Duct Size Calculator: The Complete HVAC Duct Sizing Tool

One of the greatest steps in the design of an effective HVAC system is correct duct sizing. Ducts that are too large or too small may lead to air flow issues, sound, energy wastage and imbalanced cooling or heating. To address this issue, our Professional Duct Size Calculator assists the HVAC engineers, technicians and contractors to calculate correct duct dimensions in a matter of seconds.

This is supposed to be an easy, quick and dependable tool both in residential and commercial HVAC tasks.

What Is the Duct Size Calculator?

The Duct Size Calculator is a web-based HVAC calculator, where the equivalent round duct diameter, the rectangular duct size, and air velocity cannot be found, except by their requirements alone.

The tool is based on the normal HVAC duct sizing rules in the ductulators. Through its entry of airflow (CFM) and friction rate, the calculator offers real-time accurate results without the need of manual formulas and charts.

It supports:

Round ducts

Rectangular ducts

Real-time airflow velocity calculation

This makes it ideal for HVAC design, estimation, and installation planning.

How to Use the Duct Size Calculator

Using the tool is very easy and requires only a few steps:

1. Enter Airflow (CFM)
   Example: 1200 CFM
2. Enter Friction Rate
   Default value is 0.1 in. wc / 100 ft (commonly used in HVAC systems)
3. Select Duct Shape

Choose Round for circular ducts

Choose Rectangular if you are using rectangular ducts

1. For Rectangular Ducts
   Enter Side A (one known side in inches)
2. Click “Calculate”

The tool will instantly show:

Equivalent round duct diameter (in inches)

Required Side B for rectangular ducts

Actual air velocity (FPM)

Example Calculation

Example 1: Round Duct

Airflow: 800 CFM

Friction Rate: 0.1

Result:

Equivalent round duct ≈ 12 inches

Velocity ≈ 1300 FPM

This helps you confirm that the duct size is within acceptable velocity limits.

Example 2: Rectangular Duct

Airflow: 1200 CFM

Friction Rate: 0.1

Side A: 12 inches

Result:

Equivalent round diameter ≈ 14.5 inches

Required Side B ≈ 17.5 inches

Velocity shown instantly

This saves time compared to manual duct sizing charts.

Benefits of Using This Tool

✔ Accurate HVAC duct sizing
✔ Saves time and reduces calculation errors
✔ Works for both round and rectangular ducts
✔ Instant airflow velocity check
✔ Ideal for HVAC engineers, contractors, and students
✔ Mobile-friendly and easy to use

By using this tool, you can improve HVAC system performance, reduce energy loss, and ensure balanced airflow throughout the building.

Final Thoughts

The Duct Size Calculator is an effective but easy to use HVAC tool that has removed the guesswork in duct sizing. This calculator will assist you in making the right decisions within a short period of time and with confidence whether you are creating a new HVAC system or need to make upgrades.

Try the tool today and experience faster, smarter HVAC duct sizing.