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Air duct sizing

Air flow
q - 
(Cubic Feet per Minute, cfm)
(m3/s)
Rectangular Duct Sizes
(inches)
Equivalent Diameter
Round Duct Sizes
d-
(inches)
Velocity in equivalent round Duct
v - 
(ft/min)
(m/s)
Friction Loss 
(inH2O / 100 ft duct)
200 
(0.09)
3 x 7
4 x 5
4.9
4.9
1527
(7.8)
0.88
300 
(0.14)
4 x 7
5 x 6
5.7
6.0
1635
(8.3)
0.82
400 
(0.19)
4 x 9
5 x 7
6 x 6
6.4
6.4
6.6
1736
(8.8)
0.80
500 
(0.24)
6 x 7 7.1 1819
(9.2)
0.78
750 
(0.35)
5 x 12
6 x 10
7 x 8
8.3
8.4
8.2
1996
(10.1)
0.77
1000 
(0.47)
7 x 10
8 x 9
9.1
9.3
2166
(11)
0.79
1250 
(0.59)
8 x 10
9 x 9
9.8
9.8
2386
(12.1)
0.88
1500 
(0.71)
8 x 12
10 x 10
10.7
10.9
2358
(11.9)
0.77
1750 
(0.83)
8 x 14
9 x 12
10 x 11
11.5
11.3
11.5
2469
(12.5)
0.78
2000 
(0.94)
8 x 15
10 x 12
11.8
12.0
2589
(13.2)
0.81
2500 
(1.2)
10 x 14
12 x 12
12.9
13.1
2712
(13.8)
0.8
3000 
(1.4)
12 x 14 14.1 2767
(14.1)
0.75
3500 
(1.7)
12 x 15 14.6 3010
(15.3)
0.84
4000 
(1.9)
10 x 22
14 x 15
15.9
15.8
2938
(14.9)
0.73
4500 
(2.1)
12 x 19
14 x 16
16.4
16.4
3068
(15.6)
0.76
5000 
(2.4)
10 x 25
12 x 20
15 x 16
16.9
16.8
16.9
3248
(16.5)
0.82
6000 
(2.8)
14 x 20
15 x 18
18.2
17.9
3358
(17.1)
0.8
7000 
(3.3)
12 x 26
16 x 20
19.0
19.5
3482
(17.7)
0.8
8000 
(3.8)
12 x 30
14 x 25
20.2
20.2
3595
(18.3)
0.8
9000 
(4.3)
12 x 34
15 x 25
21.4
21.0
3671
(18.6)
0.78
10000 
(4.7)
12 x 36
16 x 25
20 x 20
21.9
21.7
21.9
3858
(19.6)
0.83
12500 
(5.9)
12 x 45
16 x 30
20 x 24
24.1
23.7
23.9
4012
(20.4)
0.8
15000 
(7.1)
16 x 36
18 x 30
23 x 25
24.7
25.2
26.2
4331
(22)
0.87
17500 
(8.3)
16 x 40
20 x 32
25 x 25
27.0
27.5
27.3
4337
(22)
0.79
20000 
(9.4)
20 x 35
25 x 28
28.6
28.9
4483
(22.8)
0.79
25000 
(11.8)
16 x 55
20 x 43
25 x 38
31.0
31.5
33.5
4709
(23.9)
0.78
30000 
(14.2)
20 x 50
30 x 32
33.7
33.9
4815
(24.5)
0.74
35000 
(16.5)
20 x 55
30 x 35
35.2
35.4
5179
(26.3)
0.81
40000 
(18.9)
25 x 48
30 x 40
37.4
37.8
5243
(26.6)
0.77
45000 
(21.2)
32 x 40 39.1 5397
(27.4)
0.77
50000 
(23.6)
32 x 45
35 x 40
41.3
40.9
5222
(26.5)
0.66