Duct Sizing Calculator
Static · formula-only · worked example computed when this page was built · no live data
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 (design target) = 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. Anything the worked example does not state uses this calculator's standard default; the interactive version shows every input and lets you change it.
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 (design target) | 0.8 Pa/m |
Computed by WorkHive's calculation engine when this page was built; standard: ASHRAE 2021 Ch.21 | SMACNA | ASHRAE 62.1.
How to use this calculator
- Read the worked example. It uses a supply-air main carrying 2 m³/s (7200 m³/h) at 0.8 Pa/m friction over 20 m and shows every number in the method.
- Follow the formula with your own figures: it returns Airflow, Critical-path drop, Friction rate, computed per ASHRAE Fundamentals Ch.21 | SMACNA.
- To compute interactively, open the Duct Sizing Calculator inside WorkHive's free Engineering Design suite (link below). It runs the same method with your inputs.
FAQ
What is a duct sizing calculator equal friction CFM?
The Duct Sizing Calculator is a free online tool that 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 is free: this worked example is open to everyone, and the interactive calculator runs inside the free Engineering Design suite after a free sign-up (your work is saved to your account). 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, and a free sign-up saves your calculations, BOMs and scope-of-works to your account.