Energy & Workshop Math
Air Changes Per Hour Calculator
Compute ACH from room volume and airflow, in cubic metres per hour or CFM.
Enter your measurements
Results update as you type.
About this calculator
Enter internal room dimensions or a known volume, then airflow in m³/h or CFM. The result is hourly airflow divided by room volume, not proof that all original room air is replaced each hour. State whether the entered flow is outdoor air or recirculation; filtered-air equivalence requires a separate clean-air model.
How to measure
- Measure internal room dimensions or enter its known volume.
- Enter airflow in CFM or cubic metres per hour.
- State whether this is outdoor air or recirculating air; recirculation is not fresh-air ventilation.
How to use this calculator
- Enter internal room dimensions or a known volume.
- Choose the airflow unit and enter the airflow.
- State whether the airflow is outdoor or recirculating.
- Calculate for air changes per hour.
Formula
Room volume = length × width × height (internal dimensions)
Airflow in m³/h = CFM × 60 × 0.028316846592
ACH = airflow in m³/h ÷ room volume in m³
Assumptions
- The space is treated as a single well-mixed volume.
- Airflow is a steady rate over the period considered.
- Recirculating airflow is reported but flagged as non-ventilation.
Common mistakes
- Mixing hourly flow with per-minute flow.
- Calling recirculated flow outdoor-air ventilation.
- Treating ACH alone as an infection-control or code-compliance certificate.
Worked example
Try it: press “Load example” above the form.
300 m³/h into a 100 m³ room is 3.0 air changes per hour.
Limitations
No code requirement, ventilation design, duct sizing, pressure balance or contaminant modelling. ACH is only as meaningful as the airflow figure entered.
Frequently asked questions
What is one air change per hour?
It means an airflow equal to one room volume in one hour under the simple mixed-air model. Divide hourly flow by room volume after converting units.
Does this prove ventilation or infection-control compliance?
No. Outdoor supply, recirculated flow, filtration and mixing have different roles. Enter the stated flow context and obtain the applicable ventilation design rather than treating ACH alone as a safety certificate.
What does the worked example show?
300 m³/h into a 100 m³ room is 3.0 air changes per hour.
Which assumptions should I check before using the result?
No code requirement, ventilation design, duct sizing, pressure balance or contaminant modelling. ACH is only as meaningful as the airflow figure entered.
Sources
Sources explain the methods and any named product presets. Check your product label where coverage, yield or density differs.
- Omni: flow rate
Q = volume/time and area × mean velocity. ACH is airflow per hour divided by room volume; no required ventilation rate is recommended.
- US EPA: airflow, ACH and equivalent air changes
Hourly airflow divided by room volume, with outdoor-air versus treated-air context; no health or code certification.
Calculation documentation · Updated 2026-11-11 · Version 1.6.4. Estimates are planning aids. Confirm quantities, coverage and prices with your supplier before purchasing. Found an error? Report it.
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