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ASHRAE
Validated against the standard.
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$2.99 Pro
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How it works
Enter your values
Type in your inputs โ the app fills code tables and constants for you, so there's nothing to look up.
Get a code-checked result
Instant answer with a clear PASS / FAIL and the exact ASHRAE reference behind every number.
Save & export
Keep a history of calculations and export a clean PDF report to share or attach to a permit (Pro).
What this calculator does
Airflow in CFM (cubic feet per minute) ties the equipment load to the duct system. This app finds the CFM a system needs from its sensible load, or from duct velocity and area, so you can size ducts and verify a system is moving the right amount of air.
The core formula
CFM = sensible BTU/h รท (1.08 ร ฮT)
where 1.08 is the standard air constant and ฮT is the temperature rise or drop across the coil. You can also get CFM from velocity (fpm) ร duct area (ftยฒ).
Worked example
A 24,000 BTU/h sensible cooling load across a 20ยฐF coil split: CFM = 24,000 รท (1.08 ร 20) = 1,111 CFM, close to the common target of about 400 CFM per ton.
Related calculators
Use it after the heat load calculator and feed the result into the duct sizing calculator.
Airflow CFM FAQ
Where does the 1.08 constant come from?
It combines the density of standard air and the specific heat of air, converting BTU per hour and a temperature difference into cubic feet per minute of sensible airflow.
What is a good airflow target?
A common starting point is about 400 CFM per ton of cooling, then adjusted to the actual sensible load and the coil temperature split.
Can it work from duct velocity?
Yes. Enter duct dimensions and velocity and the app returns CFM from velocity times cross-sectional area, useful for verifying measured airflow.
Does it work offline?
Yes, it runs 100% offline with no account. Free with rewarded ads; a one-time $2.99 Pro unlock adds PDF export and saved history.
Get Airflow Cfm on your phone
Free with rewarded ads ยท $2.99 lifetime Pro ยท iOS & Android. Works 100% offline on the job site.