An extractor fan is sized by how often it needs to replace the room’s air — the air changes per hour (ACH). Multiply the room volume by the ACH for that type of room, divide by 60, and you have the airflow in CFM to look for on the fan.
The formula
Airflow (CFM) = room volume (cu ft) × ACH ÷ 60. A 13 × 10 × 8 ft bathroom is about 1,040 cu ft; at 8 ACH that is 1,040 × 8 ÷ 60 ≈ 140 CFM (about 66 L/s or 240 m³/h).
Air changes per hour by room
Wetter, smellier and busier rooms need more changes:
| Room | Typical ACH |
|---|---|
| Bedroom | ~4 |
| Living room / office | ~6 |
| Kitchen / bathroom | ~8 |
| Restaurant / gym | ~10 |
These are rules of thumb — your local building regulations set the rate that actually applies.
Ventilation Calculator
Enter the room size and pick a room type for a typical ACH — it returns the airflow in CFM (L/s, m³/h), ready to pick a fan or size a duct.
Don’t forget air per person
Occupied rooms are often sized by fresh air per person instead — commonly about 20 CFM per person (roughly 10 L/s). For a meeting room or classroom, work out both the ACH figure and the per-person figure and design for the larger of the two: a packed small room needs more air than its volume alone suggests.
Extract or supply?
The airflow is the same either way — only the direction differs. Wet and smelly rooms (bathrooms, toilets, kitchens) are extract-ventilated so odours and moisture don’t spread; living and working spaces are supplied with fresh air. Either way the fan and duct must carry the airflow above.
Pick the fan on its rated airflow — and mind the duct
Buy a fan whose rated CFM meets or beats your figure, and remember a long or kinked duct throttles it: a fan rated 140 CFM in the box may move far less through 10 ft of flexible duct and two bends. Keep runs short and straight, or size up. For the rate that’s legally required, check your local regulations or a standard such as ASHRAE 62.1.