18 Inch Duct Calculator
What is the CFM for a 18 inch duct? Calculate airflow capacity, velocity limits, and rectangular equivalents for 18" HVAC ducts. Includes metal and flex duct notes.
Airflow Capacity for 18" Round Duct
A 18 inch round duct has a cross sectional area of 254.5 square inches (1.767 sq ft). The table below shows the CFM airflow at different air velocities.
| Velocity (FPM) | Airflow (CFM) | Noise Level | Use Case |
|---|---|---|---|
| 600 FPM | 1060 CFM | ✅ Quiet | Residential supply |
| 700 FPM | 1237 CFM | ✅ Quiet | Residential supply |
| 800 FPM | 1414 CFM | ⚠️ Moderate | Commercial supply |
| 900 FPM | 1590 CFM | 🔊 May be noisy | Industrial |
Technical Specifications
Rectangular Equivalent
26x10 or 22x12 inches
Based on ASHRAE equivalent diameter formulas.
Max Recommended CFM
1590 CFM
Based on 900 FPM (max residential velocity).
18 Inch Metal vs. Flex Duct
While the nominal diameter is the same, 18" flexible duct has significantly higher air resistance than smooth 18" galvanized sheet metal duct.
- Metal Duct: Delivers approximately 1237 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 20" flex duct.
- Return Air: For a return duct, a 18" round duct is generally limited to 884 CFM to maintain quiet operation (600 FPM).
Frequently Asked Questions
How much CFM can a 18 inch duct handle?
A 18 inch round duct can handle between 884 and 1590 CFM. For residential supply branches, it is typically used for 1237 CFM at 0.08 friction rate.
What is the rectangular equivalent of 18" round?
The most common rectangular equivalents for a 18 inch round duct are 26x10 or 22x12 inches.
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