What Is a Duct Size Calculator?
A Duct Size Calculator is a specialized tool for HVAC professionals to determine the proper diameter of a duct based on required airflow (CFM) and the system's friction loss rate. Proper duct sizing is one of the most critical aspects of an efficient HVAC system, yet it's often overlooked, leading to poor performance and high energy bills. This tool simplifies the complex formulas found in a traditional ductulator.
Last updated: November 8, 2025
How to Use This Calculator
Our free duct sizing tool provides an accurate estimate based on standard engineering principles.
- Enter Air Flow (CFM): Input the required Cubic Feet per Minute for the duct run. This value is determined by a professional HVAC load calculation (Manual J).
- Enter Friction Loss Rate: Input the design friction loss for your system. A rate of 0.1 in. w.c. is standard for residential main trunk lines.
- Click "Calculate": Get an instant recommendation for the required round duct diameter.
Understanding Your Duct Size Calculation
The result shown is the recommended **internal diameter** for a straight, round metal duct to efficiently carry the specified amount of air (CFM) at the chosen friction loss rate. Using this size helps ensure that your system delivers the right amount of air to the space without being excessively noisy or inefficient.
- Recommended Duct Size: This is the diameter in inches for a standard round duct. You can use a ductulator chart to find an equivalent size for a rectangular duct.
Using ducts that are just 20% undersized can reduce total system airflow by nearly 40%, drastically increasing energy consumption and reducing comfort.
Example Duct Size Calculation
This is a common scenario for a main supply trunk line in a residential system.
| Input | Example Value |
|---|---|
| Air Flow (CFM) | 800 CFM |
| Friction Loss Rate | 0.1 in. w.c. |
| Recommended Duct Size | ~14.1-inch round duct |