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Duct Size Calculator (Round)

Round duct diameter from CFM and design velocity.

Compute the round duct diameter in inches required to carry a given airflow (CFM) at a chosen design velocity (typically 700–900 ft/min for residential, 1500+ for commercial).

Nadia Almasri By Nadia Almasri
Published Last reviewed 1 min read

Inputs

CFM
ft/min

Results

Enter values and click Calculate to see results.
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How to use this calculator

  1. Fill in the inputs above using the units you already have.
  2. Values update automatically as you type — no submit button needed.
  3. Hover any result row for the underlying formula and intermediate values.

Formula

Area (ft²) = CFM / velocity. Diameter (in) = √(4·area/π) × 12.

In depth

Compute the round duct diameter in inches required to carry a given airflow (CFM) at a chosen design velocity (typically 700–900 ft/min for residential, 1500+ for commercial).