How do I size ductwork for a given airflow?

Enter the design volume flow (l/s or m³/s) and either a circular diameter or rectangular dimensions. The calculator reports velocity, pressure drop per metre, and indicative sound power.

What duct velocity should I aim for in UK practice?

Typical design velocities: 2.5–6 m/s in mains, 2–3 m/s in branches, lower in noise-sensitive spaces. Check your project acoustic criteria and fan selection.

How is equivalent diameter calculated?

For rectangular ducts, equivalent circular diameter is derived from the hydraulic diameter relationship so you can compare against standard circular sizes.

BuildFlo Design Guide · Ventilation · CIBSE / DW144

Duct sizing calculator for ventilation ductwork

Check duct velocity, Pa/m, equivalent diameter and sound power from airflow. Free UK duct sizing calculator for circular and rectangular ductwork.

What this calculator does

Duct sizing helps UK building services engineers with check duct velocity, pa/m, equivalent ø and sound from flow and dimensions. The calculation follows CIBSE / DW144 methods used in everyday MEP design — enter your project inputs below for a first-pass result.

Whether you searched for "duct sizing", "duct sizing calculator", or "how to duct size", this page gives you a working tool plus the context to use it on a real job.

When to use it

Use this for early-stage sizing and sanity checks on UK building services work. It is not a substitute for issued calculations — verify against the project specification, drawings, and relevant standard before sign-off.

How to use it

1. Fill in the inputs for your scenario (defaults are starting points only). 2. Click Calculate — results include the method and assumptions where applicable. 3. Verify against the project specification and relevant standard before issuing.

Typical output unit: mm.

Tips

Defaults are indicative — replace with project-specific values from the architect, structural, or MEP spec.
For issued calculations, save results in your BuildFlo project and attach to the calculation folder.
Need the regulation citation? Open BuildFlo chat in the app and ask about duct sizing under UK standards.

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Frequently asked questions

How do I size ductwork for a given airflow?: Enter the design volume flow (l/s or m³/s) and either a circular diameter or rectangular dimensions. The calculator reports velocity, pressure drop per metre, and indicative sound power.
What duct velocity should I aim for in UK practice?: Typical design velocities: 2.5–6 m/s in mains, 2–3 m/s in branches, lower in noise-sensitive spaces. Check your project acoustic criteria and fan selection.
How is equivalent diameter calculated?: For rectangular ducts, equivalent circular diameter is derived from the hydraulic diameter relationship so you can compare against standard circular sizes.