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Compute duct velocity in ft/min and dynamic pressure Pv=1/2 rho V^2 from CFM and inch sizes (round diameter, or rectangular width and height).

📘 How to Use

  1. Enter the airflow rate in CFM.
  2. Select the duct shape (Circular or Rectangular).
  3. Input the duct dimensions in inches (diameter, or width and height).
  4. View the calculated velocity and dynamic pressure.

HVAC Duct Velocity Calculator

Duct Velocity
0.00 ft/min
Dynamic Pressure (Std. Air)
0.00 in.wg

The allowable duct velocity depends on the duct location and the acoustic design criterion (RC level) (ASHRAE Handbook HVAC Applications, Ch. 49, Table 8). Compare the calculated velocity with the limit for your own case.

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HVAC Duct Velocity Calculator | Airflow & Sizing Tool

This online calculator determines the air velocity and dynamic pressure within HVAC ductwork based on airflow rate and duct dimensions. HVAC engineers, technicians, and system designers can use it to check designs, size ducts, and troubleshoot airflow issues.

💡 Tool Overview

  • Comprehensive Calculations: Computes air velocity in feet per minute (ft/min) and dynamic pressure in inches of water gauge (in.wg).
  • Flexible Duct Shapes: Supports calculations for both standard circular and rectangular duct profiles.
  • Imperial Units: Designed for the US market and engineering standards, accepting inputs in CFM (Cubic Feet per Minute) and inches.

🧐 Frequently Asked Questions

Q. How should I judge the calculated velocity?

A. The allowable duct velocity depends on the duct location and the acoustic design criterion (RC level); see ASHRAE Handbook HVAC Applications, Ch. 49, Table 8. Compare the calculated velocity with the limit for your own case. Velocity affects both noise and friction loss.

Q. What is dynamic pressure (in.wg)?

A. Dynamic pressure is the kinetic pressure exerted by air in motion. It is a direct function of air velocity and density. In HVAC design, it is a key component of the total pressure a fan must overcome to move air through the system. This calculator computes it for ASHRAE standard air (0.075 lbm/ft³), using Pv = (V/4005)² with V in ft/min.

📚 Duct Sizing Principles

Proper duct sizing is a critical aspect of HVAC system design, involving a trade-off between performance, cost, and space. The air velocity within a duct is a primary factor in this balance.

  • High Velocity: Using smaller ducts results in higher air velocity. While this saves material costs and space, it also leads to increased friction loss, which requires more fan power and consumes more energy. Furthermore, high velocities are a primary source of undesirable noise (breakout noise and terminal noise at diffusers).

  • Low Velocity: Using larger ducts results in lower velocity. This minimizes noise and reduces friction loss, leading to lower energy consumption. However, it significantly increases the initial material and installation costs and requires more physical space in ceilings and walls. This tool helps you quickly assess if your chosen duct size and airflow result in a velocity that is appropriate for the target environment.