Free duct sizing calculator
HVAC Duct Size Calculator
This calculator estimates round duct diameter from airflow and target velocity so HVAC teams can start a duct sizing conversation quickly.
Use it for early planning, comfort complaints, additions, airflow reviews, and estimate preparation before a final design or duct renovation proposal.
Estimate round duct size from CFM
Enter target airflow and design velocity to calculate a planning diameter, duct area, and velocity check for round duct runs.
How it works
How the duct size estimate works
The calculator divides airflow by velocity to estimate duct area, then converts that area into an equivalent round diameter.
1
Start with target airflow
Use CFM from room load, equipment requirements, or balancing goals.
2
Choose design velocity
Lower velocities can reduce noise and friction, while higher velocities may increase restriction.
3
Convert area to diameter
The equivalent round diameter is calculated from the required duct area.
Field example
Example: under-supplied bedroom
A technician can compare target airflow with the duct run serving the room and decide whether balancing or duct changes deserve a quote.
A room needing more airflow may require a larger branch, improved return path, or duct sealing.
The estimate should include duct material, access limitations, fittings, and comfort goals.
The job record can keep airflow math beside register readings and customer symptoms.
Common mistakes
What to double-check before using the result
Sizing from diameter alone
Length, fittings, static pressure, material, and blower performance all affect delivered airflow.
Ignoring noise
High velocity may move air but can create sound complaints and comfort issues.
Skipping return air
Supply improvements may fail when the return path is undersized or blocked.
After the calculation
Turn the result into cleaner field work
Capture field readings
Attach register readings, static pressure, photos, and duct notes to the job.
Build a duct estimate
Include material, access, fittings, balancing, sealing, and cleanup details.
Follow through after install
Schedule a balancing or verification visit when the scope requires it.
Related resources
Related templates
FAQ
Questions service teams ask about this tool
Can this replace Manual D duct design?
No. It is a planning calculator. Final duct design should follow the proper design method and local requirements.
What inputs affect duct size?
Airflow, velocity, duct material, equivalent length, fittings, static pressure, and equipment performance all matter.
Why does duct velocity matter?
Velocity affects friction, noise, comfort, and the ability to deliver air through the run.