12 Inch Duct Calculator
What is the CFM for a 12 inch duct? Calculate airflow capacity, velocity limits, and rectangular equivalents for 12" HVAC ducts. Includes metal and flex duct notes.
Airflow Capacity for 12" Round Duct
A 12 inch round duct has a cross sectional area of 113.1 square inches (0.785 sq ft). The table below shows the CFM airflow at different air velocities.
| Velocity (FPM) | Airflow (CFM) | Noise Level | Use Case |
|---|---|---|---|
| 600 FPM | 471 CFM | ✅ Quiet | Residential supply |
| 700 FPM | 550 CFM | ✅ Quiet | Residential supply |
| 800 FPM | 628 CFM | ⚠️ Moderate | Commercial supply |
| 900 FPM | 707 CFM | 🔊 May be noisy | Industrial |
Technical Specifications
Rectangular Equivalent
16x8 or 14x10 inches
Based on ASHRAE equivalent diameter formulas.
Max Recommended CFM
707 CFM
Based on 900 FPM (max residential velocity).
12 Inch Metal vs. Flex Duct
While the nominal diameter is the same, 12" flexible duct has significantly higher air resistance than smooth 12" galvanized sheet metal duct.
- Metal Duct: Delivers approximately 550 CFM at 0.1 friction rate.
- Flex Duct: Usually requires a 1.5x resistance factor. To deliver the same airflow, you may need to increase to a 14" flex duct.
- Return Air: For a return duct, a 12" round duct is generally limited to 393 CFM to maintain quiet operation (600 FPM).
Frequently Asked Questions
How much CFM can a 12 inch duct handle?
A 12 inch round duct can handle between 393 and 707 CFM. For residential supply branches, it is typically used for 550 CFM at 0.08 friction rate.
What is the rectangular equivalent of 12" round?
The most common rectangular equivalents for a 12 inch round duct are 16x8 or 14x10 inches.
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