h2x Tools

Air Duct Velocity Calculator

Calculate air velocity flowing through ducts easily using our free Air Duct Velocity Calculator.

Get accurate results with this simple, yet powerful tool.

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Duct Diameter and Air Flow Rate

Air Duct Velocity Calculator - Flow Rate

Air Flow Rate

The air flow rate is the air volume that passes through an area in a unit of time. It is measured in m³/s, GPM, or l/s and is directly proportional to the circular duct flow velocity and the duct inside diameter.

Air Duct Velocity Calculator - Internal Diameter

Duct Internal Diameter

There is an inverse relationship between the internal diameter of a duct and the duct velocity. This means that the air velocity will expand as the duct diameter reduces.

Air Duct Velocity Calculator

Air Duct Velocity Formula

As the calculation can be measured in terms of feet/s and meter/s, there are two different calculation methods to establish air duct velocity:

Imperial Equation

Following the formula below, you can calculate the air duct velocity:

V = 0.408 Q/D2


  • = Air velocity inside the duct (feet/s)
  • = Air flow rate inside the duct (GPM)
  • = Air flow rate inside the duct (inches)

Metric Equation

Following the equation below, you can calculate the air duct velocity:

V = 1.274 Q/D2


  • = Air velocity inside the duct (m/s)
  • = Air flow rate inside the duct (m³/s)
  • = Air flow rate inside the duct (m)
Air Duct Velocity Calculator

What is the Air Duct Velocity Calculation Used For and Why is it Important?

Duct velocity, measured in terms of m/s or ft/s based on the duct inside diameter, represents the speed of airflow through a duct.

Furthermore, understanding your duct velocity is crucial as it directly impacts friction losses.

Higher air velocity correlates with increased friction losses and energy consumption.

Consequently, elevated friction losses not only influence the performance of fans, valves, and related equipment, but may also necessitate the selection of larger fans.

The use of larger fans and equipment can elevate both the capital and operational costs of the building.

Therefore, this calculation plays a pivotal role in system design.

h2x for Duct Velocity Calculations

  • Calculate the air duct velocity through every duct (CIBSE-verified)
  • Design to different velocities for various air systems (CIBSE-verified)
  • Automates all of the duct and fan sizing based on your design layout

h2x is CIBSE-verified design software built to improve the efficiency and quality of your design process. Read more.

Why Use a Air Duct Velocity Calculator?

Evidently, there are several benefits of using an online calculator to calculate Air Duct Velocity, including:

Increased Accuracy

Increased Accuracy:

Determine accurate values by eliminating the possibility of human error when calculating air pressure drop by hand.



Saves time by providing quick and easy calculations, putting an end to performing complex mathematical calcs manually.

Easy to Use

Easy to Use:

User-friendly and requires no special training or technical knowledge. Just provide the input and read the results.



Can save money by reducing the number of time spent on manual and long calculations, and avoiding costly mistakes.



Want to Learn More About This Topic?

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