What this calculator does
Air changes per hour (ACH) works out ventilation rate for a room, from fan airflow and room volume. Enter your own figures above and the answer updates as you type: nothing is fixed in the code, so the result reflects exactly the numbers you supply.
The formula this calculator evaluates is printed under the tool and explained below, so you can check the working by hand or reuse it in a spreadsheet.
The formula
FormulaACH = (airflow × 60) / room volume
The inputs explained
| Field | What to enter |
|---|---|
| Airflow (fan/HVAC rating) (CFM) | A number, measured in CFM. Starts at 150. |
| Room length (m) | A number, measured in m. Starts at 5. |
| Room width (m) | A number, measured in m. Starts at 4. |
| Ceiling height (m) | A number, measured in m. Starts at 2.4. |
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.
How the answer changes with airflow (fan/hvac rating)
Every other input is held at the calculator’s starting values while airflow (fan/hvac rating) varies. Select any row to load that scenario into the calculator.
How the answer changes with airflow (fan/hvac rating)
| Airflow (fan/HVAC rating) (CFM) | Air changes per hour | Room volume | Time for one full air change |
|---|---|---|---|
| 75 | 2.7 ACH | 48.0 m³ (1,695 ft³) | 22.6 min |
| 113 | 4.0 ACH | 48.0 m³ (1,695 ft³) | 15.0 min |
| 150 | 5.3 ACH | 48.0 m³ (1,695 ft³) | 11.3 min |
| 225 | 8.0 ACH | 48.0 m³ (1,695 ft³) | 7.5 min |
| 300 | 10.6 ACH | 48.0 m³ (1,695 ft³) | 5.7 min |
| 450 | 15.9 ACH | 48.0 m³ (1,695 ft³) | 3.8 min |