Duct Size Calculator
Air moves through a duct at a chosen speed, and the duct must be big enough to carry the airflow at that speed. This works out the diameter of a round duct from those two numbers.
- Free
- Works offline after your first visit
- Saves on this device
- No sign-up
- Updated
How it works
The duct's cross-section area is the airflow divided by the air speed. A round duct's diameter then follows from that area, since area equals pi times the square of the radius.
Airflow in cubic feet per minute is converted to cubic metres per second first, so the flow and speed can be given in either system. The diameter is shown in inches or centimetres.
Worked example
Airflow of 400 cubic feet per minute at 13.3 ft/s gives a round duct about 9.6 in across.
Questions
What air speed should I enter?
The right speed depends on noise, the system and the design. The person who designs the system sets the figure for it.
Why use round ducts?
A round duct carries a given area with the least surface for its size. Rectangular ducts are common where space is tight, and they need their own sizing.
Can this size a whole system?
No. It sizes one run at one speed. Full duct design also checks pressure loss, fittings and how air balances across every run.
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