Air Changes Per Hour Calculator

The Air Changes Per Hour Calculator determines how often room air is fully exchanged by ventilation equipment. Enter floor area, ceiling height, and airflow to get ACH for HVAC and indoor air quality planning.

ACH formula

Air changes per hour from CFM airflow is:

$$\mathrm{ACH} = \frac{\mathrm{CFM} \times 60}{\mathrm{Area} \times \mathrm{Height}}$$

CFM is cubic feet per minute. Multiplying by 60 converts to cubic feet per hour, then dividing by room volume gives hourly air exchange rate.

Typical ACH targets

4-6 ACH: Offices and classrooms
6-12 ACH: Kitchens and bathrooms
12-20 ACH: Workshops and garages
15-30 ACH: Hospitals and laboratories

Frequently Asked Questions

What does ACH mean?

ACH stands for air changes per hour. A value of 6 ACH means the entire room air volume is replaced six times every hour.

How do I calculate air changes per hour?

Multiply airflow in CFM by 60, then divide by room volume in cubic feet (area times ceiling height).

What ACH is needed for a bathroom?

Bathrooms typically need 6 to 12 ACH to control humidity and odors. Higher values improve moisture removal.

Can I use m³/h instead of CFM?

Yes. This calculator accepts m³/h and converts internally. 1 m³/h equals approximately 0.589 CFM.

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Air Changes Per Hour Calculator