h2x Tools

Duct Size Calculator

Calculate ductwork accurately and easily using our free Duct Size Calculator to ensure the correct velocity and pressure drop.

Get accurate results with this simple, yet powerful tool.

Circular Duct Size:


Rectangular Duct Size:


Square Duct Size:


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Flow Rate and Maximum Velocity

Duct Sizing Calculator - Flow Rate

Flow Rate

Determining the flow rate is crucial in duct sizing as it dictates the volume of air that moves through the system per unit of time, impacting comfort and efficiency.

Duct Sizing Calculator - Maximum Velocity

Maximum Velocity

Keeping air velocity within recommended limits is critical for efficient airflow and distribution in ductwork, minimising energy loss and noise levels.

Duct Sizing Calculator

Duct Size Formula

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

Imperial Equation

Following the formula below, you can calculate the duct size:

Area (ft2) = Flow rate (ft3/s) / Maximum Velocity (ft/s)

Metric Equation

Following the equation below, you can calculate the duct size:

Area (m2) = Flow rate (m3/s) / Maximum Velocity (m/s)
Air Duct Velocity Calculator

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

Duct size calculation is vital in HVAC design, determining the dimensions for sufficient air flow in buildings. Equally, properly sized ducts ensure efficient air distribution, preventing issues like inadequate air flow, pressure imbalances, and excessive noise.

Although accurate duct sizing optimises system performance, it also enhances energy efficiency and minimises operating costs. Furthermore, the calculation considers air flow volume, velocity, pressure drop, and friction losses, meeting building requirements.

However, due to site restrictions, it's crucial to consider shapes like rectangular, circular, and square to ensure proper fit and functionality of the ductwork. Thus, precise duct sizing is essential for achieving optimal thermal comfort, indoor air quality, and overall system effectiveness in residential, commercial, and industrial settings.

h2x for Duct Size Calculations

  • Calculate the flow rate for 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 Duct Size Calculator?

Evidently, there are several benefits of using a online calculator to calculate Duct Size, 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?

Webinar: Duct Sizing

Check out the unveiling of h2x software's latest Duct Sizing feature on Mechanical Ventilation.

Calculate Duct Size

Find out how to calculate duct size effectively with h2x's comprehensive guide. Optimise airflow and efficiency.

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Explore the intricacies of Mechanical Ventilation with Heat Recovery (MVHR) in our free eBook.

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