Air Changes Per Hour Calculator
Air changes per hour from a ventilation rate and a room volume — or the airflow a room needs for a target rate — against the rates typical for each kind of space.
Air changes per hour is airflow divided by room volume: 120 liters a second into a 72 m³ room is six air changes an hour.
How the air changes per hour calculator works
Air changes per hour is airflow divided by room volume: 120 liters a second into a 72 m³ room is six air changes an hour. It is the unit ventilation standards and infection-control guidance are written in.
The reverse is what a designer needs: the flow a room must be supplied with to reach a target rate. A classroom wanting six changes an hour at 200 m³ needs 333 liters a second — a real duct, not a trickle vent.
Formula: ACH = flow (m³/h) / volume; flow (L/s) = ACH × volume / 3.6
Worked examples
| Inputs | Air changes per hour | Note |
|---|---|---|
| 120 L/s into a 73 m³ room | 5.93 | about 6 ACH |
| A trickle vent | 0.49 | half an air change |
| A larger room | 1.48 | the same flow, a quarter the rate |
FAQFrequently asked questions
What is an air change per hour?
The number of times the room's entire volume of air is replaced in an hour. It is how ventilation standards state their requirements.
How many air changes does a room need?
Homes run 0.5 to 1 naturally; offices and classrooms are designed for 2 to 6; hospital isolation rooms 12 or more.
Why is per-person flow also shown?
Because standards often set both: a minimum per person, typically 8 to 10 L/s, and a minimum rate for the space. The stricter one governs.
Does a higher rate always mean fresher air?
Only if the air is fresh. Recirculated air changes the room's volume without changing its CO₂. Outdoor-air changes are what matter for air quality.
How do I measure the actual rate?
With a CO₂ decay test: fill the room, stop the source, and time how long the level takes to fall. It is the only way to know what a room really achieves.
Where these figures come from
- Sabine (1922) — Collected Papers on Acoustics — the reverberation-time equation
- CIBSE Guide A — Environmental design — daylight, comfort and ventilation criteria
- Neufert — Architects' Data — the standard reference for space and dimension
- International Code Council — International Building Code — occupant load and egress factors
- International Code Council — publisher of the model codes most US jurisdictions adopt
Last checked: September 2026. Formulas are the standard building-science and code relationships; default thresholds are common international values and should be replaced with the ones in your code.