What ACH means
Air changes per hour is the hourly volume flow divided by the enclosed room volume. It is a nominal ratio, not proof that every parcel of room air is replaced evenly or that a ventilation, filtration, contaminant-control, pressure, or comfort requirement is satisfied.
ACH and CFM formulas
CFM = room volume (ft³) × ACH ÷ 60
ACH = m³/h ÷ room volume (m³)
Use the ACH Calculator when CFM is known. Use the CFM Calculator when the target ACH and dimensions or room volume are known.
Should ACH use supply or return airflow?
There is no one correct stream for every ACH question. Use the flow that matches the decision and label the result. ASHRAE defines supply air as potentially containing outdoor, recirculated, or transfer air, so total supply is not automatically outdoor ventilation. Return air is air routed back toward the system and is not automatically the clean-air delivery or exhaust-removal rate.
| Question | Flow to use | Interpretation limit |
|---|---|---|
| Nominal mechanical supply turnover | Measured or scheduled room supply airflow | May include recirculated air; does not equal outdoor-air ACH unless documented as 100% outdoor air. |
| Outdoor-air ventilation ACH | Outdoor airflow delivered to the defined room | Do not substitute total supply when only part of it is outdoor air. |
| Exhaust-based removal ACH | Measured or scheduled room exhaust airflow | Does not show replacement-air source, distribution, or containment by itself. |
| Filtered or treated equivalent ACH | Clean-air delivery defined by the applicable method | Fan airflow alone may not equal effective clean airflow. |
| Return-system flow | Measured return airflow only when return ACH is the stated question | Return is not a universal replacement for supply, outdoor, exhaust, or clean-air flow. |
When supply and return CFM are different
Keep the calculations separate rather than averaging the readings or selecting whichever value gives the preferred ACH. At steady conditions, a difference can be balanced by exhaust, intentional transfer, door undercuts, leakage, infiltration, or exfiltration. The difference is evidence for an air-balance and pressure-path review, not automatically additional ventilation.
For example, in a 2,400 ft³ room, 500 CFM of supply corresponds to 12.5 supply ACH, 400 CFM of return corresponds to 10 return ACH, and 50 CFM of exhaust corresponds to 1.25 exhaust ACH. These values describe three paths. They should not be added into 23.75 ACH or collapsed into one unlabeled “actual ACH.” The remaining flow balance must be found from the room and adjacent-space paths and confirmed with measurements.
Worked room-volume example
A 2,400 ft³ room with an identified 240 CFM airflow has 240 × 60 ÷ 2,400 = 6 ACH. The result should be recorded as supply ACH, outdoor-air ACH, exhaust ACH, return ACH, or another defined type—not simply “6 ACH.”
What ACH can and cannot tell you
| ACH can indicate | ACH cannot establish by itself |
|---|---|
| Nominal airflow relative to a stated room volume | Uniform mixing, short-circuiting, or actual contaminant-removal effectiveness |
| A normalized comparison between rooms when the same air-stream basis is used | Outdoor-air fraction, filtration efficiency, or whether different streams may be added |
| The airflow conversion associated with a documented ACH basis | Ventilation compliance, room pressure, equipment sizing, duct design, or safety |
Use ACH in the right workflow
- Define the room boundary and volume.
- Name the airflow stream and the decision it supports.
- Use measured, scheduled, or method-defined flow from the same operating condition.
- Calculate and label the ACH.
- Review supply, return, exhaust, outdoor air, transfer, leakage, and room pressure separately when balance matters.
- Use the applicable standard, code, owner criteria, and qualified review for requirements.
FAQ
Should ACH use supply or return airflow?
Use the stream that matches the purpose. Supply is common for nominal delivered-air turnover; exhaust is used for an exhaust-removal question; outdoor air is required for an outdoor-air ACH. Return airflow is not a universal default.
Can I add supply ACH and exhaust ACH?
Not when they are opposing paths through the same room volume. CDC guidance for its clean-air application specifically says to use the larger supply or exhaust value rather than adding the two. Other applications must follow their own documented method.
Does a supply-return difference prove room pressure?
No. It identifies a balance question. Transfer openings, door position, leakage, adjacent zones, exhaust, and measurement uncertainty can affect the observed pressure relationship.
Is one ACH a complete air replacement?
No. ACH is a nominal flow-to-volume ratio. Actual mixing, distribution, leakage, and removal effectiveness can differ.
Related tools and resources
- Air Changes per Hour Calculator
- CFM Calculator
- How to Calculate CFM
- CFM vs Air Changes per Hour
- HVAC Airflow Reference Table