Airflow calculator
CFM Calculator
Calculate required airflow from room dimensions or a known room volume and a documented air-changes-per-hour target.
Calculate airflow from volume and ACH
Use dimensions or enter a known volume. Identify whether the result represents supply, outdoor, exhaust, or another airflow stream.
How to use this calculator
- Select US customary or SI units.
- Enter the inside room dimensions, or switch to Known room volume and enter the volume directly.
- Enter an ACH value with a documented project, code, process, or engineering basis.
- Read the required airflow, the volume used, and the equivalent airflow in the other unit system.
Formula
CFM = room volume (ft³) × ACH ÷ 60
m³/h = room volume (m³) × ACH
Variables and units
| Term | Meaning | US / SI unit |
|---|---|---|
| Room volume | Inside dimensions multiplied together, or a directly entered known volume | ft³ / m³ |
| ACH | Hourly airflow divided by the stated room volume | 1/h |
| Airflow | The identified stream required to produce the entered nominal ACH | CFM / m³/h |
Checked examples
A 20 ft × 15 ft × 8 ft room has 2,400 ft³ of volume. At 6 ACH, 2,400 × 6 ÷ 60 = 240 CFM, equivalent to about 407.76 m³/h.
The same workflow in SI is direct: 67.96 m³ × 6 ACH = 407.76 m³/h. Small differences after switching units come from displayed input rounding.
Which airflow does the result represent?
The arithmetic does not decide the air stream. A supply-air ACH uses measured or scheduled supply airflow; an outdoor-air ACH uses only the outdoor-air portion; an exhaust ACH uses exhaust flow; and a filtered-air or equivalent-ACH calculation needs the applicable clean-air method. Return airflow is not a universal substitute for any of these.
If supply and return differ, keep the values separate. The difference may be balanced by exhaust, transfer air, door undercuts, leakage, infiltration, or exfiltration and is part of a pressure/mass-balance question—not an extra ACH that should automatically be added to the room result.
Result interpretation
This tool converts a stated ACH basis into a volume flow. It does not determine whether the ACH is appropriate, prove uniform mixing or contaminant removal, establish outdoor-air compliance, balance a room, or select HVAC equipment.
Use the inverse tool for measured airflow
If CFM is already measured or scheduled and the question is the resulting ACH, use the Air Changes per Hour Calculator. For supply-versus-return interpretation, review Air Changes per Hour Explained.
Frequently asked questions
Does this set a required ventilation rate?
No. It converts a user-supplied ACH to airflow. Use the applicable standard, code, project criteria, and qualified design to establish requirements.
Should I enter supply or return airflow for ACH?
This calculator solves airflow from an ACH, so first label the ACH objective. Supply, outdoor, exhaust, return, and cleaned recirculated airflow describe different paths and should not be substituted without a documented method.
Can I use this result to select HVAC equipment or duct size?
No. Equipment selection requires load and manufacturer data. Duct design also requires velocity, fittings, pressure loss, noise, and available fan pressure.
Related resources
- Air Changes per Hour Calculator
- How to Calculate CFM
- Air Changes per Hour Explained
- HVAC Airflow Reference Table
- Duct Size Calculator
Sources and scope
The conversion is a volume-flow relationship. The cited sources distinguish supply, outdoor, return, exhaust, and clean-air purposes; they do not provide one universal ACH target for every room.