Duct Sizing Chart CFM

There are a few things that one must keep in mind when using the chart below.

  1. This chart should not be used if you are designing a full duct system.
  2. This chart estimates the airflow for an average system.
  3. 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 CFMRectangular 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
506 x 4550
756x 4675
100B x 46 x 66100
12510 x 46 x 67125
15010 x 48 x 67150
17512 x 48 x 68175
20014 x 48x68200
22516x410x68225
25016 x 410 x69250
27512 x68 x 89275
30012x66 x 89300
40014 x610 x 810400
50018 x 612 x 810 x 1011500
60020 x614 x 812 x 1012600
70024 x 616 x 812 x 1012700
80026 x 618 x 814 x 1012 x 1213800
90030 x 620 x 816 x 1012x 1214900
100022 x 816 x 1014 x 12141000
110024x818 x 1016x12151100
120026 x 820 x 1016 x 12151200
130028 x 820x 1018 x 12161300
140030 x 822 x 1018 x 12161400
150024 x 1020 x 12161500
160024 x 1020 x 12171600
170026 x 1022 x 12171700
180028 x 1022x 12181800
190030 x 1022x 12181900
200024 x 10182000
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