Enter the air CFM (cubic feet per minute) and the duct cross-sectional area into the calculator to determine the duct velocity.

Duct Velocity Formula

The following formula is used to calculate the linear air velocity through a duct.

V = CFM / A
  • Where V is the duct velocity (FPM)
  • CFM is the volumetric flow rate of air through the duct (typically cubic feet per minute)
  • A is the cross-sectional area (ft^2)

To calculate the duct velocity, divide the flow rate of air through the duct by the duct area.

Duct Velocity Definition

What is a duct velocity? A duct velocity is defined as the linear speed at which air is moving through the opening of a duct or air vent. This is the average linear speed of the air since normal airflow is not constant across an entire area.

Example Problem

How to calculate duct velocity?

  1. First, determine the volumetric flow rate.

    The volumetric flow rate, often denoted CFM (cubic feet per minute) is the total volume of air moving through an area per unit of time. For this problem, the flow rate is 1,500 ft^3/min.

  2. Next, determine the cross-sectional area.

    For this example, the cross-sectional area of the duct is 5 ft^2.

  3. Finally, calculate the duct velocity.

    The air velocity is then calculated using the formula above. V = 1500 / 5 = 300 ft/min = 5 ft/s.

About Duct Velocity

What does fpm stand for? FPM stands for feet per minute and is the most common unit for describing duct velocity.