Duct Sizing Calculator
Estimate an equivalent round or rectangular duct size from airflow and the selected velocity assumptions. Review noise, pressure loss and fitting effects in the full system design.
How the estimate works
Required area = airflow ÷ target air velocity. Round diameter follows A = πd² ÷ 4.
Metric worked example
Inputs: 0.5 m³/s airflow at 4 m/s
Working: Area = 0.5 ÷ 4 = 0.125 m²; equivalent diameter = √(4A ÷ π) ≈ 0.399 m
Result: About 399 mm equivalent diameter
US customary worked example
Inputs: 1,060 cfm airflow at 787 ft/min
Working: Area = 1,060 ÷ 787 ≈ 1.35 ft²; equivalent diameter = √(4 × 1.35 ÷ π) ≈ 15.7 in
Result: About 15.7 in equivalent diameter
Units and regional checks
Check airflow units, duct dimensions and local mechanical requirements. Equivalent area alone does not validate pressure, acoustics or system performance.
Frequently asked questions
Is equivalent diameter a final duct design?
No. It is a geometry estimate; fittings, friction, noise, leakage and equipment requirements need system-level design.
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