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Choosing duct size from airflow and a velocity you can live with

Turning airflow and a velocity budget into a standard duct size, and the rectangular equivalent that fits the ceiling void.

The engineering question

How big does the duct need to be, and what happens when the standard size does not fit the space?

Worked example

The figures below were produced by running the Duct Sizing Calculator with the same inputs — during the build, from its authoritative engine.

Results
Nominal round duct 22 in
Actual velocity 757.6301 fpm
Required free area 2.5 ft²
Rectangular equivalent width 26.8328 in
Rectangular equivalent height 13.4164 in
Assessment Round duct 22" (758 fpm actual). Rectangular equivalent ≈ 26.8" × 13.4".
  1. Free area is airflow divided by design velocity.
  2. The next standard round size always gives a slightly lower velocity than the target.
  3. The rectangular equivalent at 2:1 is what usually decides whether it fits.

Sanity check

Actual velocity must never exceed the design target. If it does, the wrong nominal size has been selected.

Tools for this problem

These are the EngiMetric calculators that compute the numbers in this topic. Each one shows its formula, its units and its limits rather than returning a bare result.

References & standards

  • ASHRAE Handbook — duct design
  • SMACNA — duct construction standards

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