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HVAC duct size calculator

Estimate a round duct diameter and rectangular equivalent from airflow and design limits.

Design airflowUse the CFM for this duct section.CFM

Target frictionin/100 ftVelocity limitft/min

Effective lengthftRectangle shape

Two limits set the minimum size

The calculator checks pressure loss and air speed. The larger required diameter wins.

DIAMETER

Friction ratePressure loss per 100 ft

Velocity limitAir speed inside the duct

Use total effective length, including fitting losses, not only the straight measured distance.

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HVAC Judge provides general information and planning estimates, not a diagnosis, installation design, warranty decision, or contractor quote. Check equipment details with the manufacturer and use a qualified HVAC professional before repair, replacement, or installation.