Enter the room type, dimensions and measured extract rate. The calculator checks your ventilation against the UK Approved Document F (2021) minimum extract rates and converts it to air changes per hour.
Enter room details
Anemometer reading at the fan grille, or the fan’s rated duty from the dataplate. Leave at 0 to see the Part F minimum only.
Rates are indicative intermittent extract minimums under Approved Document F (England, 2021). Always verify against the current edition for your project. Results update automatically as you adjust values.
Approved Document F extract rates (England, 2021)
| Room | Minimum extract (l/s) |
|---|---|
| Bathroom / Shower Room | 15 l/s |
| Kitchen (fan adjacent to hob) | 30 l/s |
| Kitchen (fan elsewhere) | 60 l/s |
| Utility Room | 30 l/s |
| WC / Cloakroom | 6 l/s |
Frequently asked questions
How do I measure the extract rate of my fan?
The most accurate method is a vane anemometer held at the face of the grille. As a rough alternative, fan dataplates often quote a rated duty in m³/hr — divide by 3.6 to convert to l/s. Note that real-world performance is often lower than the rated duty due to duct resistance, especially with flexible or long ducting runs.
Why does inadequate ventilation cause condensation and mould?
Every person, shower, kettle and drying cycle adds moisture to the air. If extract fans cannot remove this moisture fast enough, humidity builds up. When moist air contacts a cold surface — a window, an uninsulated wall, a corner — the air cools to its dew point and water condenses. Sustained high humidity at a surface creates the conditions for mould growth.
My ventilation passes Part F but I still have mould — why?
Part F sets minimum extract rates, but passing that minimum doesn’t guarantee a condensation-free property. Other factors include thermal bridging (cold spots in walls, lintels, corners), heating levels, trickle ventilation (window vents) being blocked, high occupancy generating more moisture than the minimum fan can handle, and the fan only running intermittently. A full survey will check all of these.
Does this apply to all UK properties?
Approved Document F (England) covers new builds and material changes of use in England. Scotland, Wales and Northern Ireland have separate regulations with similar intent. Existing properties are not automatically subject to the current Part F rates, but they provide a useful benchmark for whether ventilation is adequate.
What if my ducting runs through a cold loft?
Uninsulated flexible ducting running through a cold loft is a common problem — warm, moist extract air cools as it travels, water condenses inside the duct, and the duct can drip back into the room or become a source of biological growth. The fix is smooth, rigid, pre-insulated ducting. A survey will flag this if found.
Ventilation failing — or not sure what’s causing the damp?
Damp Detectives provides independent professional damp surveys for homeowners, landlords and home buyers. We assess ventilation, condensation risk, thermal bridging, and structural damp — and give you a written report with the real cause and what to do about it.