Professional HVAC Duct Sizing Calculator
Calculate round and rectangular duct dimensions based on airflow (CFM) and velocity.
Cubic Feet per Minute
Feet per Minute (Res: 600-900)
If Side A is:
inches
Required Duct Sizes
Round Duct Diameter
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Rectangular Size
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Note: Calculated sizes are theoretical. Always round up to the nearest standard commercial size.
How to Use the HVAC Duct Sizing Calculator
Proper duct sizing is critical for ensuring efficient airflow, minimizing noise, and maintaining the longevity of your HVAC equipment. This calculator helps you determine the necessary diameter for round ducts and the dimensions for rectangular ducts based on your system's airflow requirements.
Understanding the Inputs
- CFM (Cubic Feet per Minute): This is the volume of air your blower is moving. Most residential systems require roughly 400 CFM per ton of cooling.
- Velocity (FPM): This is how fast the air travels. For residential branch ducts, 600 FPM is common. For main trunks, 900 FPM is typical. High velocity causes noise (whistling).
- Friction Rate (Standardized): While this calculator uses the Velocity Method, most HVAC technicians design for a 0.1 friction loss per 100 feet of ducting.
The Math Behind Duct Sizing
The fundamental formula used in this calculator is based on the Continuity Equation:
Area (sq. ft.) = CFM / Velocity (FPM)
To find the diameter of a round duct in inches, we convert the area into a circle:
Diameter = √((Area × 144 × 4) / π)
Example Calculation
Suppose you have a 2.5-ton AC unit requiring 1,000 CFM. If you want a quiet system with a design velocity of 700 FPM:
- Area = 1000 / 700 = 1.428 sq. ft.
- Round Diameter = √((1.428 × 144 × 4) / 3.14159) ≈ 16.2 inches.
- You would likely install a 16″ or 18″ round duct depending on available space and static pressure requirements.
Standard Duct Velocity Recommendations
| Application | Recommended Velocity |
|---|---|
| Residential Main Trunk | 700 – 900 FPM |
| Residential Branch Ducts | 600 FPM |
| Commercial Main Trunk | 1,200 – 1,500 FPM |