CFM Duct Size Calculator
CFM Duct Size Calculator
Enter your design airflow and velocity, then choose the duct shape.
• Area (ft²) = CFM ÷ Velocity (FPM)
• Round Duct Diameter (ft) = √[4 × Area / π] → × 12 to get inches
• Rectangular Duct: W = √(Area × Ratio), H = Area / W → convert ft to inches (× 12)
Disclaimer: This is a simplified calculator ignoring fittings, friction, and real‐world losses.
Introduction: CFM Duct Size Calculator for Round and Rectangular Ducts
In HVAC system design, proper duct sizing is essential to ensure efficient airflow, optimal comfort, and system performance. One of the most critical parameters in duct design is CFM (Cubic Feet per Minute)—a measure of airflow volume. Whether you’re dealing with a residential air handler, a commercial rooftop unit, or an industrial ventilation system, calculating the correct duct size based on CFM ensures that air reaches all zones evenly without excessive noise or energy loss.
To simplify this task, our CFM Duct Size Calculator provides a fast, accurate, and user-friendly way to determine the ideal duct dimensions—either round or rectangular—based on airflow requirements and velocity.
Why Accurate Duct Sizing Matters:
- Prevents air pressure drops and noisy operation
- Reduces energy consumption and load on fans
- Ensures even temperature distribution throughout the system
- Complies with ASHRAE and SMACNA design standards
How to Use the CFM Duct Size Calculator
Using this calculator is straightforward. Just follow the steps below:
🔄 For Round Ducts:
- Enter the CFM value – the airflow required for your zone or application.
- Choose the desired air velocity (typically between 600–1000 FPM for comfort cooling).
- The calculator will instantly compute the duct diameter (in inches) suitable for the input values.
🟦 For Rectangular Ducts:
- Enter the CFM value and desired air velocity.
- Specify either:
- One fixed dimension (e.g., duct width), and let the calculator find the matching height
- Or allow the calculator to suggest standard size combinations with equal aspect ratios
- The result will show the ideal width and height (in inches) for the given airflow.
This tool is particularly useful for:
- HVAC engineers sizing new systems
- Contractors reviewing retrofit options
- Energy auditors optimizing existing ductwork
- Students and educators in mechanical engineering or HVAC design
Use the calculator below to simplify your ductwork planning and ensure system reliability from design to commissioning.