Duct velocity and friction loss, for the duct you have.

Short answer

Duct velocity in ft/min is CFM divided by the duct cross-section in square feet. 400 CFM through a 14 x 8 in rectangular duct (0.778 sq ft) moves at 514 ft/min, and the friction rate is about 0.044 in. w.g. per 100 ft at an equivalent round diameter of 11.5 in.

Worked example

A branch carries 400 CFM in a 14 x 8 in rectangular duct. The area is 14 x 8 / 144 = 0.778 sq ft, so the air moves at 400 / 0.778 = 514 ft/min. The equal-friction round equivalent is 11.5 in, and the model gives 0.044 in. w.g. per 100 ft.

Formula, assumptions and sources

Velocity is the continuity relation, velocity = airflow / area, with area in square feet and airflow in CFM. Friction uses the same model as the Ductulator: the Darcy equation with the Colebrook friction factor, standard air at 0.075 lbm/ft3 and galvanized-steel roughness of 0.0003 ft, the values ASHRAE uses for its round-duct friction chart in the Duct Design chapter. For a rectangular duct the friction rate is evaluated at the Huebscher equal-friction equivalent diameter and the same airflow. The DOE Building America air distribution guideline lists duct performance as defined by four variables: diameter, flow rate, velocity and friction rate. Real systems also lose pressure in fittings, transitions and flex duct, which this tool does not model, and the tool offers no velocity limit because the right limit depends on noise and your designer.

Source: ASHRAE Handbook - Fundamentals (2017), Chapter 21: Duct Design (retrieved 2026-10-15)

Source: U.S. DOE Building America, Advanced Strategy Guideline: Air Distribution Basics and Duct Design (IBACOS, December 2011) (retrieved 2026-10-15)

Safety and limits

This is a planning aid, not professional advice. Duct, furnace and air-conditioner design affects comfort, combustion safety and code compliance; a licensed HVAC contractor should perform a load calculation, size the system and verify airflow on site.

FAQ

How do I calculate duct velocity?

Divide the airflow in CFM by the duct cross-section in square feet. For a round duct the area is pi x diameter squared / 4, with the diameter in feet.

What is a good duct velocity?

There is no single number. Velocity trades noise against duct size and pressure loss, and the limit depends on where the duct runs. Your HVAC designer or the equipment manufacturer sets it.

How is friction loss calculated here?

With the Darcy equation and the Colebrook friction factor for galvanized steel and standard air, the same basis as ASHRAE's round-duct friction chart.

Why does a rectangular duct show an equivalent round diameter?

Friction in a rectangular duct is evaluated at the round diameter that gives the same friction at the same airflow, from the Huebscher equation.