HVAC CFM Calculator — Airflow for Any Room
CFM — cubic feet per minute — is the number every HVAC job starts with. If the airflow is wrong, nothing else matters: undersized ducts starve the system, oversized ones make it noisy and short-cycle. This calculator gives you the target CFM for any room in seconds.
It's the same math HVAC techs do on every load calc: room volume × air changes per hour, divided by 60. You bring the tape measure; it does the arithmetic.
Required CFM
240
Room volume
3600 ft³
ACH setting
4/hr
Formula: (sq ft × ceiling × ACH) ÷ 60. Living areas run 3–4 ACH; kitchens and baths 6–8. Always size the duct run to match.
The formula
CFM = (Room sq ft × Ceiling height × ACH) ÷ 60. Standard guidance: living areas 3–4 ACH, kitchens and bathrooms 6–8 ACH, bedrooms 3–4 ACH.
Pro tips
- →Measure to the actual ceiling — vaulted ceilings change the volume and the answer.
- →When in doubt, size up one duct size. Undersizing causes the airflow complaints that come back to bite you.
- →Use the result with your duct sizing calculator to pick the right round duct diameter.
Frequently Asked Questions
What CFM do I need for a 400 sq ft room?+
With a 9-ft ceiling at 4 air changes per hour: (400 × 9 × 4) ÷ 60 = 240 CFM. For a kitchen at 8 ACH, the same room needs 480 CFM.
How many CFM per square foot?+
Roughly 1 CFM per sq ft at standard 8-9 ft ceilings with 4 ACH. That's the common rule of thumb before load calculations.
Is CFM the same as BTUs?+
No. CFM measures airflow volume; BTUs measure heating/cooling output. You need both right — the air moves the energy.
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