recirculates.
How do I convert l/s to m³/h?
Multiply by 3.6. 15 l/s is 54 m³/h and 30 l/s is 108 m³/h.
What is the minimum extract rate for a toilet?
6 l/s (21.6 m³/h) for a separate WC, or use purge ventilation such as an openable window.
What is the whole-dwelling ventilation rate?
The higher of 0.3 l/s per m² of floor area and the bedroom figure in Table 1.3 (19 l/s for one bedroom, plus 6 for each extra). A 3-bedroom, 90 m² home needs 31 l/s.
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Estimates only. Results assume standard materials and simple shapes. Supplier products vary, and the final call on quantity, structure and safety is yours. Check your local code and your supplier's guidance before ordering.