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Duct Velocity Calculator

Find the air velocity inside a duct from the airflow and the duct size. Works for round ducts (diameter) or rectangular ducts (width and height), and reports the cross-sectional area used.

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How to use this calculator

Enter Values

Minimum 0

Minimum 0

Minimum 0

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Enter your values and press Calculate to see results.

Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.

How to use this calculator

  1. Enter the airflow Q in litres per second (L/s).
  2. Enter EITHER the round duct diameter, OR both the width and height for a rectangular duct.
  3. Read the resulting air velocity in m/s and check it against typical duct limits.

How it works

Velocity is airflow divided by area: v = Q / A. The airflow is converted from L/s to m³/s (÷1000). For a round duct the area is A = π/4 × d²; for a rectangular duct it is A = w × h, with all dimensions converted from millimetres to metres. Dividing the flow by that area gives the mean velocity.

Worked example

Worked example. For 500 L/s through a 400 mm round duct, the area is π/4 × 0.4² = 0.1257 m². Velocity = 0.5 ÷ 0.1257 = 3.98 m/s — comfortably within the usual main-duct range.

Common mistakes

  • Entering only one of width or height for a rectangular duct — both are needed to form the area.
  • Mixing a round diameter and rectangular dimensions at the same time; pick one duct shape.
  • Reading the mean velocity as the peak — real profiles are higher at the centre than at the walls.

Frequently asked questions

What is a typical duct velocity?

Comfort air systems usually run main ducts around 4–7 m/s, with branches and terminal connections slower to keep noise down near occupied rooms. Higher velocities are reserved for space-constrained or industrial runs where noise is less critical.

Why does high velocity matter?

Faster air means more friction loss, higher fan power and more noise. Sizing to a sensible velocity balances duct size against running cost and acoustic comfort.

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Cite this tool

Zerdly. (2026). Duct Velocity Calculator. Retrieved from https://www.zerdly.com/tools/duct-velocity-calculator/

Tip: Enter any known values to calculate the remaining results.

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