CFM Calculator
Use this CFM calculator to estimate the airflow needed for a room from its dimensions and the target air changes per hour. It follows the simple Omni relationship between room volume, ACH, and CFM, which makes it a practical starting point for ventilation planning, air-cleaner sizing, and quick HVAC checks.
Airflow Result
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Quick Answer: First calculate room volume as length x width x height. Then multiply by air changes per hour to get hourly airflow, and divide by 60 to convert that result into cubic feet per minute.
How to Calculate
- Measure the room: Enter the room length, width, and height in consistent units.
- Choose the target ACH: Use the air-change rate you want the room or enclosure to achieve.
- Calculate the room volume: The calculator multiplies the three dimensions to get the enclosed volume.
- Convert airflow to CFM: The hourly airflow is divided by 60 so you can compare it with fan and HVAC ratings.
Formula
CFM = room volume x ACH / 60
| Variable | Meaning | Unit |
|---|---|---|
| room volume | Length x width x height | cu ft |
| ACH | Air changes per hour | changes per hour |
| CFM | Airflow rate | cubic feet per minute |
Worked Examples
Ventilation - Workshop airflow estimate
- Room: 20 ft x 15 ft x 10 ft
- ACH: 6
Result: Volume = 3,000 cu ft; CFM = 300
That means a 300 CFM airflow rate would deliver roughly six air changes per hour in that room.
Interpretation Table
| Range | Meaning | Action |
|---|---|---|
| Lower CFM | Small room, low ceiling, or low ACH target | This can be enough for light ventilation or low-demand spaces. |
| Mid CFM | Average room with a moderate ACH target | Compare the result with fan curves, filter losses, and duct pressure before final selection. |
| Higher CFM | Large room or high ACH target | A larger fan, multiple units, or a more careful duct design may be needed. |
Frequently Asked Questions
CFM stands for cubic feet per minute. It is a common way to describe airflow rate for fans, HVAC equipment, and ventilation systems.
ACH means air changes per hour. It tells you how many times the total room air volume is replaced or circulated in one hour.
It is a useful starting point, but final HVAC design should also consider pressure losses, filtration, heat loads, noise, and equipment performance curves.
Note: This is a planning estimate. Final ventilation and HVAC design should consider actual equipment performance, losses, and applicable code requirements.
References
Last reviewed: March 14, 2026