Airflow / ACH Calculator
Enter room volume and target air changes per hour to get the required airflow in CFM, L/s and m³/h — or verify the ACH an existing fan delivers.
Last updated: 2026-09-15
How the calculation works
- One air change replaces the full room volume once; per minute that is volume/60, multiplied by the ACH target.
- The result is shown in CFM, litres per second and cubic metres per hour for international fans.
Formula
CFM = (Volume × ACH) / 60 1 CFM ≈ 0.472 L/s ≈ 1.699 m³/h
| Symbol | Meaning | Unit |
|---|---|---|
CFM | Airflow | ft³/min |
Volume | Room volume | ft³ |
ACH | Air changes per hour | 1/h |
Worked example
Interpreting the result
The ACH target depends on the room's job: bathrooms and kitchens need high rates to remove moisture and odors quickly; bedrooms need moderate rates for air quality; whole-house ventilation runs at a low continuous rate. Exceeding the target wastes heating and cooling energy, so pick the target for the room type, not the maximum.
Assumptions
- Well-mixed room air — real rooms stratify, and effective ventilation is lower than the theoretical value.
- Fan rating at typical static pressure; real flow drops with ducting resistance.
Limitations
- Does not account for duct losses, backdraft dampers or filter loading.
- Codes (IMC/ASHRAE 62.2) may specify airflow directly rather than ACH — check local requirements.
Frequently asked questions
How many CFM do I need for a bathroom?
The common rule is 1 CFM per ft² of floor area, which for a standard bathroom equals about 8 ACH. A 10 × 10 bathroom needs ~100 CFM.
What ACH should a bedroom have?
5–6 ACH with the door closed is a common target for air quality; whole-house continuous ventilation is much lower, around 0.35 ACH.