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