Duct Size Calculator
Size round and rectangular ducts from airflow and target velocity.
About Duct Size Calculator
Duct area follows directly from flow divided by velocity, and the round diameter comes from that area. Because pressure drop rises with the square of velocity, undersizing a duct is penalised twice. Velocity above about 7 m/s in residential ducts becomes audible, which is why quiet systems use larger ducts rather than more fan power.
Formula
Duct Size
Area = flow ÷ velocity; round diameter = √(4A ÷ π)
- velocity
- Around 5 m/s for branch ducts, higher velocities cause noise
- pressure drop
- Friction loss along the run, which the fan must overcome
Worked example
With the default values below, Duct Size Calculator returns Round Duct Diameter of 252.3. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Airflowminimum 1 L/s
- 250 L/s
- Target velocityaccepts 0.5–20 m/s
- 5 m/s
- Rectangular duct heightminimum 50 mm
- 200 mm
- Duct run lengthminimum 0 m
- 15 m
Results
- Round Duct Diameter
- 252.3 mm
- Required Cross-Sectional Area
- 0.05000 m2
- Rectangular Duct Width
- 250.0 mm
- Airflow
- 529.7 cfm
- Estimated Pressure Drop
- 17.84 Pa
Step by step
- 1Values used
- 2Duct Size
- 3Round Duct Diameter= 252.3 mm
- 4Required Cross-Sectional Area= 0.05000 m2
- 5Rectangular Duct Width= 250.0 mm
- 6Airflow= 529.7 cfm
- 7Estimated Pressure Drop= 17.84 Pa
Assumptions and limits
- The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
- Figures are rounded for display to 5 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
Frequently Asked Questions
How is Duct Size calculated?
Area = flow ÷ velocity; round diameter = √(4A ÷ π) Duct area follows directly from flow divided by velocity, and the round diameter comes from that area. Because pressure drop rises with the square of velocity, undersizing a duct is penalised twice.
Why does Duct Size matter?
Velocity above about 7 m/s in residential ducts becomes audible, which is why quiet systems use larger ducts rather than more fan power.
What values do I need to enter?
This calculator takes 4 inputs: Airflow, Target velocity, Rectangular duct height, Duct run length. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.