Round & Rectangular Duct Sizing
Enter airflow (CFM) and a design friction rate, and get the required round diameter or rectangular dimensions using the equal friction method.
Velocity Calculator
Enter CFM and duct size to get velocity in FPM, checked against typical recommended ranges for residential and commercial applications.
Reverse Lookup (Velocity → CFM)
Have a target velocity and a duct size? Work backwards to the achievable airflow.
Round ⇄ Rectangular Equivalent Diameter
Convert between rectangular duct dimensions and their round equivalent diameter using the ASHRAE (Huebscher) equation.
Calculation History
Every calculation is saved locally. Tap any past entry to instantly reload it and adjust.
Built for the field
Large, glove-friendly buttons. A calculator-style layout you can operate one-handed on a ladder or in a mechanical room. Every result screen shows the formula and standard behind the number, so you always know where it came from.
Results are estimates for preliminary sizing. Final design must follow applicable codes and engineering judgment. Duct Calc Pro is a field reference tool and does not replace a full engineered duct system design.