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.
Enter Values
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
- Enter the airflow Q in litres per second (L/s).
- Enter EITHER the round duct diameter, OR both the width and height for a rectangular duct.
- 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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Tip: Enter any known values to calculate the remaining results.
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