Airflow calculator
CFM Calculator
Calculate a volume-flow value in CFM from room dimensions and a documented air-changes-per-hour target.
Calculate CFM from volume and ACH
Identify whether your target refers to supply, outdoor, exhaust, or recirculated airflow before using it.
How to use this calculator
- Enter the inside length, width, and ceiling height of the space.
- Enter an ACH value with a documented project, code, or engineering basis.
- Read the implied airflow in cubic feet per minute.
Formula
CFM = room volume (ft³) × ACH ÷ 60
Variables and units
| Term | Meaning | Unit |
|---|---|---|
| Room volume | Inside length × width × height | ft³ |
| ACH | Volume changes per hour | 1/h |
| CFM | Volume flow rate | ft³/min |
Example
A 20 ft × 15 ft × 8 ft room has 2,400 ft³ of volume. At 6 ACH, 2,400 × 6 ÷ 60 = 240 CFM.
Result interpretation
The calculation converts an ACH assumption into a volume flow. It does not determine that the ACH is appropriate, nor does it distinguish total supply, outdoor air, exhaust, return, or filtered recirculated air. Those streams can have different requirements and design purposes.
Continue with the air path
After a documented airflow target is established, use the Duct Size Calculator to check an area/velocity relationship and the Duct Velocity Calculator for a separate velocity check.
Frequently asked questions
Does this set a required ventilation rate?
No. It only converts a chosen ACH to CFM. Use applicable requirements and qualified design for ventilation decisions.
Does ACH mean all air is perfectly replaced?
No. ACH is a volumetric rate. Actual mixing, distribution, short-circuiting, filtration, and contaminant removal depend on the system and space.
Can I use this result to select HVAC equipment?
No. Equipment selection needs a load calculation and manufacturer performance data. Airflow and capacity are related but are not interchangeable.