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Free Mechanical / HVAC calculator

Duct size calculator from airflow and velocity

Calculate theoretical circular diameter or rectangular dimensions from airflow and target air velocity.

Enter the project values

All values remain in your browser. Review the assumptions before using the result.

For example, 2 means width = 2 × height.
Your result will appear here.Calculate to see the outputs, formula and substituted working.

How the calculation works

Continuity relationship using theoretical internal free area.

Formula

A = Q ÷ v; D = √(4A ÷ π)

Worked example

Airflow1,000 L/s
Target velocity5 m/s
ShapeCircular
Expected diameter504.6 mm

Assumptions and limitations

Assumptions

  • The airflow is steady.
  • Dimensions are internal free dimensions.

What this does not check

  • Does not calculate pressure loss, sound, leakage or fitting effects.
  • Round to practical sizes and verify the complete duct design.
Formula version: 1.0 · Last reviewed: 16 August 2026

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Engineering basis: first-principles calculation using user-entered project data. The calculator does not choose design criteria or replace the complete project-specific engineering design.
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