Duct Sizing Calculator
The Duct Sizing Calculator uses the equal-friction method to size supply ductwork and find the critical-path pressure drop. Example: for a supply-air main carrying 2 m³/s (7200 m³/h) at 0.8 Pa/m friction over 20 m, Airflow = 7200 m³/h, Critical-path drop = 12.2 Pa, Friction rate = 0.8 Pa/m (per ASHRAE 2021 Ch.21 | SMACNA | ASHRAE 62.1).
How it works
Computed from your inputs per ASHRAE 2021 Ch.21 | SMACNA | ASHRAE 62.1.
Worked example (HVAC & Cooling)
Inputs: a supply-air main carrying 2 m³/s (7200 m³/h) at 0.8 Pa/m friction over 20 m.
| Result | Value |
|---|---|
| Airflow | 7200 m³/h |
| Critical-path drop | 12.2 Pa |
| Friction rate | 0.8 Pa/m |
Computed live by WorkHive's calculation engine; standard: ASHRAE 2021 Ch.21 | SMACNA | ASHRAE 62.1.
How to use this calculator
- Enter the figures for the duty you are sizing: the worked example below uses a supply-air main carrying 2 m³/s (7200 m³/h) at 0.8 Pa/m friction over 20 m.
- The calculator returns Airflow, Critical-path drop, Friction rate, computed per ASHRAE Fundamentals Ch.21 | SMACNA.
- Check the worked example to confirm the method matches how you would do it by hand, then run your own numbers in the interactive tool.
FAQ
What is a duct sizing calculator equal friction CFM?
The Duct Sizing Calculator is a free online tool that computes uses the equal-friction method to size supply ductwork and find the critical-path pressure drop. It shows the formula and a fully worked example so you can check the method, not just the number.
How is it calculated?
The result is computed from your inputs following ASHRAE Fundamentals Ch.21 | SMACNA. The worked example on this page shows a real computation with real numbers.
Is the calculator free?
Yes. WorkHive's engineering calculators are free to use, no sign-up needed for the tools. WorkHive is a free, offline-first maintenance platform built for Philippine industrial plants.
Run it on your own numbers
Open the interactive Duct Sizing Calculator in WorkHive: free, no sign-up needed for the calculators.