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Free HVAC Duct Sizing Calculator

HVAC Duct Size Calculator

Estimate round or rectangular HVAC duct dimensions from required airflow in CFM and a target air velocity. Use it for preliminary supply, return and ductwork planning before a complete residential duct design.

Calculate Duct Size
CFM to Duct Size

Turn HVAC airflow into a preliminary duct dimension.

If you do not know the required airflow yet, calculate it first with the HVAC CFM calculator . If you are planning a larger duct project, compare the result with our ductwork cost calculator .

Calculate Round or Rectangular Duct Size

Enter airflow and a target velocity to estimate the required duct cross-sectional area.

Enter airflow in cubic feet per minute (CFM).
Lower or higher velocities may be appropriate depending on the duct section and design.
More compact aspect ratios generally use cross-sectional area more efficiently.
Preliminary Duct Dimension - -

The result is based on airflow divided by target velocity. A complete residential duct design also considers friction rate, effective duct length, fittings, duct material, available static pressure, blower performance and air balancing.

- Approx. Velocity
Airflow -
Required Area -
Calculated Size -
Duct Shape -

Why can the practical size be larger than the exact calculation?

The calculator rounds dimensions upward to a more practical nominal reference instead of rounding down and unintentionally increasing air velocity. This still does not replace friction-rate calculations or a complete duct-system design.

Planning duct installation or replacement?

Request HVAC help below after reviewing your preliminary airflow and duct dimensions.

Duct-design warning: this is a velocity-and-area calculator, not a Manual D duct design. A duct can have enough cross-sectional area and still perform poorly if the system has excessive pressure drop, restrictive fittings, long runs, leakage, flex-duct compression or inadequate blower capacity.