What this calculator does
Air changes per hour is the airflow rate divided by the room volume. A 150 m³/h system serving a 54 cubic metre room delivers 2.78 changes an hour, replacing the air every 21.6 minutes.
That rate sounds respectable and is not. Living spaces generally want 4 to 6 changes an hour, so 2.78 leaves the room stuffier than intended. Getting there in this room needs about 300 m³/h, double what is installed, which is the kind of gap that only shows up once the arithmetic is done.
The formula
The airflow in cubic metres per hour is divided by the room volume in cubic metres. This calculator takes airflow in m³/h, which is how most European and Australian fans are rated. If yours is rated in cubic feet per minute, use the companion calculator that takes CFM directly rather than converting by hand.
| Term | Meaning |
|---|---|
| ACH | Air changes per hour: room volumes of air delivered each hour. |
| m³/h | Cubic metres per hour, the metric airflow rating. 1 m³/h is about 0.589 CFM. |
| Rated against delivered airflow | Ductwork, bends and grilles reduce what a fan actually achieves. |
| Make-up air | Air entering to replace what is extracted, without which the fan cannot perform. |
The inputs explained
| Field | What to enter |
|---|---|
| Room length (m) | Room length in metres. |
| Room width (m) | Room width. |
| Ceiling height (m) | Ceiling height. |
| Airflow rate (m³/h) | Airflow rate in cubic metres per hour, from the fan or system rating. |
When to use it
Specifying mechanical ventilation
Design rates are given in ACH, and this converts a fan rating into that figure.
Checking a heat recovery unit
MVHR units are rated in m³/h and must deliver an adequate change rate across the dwelling.
Diagnosing a stuffy room
A room that feels stale usually has a change rate well below what it needs.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
What does each airflow rate deliver?
A range of airflow rates in the same room.
| Airflow rate | Air changes per hour | Time for one full air change | Rating |
|---|---|---|---|
| 50 m³/h | 0.93 | 64.8 minutes | Low (typical closed room) |
| 150 m³/h | 2.78 | 21.6 minutes | Low (typical closed room) |
| 300 m³/h | 5.56 | 10.8 minutes | Moderate (typical occupied room) |
| 600 m³/h | 11.11 | 5.4 minutes | High (industrial/clean-room level) |
Questions
What is a good air change rate?
Around 4 to 6 per hour for living spaces and 8 to 15 for bathrooms and kitchens. Bedrooms can sit lower if the air is fresh. Requirements vary by jurisdiction and by whether ventilation runs continuously or on demand, so check the local standard.
How do I convert CFM to m³/h?
Multiply CFM by 1.699. A 150 CFM fan is about 255 m³/h. Rather than converting by hand, the companion calculator takes CFM directly, which avoids an error that is easy to make and hard to notice.
Why does my system not achieve its rated flow?
Because ratings are usually measured in free air. Duct runs, bends, filters and external grilles all add resistance, and the delivered flow can be a third below the rating or worse. Size against the target with margin rather than exactly at it.
Is higher always better?
No. Over-ventilating wastes heat in winter and cooling in summer, since every air change brings in outside air that must be conditioned. This is why heat recovery ventilation exists: it achieves the change rate while recovering most of the energy that would otherwise go out with the stale air.
For airflow rated in CFM, see the air changes per hour calculator. For heating capacity, see the furnace sizing calculator.