HVAC Duct Sizing Calculator
Calculate round and rectangular duct dimensions based on CFM and Static Pressure
Comprehensive Guide to HVAC Duct Sizing
Proper duct sizing is critical for ensuring that an HVAC system operates efficiently, quietly, and provides consistent comfort throughout a building. If ducts are too small, the system will be noisy and the blower motor may fail prematurely due to high static pressure. If they are too large, air velocity drops, leading to poor air distribution and temperature stratification.
Understanding the Key Metrics
- CFM (Cubic Feet per Minute): The volume of air moving through the duct. Typically, a cooling system requires 400 CFM per ton of refrigeration.
- Friction Loss: The resistance air faces as it moves through the ductwork. It is usually expressed as inches of water gauge (in. wg) per 100 feet of duct. 0.1 in. wg is the most common industry standard for sizing.
- Velocity (FPM): Feet per minute represents how fast the air is moving. Residential branch ducts should stay below 600-700 FPM, while main trunks can go up to 900 FPM to minimize noise.
How to Use the Duct Sizing Calculator
To calculate your duct size, follow these steps:
- Determine CFM: Calculate the heat load of the room to find the required airflow. For example, a 3-ton unit needs approximately 1,200 CFM.
- Select Friction Rate: For a standard residential installation, use 0.08 or 0.10. Use a lower value for quieter systems.
- Round vs. Rectangular: The calculator provides the ideal round diameter first. If you are installing rectangular ducts, input one side (e.g., the height restricted by your ceiling joists) to find the required width.
Duct Sizing Chart (Standard Round)
| CFM Range | Round Duct Diameter | Velocity (Approx) |
|---|---|---|
| 50 – 100 | 6″ | 500 FPM |
| 150 – 250 | 8″ | 650 FPM |
| 350 – 500 | 10″ | 800 FPM |
| 600 – 900 | 12″ | 950 FPM |
Common Calculation Examples
Example 1: A bedroom requires 150 CFM. Using a friction rate of 0.1, the calculator suggests a 7-inch round duct. If using a 6-inch high rectangular duct, you would need it to be roughly 7 inches wide.
Example 2: A main trunk line for a 4-ton unit (1600 CFM) at a 0.08 friction rate. The result would be approximately a 16-inch round duct or a 20″ x 10″ rectangular duct.