Air changes per hour calculator
To calculate air changes per hour, enter the room dimensions and the airflow, or the other way round the wanted rate; the fresh air needed for a number of people based on CO₂ is possible too.
Example: What do you know: Airflow (fan, ventilation) · Room length 5 m · Room width 4 m · Room height 2.5 m · Airflow 100 m³/h → Result: 2 air changes per hour at 100 m³/h airflow. Source: Wikipedia – Luftwechselrate (air change rate): n = V̇ / V, time until air exchange with ideal mixing. Updated: .
How it is calculated
The air change rate is the airflow per hour divided by the room volume: n = V̇ / V. A room of 50 m³ in which a fan moves 100 m³/h has 2 air changes per hour; in theory all the air is exchanged every 30 minutes.
How it is calculated
The volume is length · width · height. Airflow and volume give n; the reverse gives the airflow needed for a wanted rate. With ideal mixing, only the fraction e−n·t of the old air remains after time t: after 1/n, 63 % is exchanged; after ln(20)/n (about 3/n), 95 %. For fresh air by number of people the Pettenkofer calculation uses CO₂ output, limit and outdoor value.
Good to know
- A seated person gives off about 20 litres of CO₂ per hour; to stay at 1000 ppm with 420 ppm outdoors, that takes about 34 m³ of fresh air per hour.
- Opening windows wide for a short time exchanges air much faster than leaving them tilted.
- For legally binding ventilation concepts (residential, workplaces) national standards apply; the calculator only provides the physics.
Frequently asked questions
How do you calculate air changes per hour?
Divide the airflow in m³/h by the room volume in m³: n = V̇ / V. With 100 m³/h and a 50 m³ room that is 2 per hour.
How much fresh air does a person need?
Per Pettenkofer, with 20 l/h CO₂ output, a 1000 ppm limit and 420 ppm outdoors, about 34 m³/h. The calculator uses your values.
How long does a complete air change take?
At n = 1 per hour, 63 % of the air is exchanged after an hour and 95 % after about three hours – with ideal mixing.
What air change rate should a living room have?
That depends on use and standards. A hygienic minimum of about 0.5 per hour is often quoted; the actual requirements are set by national standards and building codes.
Sources and legal basis
- Wikipedia – Luftwechselrate (air change rate): n = V̇ / V, time until air exchange with ideal mixing
- Umweltbundesamt – Gesundheitliche Bewertung von Kohlendioxid in der Innenraumluft (1000 ppm guideline value I, 2000 ppm guideline value II)
- Wikipedia – Pettenkoferzahl: fresh-air requirement based on CO₂ content
As of: · Editorial policy
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