Air changes per hour calculator
Airflow over volume, in metric and imperial units.
Your inputs
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There is no figure yet: nothing is calculated until you ask for it.
- Enter your measurements.
- Check the units and the assumptions.
- Press Calculate, or load the example.
Rather see a worked case? Press “Load example” and change any value afterwards.
Next step
The air coming in is thermal load: it belongs in the sizing of the unit.
Where each number comes from Conversion constants are exact; densities and coverage rates come from cited sources; everything else is an assumption you can change. Methodology · Sources and updates
How to use it
- Choose whether you are working out air changes or the airflow needed.
- Enter the room dimensions, or its volume if you already know it.
- Enter the airflow in CFM, m3/h or L/s.
- The result includes the minutes one full air change takes.
How it is calculated
Air changes per hour is a division of airflow by volume:
The imperial formula multiplies by 60 simply because CFM is cubic feet per minute while air changes are counted per hour. Same calculation, different time unit.
An ACH of 3 means the room's air is replaced three times an hour, that is, once every twenty minutes.
Worked example
A 16 x 13 ft room with an 8 ft ceiling and an 88 CFM extractor:
- Volume: 16 x 13 x 8 = 1,664 ft³ (47 m³)
- ACH: 88 x 60 / 1,664 = 3.2 air changes/h
- In metric: 150 m³/h
- One full change every 19 minutes
Good to know
- Required air changes depend on use. A bedroom, a commercial kitchen and a laboratory are not comparable, and the applicable code decides.
- Rated and real airflow differ. An extractor loses flow to duct length, bends and grilles.
- Ventilation costs energy too. Incoming air has to be conditioned: more air changes means more cooling load.
- Check your regulations. This calculator does the arithmetic; it does not tell you how many air changes your case requires.
Frequently asked questions
How do you calculate air changes per hour?
Divide the airflow by the room volume. With 150 m³/h in a 50 m³ room, that is 3 air changes per hour.
What is the CFM formula for ACH?
ACH = CFM x 60 / cubic feet. The 60 is there because CFM is per minute and air changes are counted per hour.
How many air changes per hour do I need?
It depends on the use of the space and the code that applies. This calculator does the arithmetic; the required figure comes from your local regulations.
What airflow do I need for 4 air changes?
Multiply 4 by the room volume. For 1,664 cubic feet you would need about 111 CFM, or 188 m³/h.
Does ventilation affect the size of the air conditioner?
Yes: incoming outdoor air has to be cooled. More air changes means more cooling load and therefore more capacity needed.