Duct Sizing Chart CFM
There are a few things that one must keep in mind when using the chart below.
- This chart should not be used if you are designing a full duct system.
- This chart estimates the airflow for an average system.
- Use volume dampers for every branch to ensure proper distribution of air.
The duct sizes in the chart below have been based on a friction rate of around 0.10 inches per 100 feet of a lineal duct. These duct sizings should be more than sufficient residential air conditioning and furnace heating applications. Higher static pressures and larger air volumes should be dealt with on a job to job basis.
| Air Volume CFM | Rectangular Duct Height 4" | Rectangular Duct Height 6" | Rectangular Duct Height 8" | Rectangular Duct Height 10" | Rectangular Duct Height 12" | Equivalent Round Duct (inches) | Air Volume CFM |
|---|---|---|---|---|---|---|---|
| 50 | 6 x 4 | 5 | 50 | ||||
| 75 | 6x 4 | 6 | 75 | ||||
| 100 | B x 4 | 6 x 6 | 6 | 100 | |||
| 125 | 10 x 4 | 6 x 6 | 7 | 125 | |||
| 150 | 10 x 4 | 8 x 6 | 7 | 150 | |||
| 175 | 12 x 4 | 8 x 6 | 8 | 175 | |||
| 200 | 14 x 4 | 8x6 | 8 | 200 | |||
| 225 | 16x4 | 10x6 | 8 | 225 | |||
| 250 | 16 x 4 | 10 x6 | 9 | 250 | |||
| 275 | 12 x6 | 8 x 8 | 9 | 275 | |||
| 300 | 12x6 | 6 x 8 | 9 | 300 | |||
| 400 | 14 x6 | 10 x 8 | 10 | 400 | |||
| 500 | 18 x 6 | 12 x 8 | 10 x 10 | 11 | 500 | ||
| 600 | 20 x6 | 14 x 8 | 12 x 10 | 12 | 600 | ||
| 700 | 24 x 6 | 16 x 8 | 12 x 10 | 12 | 700 | ||
| 800 | 26 x 6 | 18 x 8 | 14 x 10 | 12 x 12 | 13 | 800 | |
| 900 | 30 x 6 | 20 x 8 | 16 x 10 | 12x 12 | 14 | 900 | |
| 1000 | 22 x 8 | 16 x 10 | 14 x 12 | 14 | 1000 | ||
| 1100 | 24x8 | 18 x 10 | 16x12 | 15 | 1100 | ||
| 1200 | 26 x 8 | 20 x 10 | 16 x 12 | 15 | 1200 | ||
| 1300 | 28 x 8 | 20x 10 | 18 x 12 | 16 | 1300 | ||
| 1400 | 30 x 8 | 22 x 10 | 18 x 12 | 16 | 1400 | ||
| 1500 | 24 x 10 | 20 x 12 | 16 | 1500 | |||
| 1600 | 24 x 10 | 20 x 12 | 17 | 1600 | |||
| 1700 | 26 x 10 | 22 x 12 | 17 | 1700 | |||
| 1800 | 28 x 10 | 22x 12 | 18 | 1800 | |||
| 1900 | 30 x 10 | 22x 12 | 18 | 1900 | |||
| 2000 | 24 x 10 | 18 | 2000 |
