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Free tools › Contractors › Duct size calculator

Free calculator · no sign-up

Duct size calculator

Size a duct from the airflow and the air speed you’re designing for — round diameter or rectangular equivalent.

✓ Free✓ Works on your phone✓ Any tax rate

Airflow

Round duct diameter
Duct area
Round size to use (next size up)
Rectangular (same area)
Equal-area sizing. Friction and fittings matter too — confirm with a duct design method (e.g. ACCA Manual D).
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How it’s calculated

  • Area (sq ft) = CFM ÷ air speed (ft/min).
  • Round diameter (in) = √(4 × area ÷ π) × 12.
  • Rectangular height = area × 144 ÷ the side you chose.

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Questions

Why does air speed matter?

Faster air needs a smaller duct but makes more noise and pressure loss; slower air needs a bigger duct.