Round Duct Size Chart (CFM at Standard Friction Rate)
This chart shows the airflow capacity of standard round sheet metal ducts at a friction rate of 0.08 inches of water column (IWC) per 100 feet, which is the standard design friction rate for most residential HVAC systems. Actual capacity varies with duct material, fittings, and total equivalent length.
| Round Duct Diameter | Airflow Capacity (CFM) | Cross-Sectional Area (sq in) | Airflow Velocity (FPM) | Typical Use |
|---|---|---|---|---|
| 4 inch | 50 CFM | 12.6 | ~575 | Small bathroom, laundry |
| 5 inch | 80 CFM | 19.6 | ~590 | Bathroom, small closet |
| 6 inch | 120 CFM | 28.3 | ~610 | Small bedroom, bathroom |
| 7 inch | 170 CFM | 38.5 | ~640 | Medium bedroom |
| 8 inch | 250 CFM | 50.3 | ~720 | Large bedroom, office |
| 9 inch | 320 CFM | 63.6 | ~740 | Large room, living room branch |
| 10 inch | 400 CFM | 78.5 | ~765 | Living room, 1-ton trunk |
| 12 inch | 600 CFM | 113.1 | ~795 | Large living area, main trunk |
| 14 inch | 900 CFM | 153.9 | ~870 | 2-ton trunk line |
| 16 inch | 1,200 CFM | 201.1 | ~890 | 3-ton trunk (main supply) |
| 18 inch | 1,600 CFM | 254.5 | ~930 | 4-ton trunk, large return |
| 20 inch | 2,000 CFM | 314.2 | ~960 | 5-ton trunk, commercial |
Rectangular Duct Equivalent Sizes
When round ducts won't fit, rectangular ducts provide the same airflow capacity in a different shape. The following table shows common rectangular duct dimensions and their round duct equivalents:
| Rectangular Duct (inches) | Equivalent Round Duct | Approximate CFM |
|---|---|---|
| 4 × 6 | 5″ round | ~75 CFM |
| 4 × 8 | 6″ round | ~100 CFM |
| 4 × 10 | 6″ round | ~120 CFM |
| 6 × 8 | 8″ round | ~200 CFM |
| 6 × 10 | 9″ round | ~280 CFM |
| 8 × 10 | 10″ round | ~380 CFM |
| 8 × 12 | 12″ round | ~500 CFM |
| 10 × 12 | 14″ round | ~700 CFM |
| 10 × 14 | 14″ round | ~850 CFM |
| 12 × 14 | 16″ round | ~1,050 CFM |
| 12 × 16 | 16″ round | ~1,200 CFM |
| 14 × 16 | 18″ round | ~1,400 CFM |
| 16 × 18 | 20″ round | ~1,800 CFM |
| 18 × 20 | 22″ round | ~2,200 CFM |
How to Use This Chart
Step 1: Calculate Total CFM
Start with your system's total tonnage. Multiply tons × 400 CFM to get total airflow. For example, a 3-ton system = 1,200 CFM.
Step 2: Size the Main Trunk
Look up the total CFM in the round duct chart to find the main trunk size. For 1,200 CFM, use a 16-inch round duct or 12×16 rectangular duct.
Step 3: Size Each Branch
Determine the CFM for each room (1 CFM per sq ft is a good starting estimate) and look up the corresponding branch duct size. A 200 sq ft room needs about 200 CFM, so use an 8-inch duct.
Step 4: Taper the Trunk
After each branch takeoff, subtract that branch's CFM from the remaining trunk airflow and downsize the trunk if needed. This maintains consistent velocity throughout the system.
Understanding Friction Rate and Air Velocity
The CFM values in this chart are based on a standard residential friction rate of 0.08 IWC per 100 feet of equivalent duct length. Key factors that affect actual performance:
- Duct material: Sheet metal (galvanized steel) has the lowest friction. Flex duct adds 50-100% more friction. Always upsize flex duct by one inch.
- Fittings: Each 90° elbow adds approximately 10-15 feet of equivalent length. Use smooth radius elbows when possible.
- Air velocity: Keep supply trunk velocity below 900 FPM and branch velocity below 600 FPM for quiet operation. Return air can run slightly higher.
- Altitude and temperature: At high altitudes or extreme temperatures, air density changes. Adjust CFM calculations accordingly.
Common Residential Duct Size Applications
Here's a quick reference for the most common residential HVAC system sizes:
| System Size | Total CFM | Main Trunk (Round) | Main Trunk (Rect) | Typical Branches |
|---|---|---|---|---|
| 1.5 Ton | 600 CFM | 12″ | 8×12″ | 6″ for most rooms |
| 2 Ton | 800 CFM | 14″ | 10×12″ | 6–8″ for most rooms |
| 2.5 Ton | 1,000 CFM | 14″ | 10×14″ | 6–8″ for most rooms |
| 3 Ton | 1,200 CFM | 16″ | 12×16″ | 6–8″ for most rooms |
| 3.5 Ton | 1,400 CFM | 16″ | 12×18″ | 7–8″ for most rooms |
| 4 Ton | 1,600 CFM | 18″ | 14×18″ | 8–10″ for most rooms |
| 5 Ton | 2,000 CFM | 20″ | 16×20″ | 8–10″ for most rooms |
Get Precise Duct Sizes for Your Home
This chart provides general guidelines. For exact sizing based on your home's layout, room sizes, and duct run lengths, use our free duct sizing calculator.
Open Duct Sizing Calculator →Frequently Asked Questions
How many CFM can a 6-inch duct handle?
A 6-inch round duct can handle approximately 100 to 120 CFM at standard residential friction rates (0.08 IWC per 100 ft). This is suitable for a small bedroom or bathroom.
What size duct is needed for 400 CFM?
For 400 CFM, you need a 10-inch round duct. The rectangular equivalent is approximately 8×10 inches. This handles one ton of cooling capacity.
What is the maximum CFM for a 12-inch duct?
A 12-inch round duct can handle approximately 600 CFM at standard friction rates. This is equivalent to 1.5 tons of cooling capacity.
How do I convert rectangular duct to round duct size?
Use the formula: D = 1.3 × (A×B)^0.625 ÷ (A+B)^0.25, where A and B are the rectangular duct dimensions and D is the equivalent round diameter. Or use our online calculator for instant conversion.
Is round duct better than rectangular duct?
Round duct is generally more efficient because it has less surface area per unit of airflow, resulting in lower friction losses and quieter operation. Rectangular duct is used where space constraints prevent round duct installation.