Ventilation · Airflow
CFM Calculator
Exhaust fan airflow = room volume × air changes per hour. A standard bathroom with a shower needs 15–25 air changes per hour — pick your room type below and get the fan size in CFM and m³/h.
Room
- Minimum airflow
- 71 m³/h
- Maximum airflow
- 177 m³/h
- Room area
- 4.83 m²
- Room volume
- 11.78 m³
Exhaust fan · Bathroom (Toilet Only)
42–104CFM
6–15 air changes per hour
Air changes per hour by room type
Recommended ventilation rates used by the calculator.
| Room type | Min ACH | Max ACH |
|---|---|---|
| Bathroom (Toilet Only) | 6 | 15 |
| Bathroom with Shower | 15 | 25 |
| Bedroom | 5 | 8 |
| Cafe | 15 | 25 |
| Computer Room | 6 | 10 |
| Factory / Workshop | 6 | 10 |
| Garage | 6 | 8 |
| Commercial Kitchen | 20 | 30 |
| Domestic Kitchen | 15 | 25 |
| Laundry (No Dryer) | 6 | 15 |
| Laundry (With Dryer) | 15 | 25 |
| Commercial Laundry | 11 | 20 |
| Office | 4 | 6 |
| Domestic Roof Cavity | 10 | 10 |
| Sub Floor | 6 | 10 |
| Spa Bathroom | 15 | 25 |
The formula
volume(m³) = length × width × height
CFM = volume × ACH ÷ 1.698
m³/h = CFM × 1.699Example: a 2.5 × 2 × 2.5 m shower bathroom is 12.5 m³. At 15–25 ACH that is 110–184 CFM (188–313 m³/h).
FAQ
- How is exhaust fan CFM calculated?
- Multiply the room volume in cubic meters by the recommended air changes per hour (ACH) for that room type, then divide by 1.698 to convert m³/h to CFM. A bathroom with a shower needs 15–25 ACH; an office only 4–6.
- What are air changes per hour (ACH)?
- ACH is how many times the fan replaces the room's entire air volume in one hour. Humid or odor-prone rooms (showers, kitchens, laundries with dryers) need 15–30 ACH, while dry occupied spaces like bedrooms and offices need 4–8.
- Should I pick the minimum or maximum CFM?
- Size toward the maximum when the room is heavily used, poorly ventilated otherwise, or in a humid climate. The minimum is acceptable for lightly used rooms with some natural ventilation.
- What size fan does a typical bathroom need?
- An 8 × 6.5 ft bathroom with an 8 ft ceiling (about 12 m³) with a shower needs roughly 105–175 CFM. Toilet-only bathrooms need less — about 40–105 CFM for the same size.