Duct Pressure Drop Calculator

Find the friction loss per 100 ft of straight duct for a given airflow and duct size, round or rectangular, with the air velocity it produces.

Airflow (CFM)

Duct Shape

Diameter (inches)

Friction Loss

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Enter values above and click calculate.

How It Works

This is the reverse of the Duct Size calculator: instead of choosing a friction rate and getting a size, you enter an existing size and get the friction rate it runs at.

Friction (in w.g./100 ft) = 0.109136 × CFM^1.9 ÷ D^5.02

The ASHRAE Fundamentals friction equation for galvanized sheet metal duct and standard air. For rectangular duct, D is the Huebscher equivalent diameter; velocity uses the actual W × H area.

What the numbers mean

0.08–0.10 in w.g./100 ft is a typical design friction rate for supply and return trunks, and 0.10–0.20 is common on commercial systems. In metric, 0.1 in w.g./100 ft is about 0.8 Pa/m. The velocity check flags anything above 900 FPM (about 4.6 m/s), where duct noise becomes noticeable.

Multiply by the equivalent length of the run (÷ 100) to get its pressure loss, then total the runs in the Static Pressure calculator.