Air Change Rate Calculator
Written by Thierno Sadou Diallo, formula verified per our methodology • Last checked on 10/10/2026
The air change rate (ACH, air changes per hour) is calculated with airflow ÷ room volume. A 25 m³ living room ventilated at 100 m³/h has an ACH of 4 (the air is replaced 4 times per hour), which halves a pollutant concentration in about 10.4 minutes.
Explanation
The air change rate, expressed in ACH (air changes per hour), indicates how many times a room's total air volume is theoretically replaced by fresh air in one hour. It is the reference indicator for assessing the quality of ventilation in a space, whether a living room, an office, or a classroom: an ACH of 4 to 6 per hour is generally considered good ventilation for a home, while a lower ACH can favor the buildup of CO2, humidity, and indoor pollutants. This calculator goes beyond the simple ratio by modeling, in a simplified way, the dynamics of pollutant dilution in the room's air: assuming fresh air mixes instantly and uniformly with the existing air (a standard approximation in airflow engineering, called the perfectly-mixed single-volume model), a pollutant's concentration decays exponentially over time, at a rate directly determined by the ACH. The time needed to halve this concentration — a far more concrete and intuitive figure than the ACH alone for assessing how long a room needs to be ventilated — follows directly from the air change rate via the natural logarithm of 2, the same mathematical relationship used for the half-life of a medication that breaks down progressively. This calculator intentionally does not cover the minimum airflow rates required by code for mechanical ventilation depending on room type (kitchen, bathroom, bedroom), which vary according to the applicable regulation and the number of rooms in the home — only the universal physical calculation of the air change rate and its effect on pollutant dilution is covered here.
Example: a 25 m³ living room ventilated at 100 m³/h
Inputs
Airflow: 100 m³/h. Room volume: 25 m³.
Calculation
ACH = 100 ÷ 25 = 4 air changes per hour. Half-life time = (ln(2) ÷ 4) × 60 ≈ 10.4 minutes.
Result
In this living room, the air is replaced 4 times per hour, which halves a pollutant concentration in about 10 minutes and 24 seconds.
Frequently asked questions
Why does the half-life time depend only on the ACH, not on airflow or volume separately?
Because this time depends only on the RATIO between airflow and volume (the ACH itself), not on their individual values: the third test case for this calculator illustrates this well — a living room twice as large ventilated with twice the airflow keeps exactly the same ACH, and therefore the same half-life time, as the first case. A small, weakly ventilated room can thus have the same air change rate as a large, strongly ventilated room.
Does this model exactly match the reality of a ventilated room?
It is a useful but simplified approximation: in practice, fresh air never mixes instantly or perfectly with existing air, which creates zones where renewal is slower (room corners, poorly mixed areas) and others where it is faster (close to ventilation outlets). The perfectly-mixed model nonetheless gives a reliable order of magnitude and is widely used as a reference calculation in building engineering.
What ACH should I aim for in good indoor air quality?
Commonly cited benchmarks sit around 4 to 6 air changes per hour for a normally occupied home, with higher needs in rooms with heavy moisture or pollutant production (kitchen, bathroom) and lower needs in a bedroom at night, where too-fast renewal can also create uncomfortable drafts. These general benchmarks do not replace the minimum airflow rates set by the regulation applicable to your home, to be checked separately depending on the room type concerned — in the same way that good insulation (see our thermal resistance calculator) must be balanced with sufficient ventilation, or indoor moisture gets trapped.