Properly sizing HVAC ducts is critical to ensuring the efficient and effective operation of your HVAC system.
In this article, we you will find HVAC duct size charts that provide guidelines for selecting the appropriate duct size for the airflow of your air conditioning / HVAC system.
How To Choose The Correct Size Duct
While fully designing your ductwork system can be quite complex, a lot of the work can be taken out of the equation by using simple rules and charts.
This page focuses on understanding the appropriate information required for the Ductwork Calculator. It includes aspects such as air flow CFM, friction loss (essentially loss resulting from friction), and velocity.
If you want to save yourself even more time, then head on over to our duct size calculator and have it do the hard work for you.
But if you want to take a look at the charts and data behind the calculator, then you have come to the right spot.
How To Use These HVAC Duct Size Charts
- Identify the volume of air (in CFM) that will be passing through the duct.
- Choose the appropriate duct size from the table based on the identified volume of air.
- In case the desired airflow exceeds the maximum CFM rating provided in the table, increase to the next larger size duct.
- Keep in mind that the listed CFM ratings are based on typical field results, and variations may occur. In such cases, it’s advisable to install dampers for better airflow control.
- If the duct run is longer than 25 feet or has numerous transitions, consider using the next larger size duct.
- You can easily print these charts out using the “Print Chart” button at the bottom of each table.
We have listed the friction rate used in the calculation on each chart. These friction rates are considered adequate for residential HVAC systems.
Standard Air Duct Sizes
We have included charts for most of the popular standard air duct sizes.
These include:
- Round flex duct: 5 inch – 20 inch
- Metal round pipe: 5 inch – 20 inch
- 4 inch rectangular duct: 6 inch – 24 inch diameter
- 6 inch rectangular duct: 4 inch – 30 inch diameter
- 8 inch rectangular duct: 4 inch – 36 inch diameter
- 10 inch rectangular duct: 4 inch – 40 inch diameter
- 12 inch rectangular duct: 4 inch – 42 inch diameter
Round Flex Duct Sizing Chart
| Duct Size | Max Airflow (0.05 Friction Rate) | Max Airflow (0.1 Friction Rate) |
|---|---|---|
| 4 inch | 20 CFM | 20 CFM |
| 5 inch | 50 CFM | 50 CFM |
| 6 inch | 75 CFM | 80 CFM |
| 7 inch | 110 CFM | 120 CFM |
| 8 inch | 160 CFM | 170 CFM |
| 9 inch | 225 CFM | 230 CFM |
| 10 inch | 300 CFM | 300 CFM |
| 12 inch | 480 CFM | 500 CFM |
| 14 inch | 700 CFM | 740 CFM |
| 16 inch | 1000 CFM | 1050 CFM |
| 18 inch | 1300 CFM | 1400 CFM |
| 20 inch | 1700 CFM | 1875 CFM |
Take note of this, and the following table. You will notice that round flexible duct does not handle as much airflow as metal round duct of the same size.
This is due to the fact that flex ducts often have obstruction issues, especially around bends.
Metal Round Duct CFM Chart (Max 1700 CFM)
| Duct Size | Max Airflow (0.06 Friction Rate) | Max Airflow (0.1 Friction Rate) |
|---|---|---|
| 4 inch | 20 CFM | 20 CFM |
| 5 inch | 50 CFM | 70 CFM |
| 6 inch | 85 CFM | 115 CFM |
| 7 inch | 125 CFM | 170 CFM |
| 8 inch | 180 CFM | 240 CFM |
| 9 inch | 240 CFM | 320 CFM |
| 10 inch | 325 CFM | 420 CFM |
| 12 inch | 525 CFM | 680 CFM |
| 14 inch | 750 CFM | 1050 CFM |
| 16 inch | 1200 CFM | 1450 CFM |
| 18 inch | 1500 CFM | 2000 CFM |
| 20 inch | 2000 CFM | 2600 CFM |
4 Inch Rectangular Duct Size Chart (Max 2000 CFM)
| Duct Size (inches) | Max Airflow (0.07 Friction Rate) |
|---|---|
| 6×4 | 60 CFM |
| 8×4 | 90 CFM |
| 10×4 | 120 CFM |
| 12×4 | 150 CFM |
| 14×4 | 180 CFM |
| 16×4 | 210 CFM |
| 18×4 | 240 CFM |
| 20×4 | 270 CFM |
| 22×4 | 300 CFM |
| 24×4 | 330 CFM |
6 Inch Rectangular Ducts Size Chart (Max 775 CFM)
| Duct Size (inches) | Max Airflow (0.07 Friction Rate) |
|---|---|
| 4×6 | 60 CFM |
| 6×6 | 110 CFM |
| 8×6 | 160 CFM |
| 10×6 | 215 CFM |
| 12×6 | 270 CFM |
| 14×6 | 320 CFM |
| 16×6 | 375 CFM |
| 18×6 | 430 CFM |
| 20×6 | 490 CFM |
| 22×6 | 540 CFM |
| 24×6 | 600 CFM |
| 26×6 | 650 CFM |
| 28×6 | 710 CFM |
| 30×6 | 775 CFM |
8 Inch Rectangular Ducts Size Chart (Max 1500 CFM)
| Duct Size (inches) | Max Airflow (0.07 Friction Rate) |
|---|---|
| 4×8 | 90 CFM |
| 6×8 | 160 CFM |
| 8×8 | 230 CFM |
| 10×8 | 310 CFM |
| 12×8 | 400 CFM |
| 14×8 | 490 CFM |
| 16×8 | 580 CFM |
| 18×8 | 670 CFM |
| 20×8 | 750 CFM |
| 22×8 | 840 CFM |
| 24×8 | 930 CFM |
| 26×8 | 1,020 CFM |
| 28×8 | 1,100 CFM |
| 30×8 | 1,200 CFM |
| 32×8 | 1,300 CFM |
| 34×8 | 1,400 CFM |
| 36×8 | 1,500 CFM |
10 Inch Rectangular Duct Sizing Chart (Max 2350 CFM)
| Duct Size (inches) | Max Airflow (0.07 Friction Rate) |
|---|---|
| 4×10 | 120 CFM |
| 6×10 | 215 CFM |
| 8×10 | 310 CFM |
| 10×10 | 430 CFM |
| 12×10 | 550 CFM |
| 14×10 | 670 CFM |
| 16×10 | 800 CFM |
| 18×10 | 930 CFM |
| 20×10 | 1,060 CFM |
| 22×10 | 1,200 CFM |
| 24×10 | 1,320 CFM |
| 26×10 | 1,430 CFM |
| 28×10 | 1,550 CFM |
| 30×10 | 1,670 CFM |
| 32×10 | 1,800 CFM |
| 34×10 | 1,930 CFM |
| 36×10 | 2,060 CFM |
| 38×10 | 2,200 CFM |
| 40×10 | 2,350 CFM |
12 Inch Rectangular Ducts Sizing Chart (max 3050 CFM)
| Duct Size (inches) | Max Airflow (0.07 Friction Rate) |
|---|---|
| 4×12 | 150 CFM |
| 6×12 | 270 CFM |
| 8×12 | 400 CFM |
| 10×12 | 550 CFM |
| 12×12 | 680 CFM |
| 14×12 | 800 CFM |
| 16×12 | 950 CFM |
| 18×12 | 1,100 CFM |
| 20×12 | 1,250 CFM |
| 22×12 | 1,400 CFM |
| 24×12 | 1,600 CFM |
| 26×12 | 1,750 CFM |
| 28×12 | 1,950 CFM |
| 30×12 | 2,150 CFM |
| 32×12 | 2,300 CFM |
| 34×12 | 2,450 CFM |
| 36×12 | 2,600 CFM |
| 38×12 | 2,750 CFM |
| 40×12 | 2,900 CFM |
| 42×12 | 3,050 CFM |
Frequently Asked Questions
what size duct do i need for 1600 cfm?
For an airflow of 1600 CFM then your options are:
- 20 Inch Round Flex
- 20 Inch Round Metal Pipe
- 30″ x 10″ Rectangular duct
- 24″ x 12″ Rectangular duct
For round flex we use a friction rate of 0.05, for round solid we use 0.06, for rectangular we use 0.07
what size duct do i need for 2000 cfm?
For an airflow of 2000 CFM then your options are:
- 20 Inch Round Metal Pipe
- 36″ x 10″ Rectangular duct
- 30″ x 12″ Rectangular duct
For round solid duct we use a friction rate of 0.06, for rectangular we use 0.07
what size duct do i need for 2500 cfm?
For an airflow of 2500 CFM then your options are:
- 36″ x 12″ Rectangular duct
For rectangular duct we use a friction rate of 0.07
what size duct do i need for Larger Airflow Amounts Like 3000, 4000, or 6000 CFM?
For larger systems with CFM ratings of 3000, 4000, 6000 or higher then you should contact a professional HVAC installer to help you plan out the system correctly. Making even a small mistake in planning for a high airflow system can have expensive ramifications down the track.
Why is Velocity Important?
When determining the correct duct size for an HVAC system, velocity plays a crucial role. Velocity refers to the speed at which air is circulated, or ‘flows,’ through the ductwork.
A well-calculated velocity ensures optimal efficiency in heat distribution and air circulation, maintaining a comfortable environment in your home.
Too high a velocity could lead to noise, increased wear and tear, and potential damage to your HVAC system. In contrast, too low may result in inadequate air distribution, causing uneven temperature and issues with heating and cooling efficiency.
Therefore, velocity must be carefully calculated and considered during duct sizing to ensure the HVAC system functions at its best.
