8 Inch Duct Calculator
What is the CFM for a 8 inch duct? Calculate airflow capacity, velocity limits, and rectangular equivalents for 8" HVAC ducts. Includes metal and flex duct notes.
Airflow Capacity for 8" Round Duct
A 8 inch round duct has a cross sectional area of 50.3 square inches (0.349 sq ft). The table below shows the CFM airflow at different air velocities.
| Velocity (FPM) | Airflow (CFM) | Noise Level | Use Case |
|---|---|---|---|
| 600 FPM | 209 CFM | ✅ Quiet | Residential supply |
| 700 FPM | 244 CFM | ✅ Quiet | Residential supply |
| 800 FPM | 279 CFM | ⚠️ Moderate | Commercial supply |
| 900 FPM | 314 CFM | 🔊 May be noisy | Industrial |
Technical Specifications
Rectangular Equivalent
10x6 or 8x8 inches
Based on ASHRAE equivalent diameter formulas.
Max Recommended CFM
314 CFM
Based on 900 FPM (max residential velocity).
8 Inch Metal vs. Flex Duct
While the nominal diameter is the same, 8" flexible duct has significantly higher air resistance than smooth 8" galvanized sheet metal duct.
- Metal Duct: Delivers approximately 244 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 10" flex duct.
- Return Air: For a return duct, a 8" round duct is generally limited to 175 CFM to maintain quiet operation (600 FPM).
Frequently Asked Questions
How much CFM can a 8 inch duct handle?
A 8 inch round duct can handle between 175 and 314 CFM. For residential supply branches, it is typically used for 244 CFM at 0.08 friction rate.
What is the rectangular equivalent of 8" round?
The most common rectangular equivalents for a 8 inch round duct are 10x6 or 8x8 inches.
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