About this calculator
Size an extractor fan in l/s against the Approved Document F minimum extract rates and an air-change check.
How it’s calculated
Approved Document F intermittent extract minimums: bathroom 15 l/s, kitchen 30 l/s with a cooker hood or 60 l/s elsewhere, utility 30 l/s, WC 6 l/s.
Worked example
With these inputs:
- Room
- Bathroom
- Room length
- 3 m
- Room width
- 2.5 m
- Ceiling height
- 2.4 m
the result is:
- Extract rate
- 15 l/s
- Approved Doc F intermittent minimum
- 15 l/s
- 8 air changes per hour
- 40 l/s
- Suggested duct
- 100 mm
Tips from the trade
- Rated airflow falls off with long or flexible ducts. Keep runs short and smooth.
- Duct to outside, never into the loft or attic.
Results are estimates for planning and pricing. Check quantities against your drawings, the manufacturer’s data and the Building Regulations before ordering.