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Equivalent Diameters for Rectangular and Round Circular Ducts
The rectangular dimensions and the air flow volume are adapted to the equal friction loss method of sizing ventilation duct systems.  An approximate friction loss of 0.8 inches water gauge per 100 ft duct (6.6 Pa/m) is used.
Air Flow
-q-
(Cubic Feet per Minute, CFM)
(m3/s)
Rectangular Duct Sizes
(Inches)
Equivalent Diameter
Round Duct Sizes

- de -
(Inches)
Velocity
- v -
(ft/min)
(m/s)
Friction Loss
(Inches water gauge per 100 ft duct)
200
-0.09
300
-0.14
400
-0.19
 
500
-0.24
750
-0.35
 
1000
-0.47
1250
-0.59
1500
-0.71
1750
-0.83
 
2000
-0.94
2500
-1.2
3000
-1.4
3500
-1.7
4000
-1.9
4500
-2.1
5000
-2.4
 
6000
-2.8
7000
-3.3
8000
-3.8
9000
-4.3
10000
-4.7
 
12500
-5.9
 
15000
-7.1
 
17500
-8.3
 
20000
-9.4
25000
-11.8
 
30000
-14.2
35000
-16.5
40000
-18.9
45000
-21.2
50000
-23.6
3 x 7
4 x 5
4 x 7
5 x 6
4 x 9
5 x 7
6 x 6
6 x 7
5 x 12
6 x 10
7 x 8
7 x 10
8 x 9
8 x 10
9 x 9
8 x 12
10 x 10
8 x 14
9 x 12
10 x 11
8 x 15
10 x 12
10 x 14
12 x 12
12 x 14
12 x 15
10 x 22
14 x 15
12 x 19
14 x 16
10 x 25
12 x 20
15 x 16
14 x 20
15 x 18
12 x 26
16 x 20
12 x 30
14 x 25
12 x 34
15 x 25
12 x 36
16 x 25
20 x 20
12 x 45
16 x 30
20 x 24
16 x 36
18 x 30
23 x 25
16 x 40
20 x 32
25 x 25
20 x 35
25 x 28
16 x 55
20 x 43
25 x 38
20 x 50
30 x 32
20 x 55
30 x 35
25 x 48
30 x 40
32 x 40
32 x 45
35 x 40
4.9
4.9
5.7
6
6.4
6.4
6.6
7.1
8.3
8.4
8.2
9.1
9.3
9.8
9.8
10.7
10.9
11.5
11.3
11.5
11.8
12
12.9
13.1
14.1
14.6
15.9
15.8
16.4
16.4
16.9
16.8
16.9
18.2
17.9
19
19.5
20.2
20.2
21.4
21
21.9
21.7
21.9
24.1
23.7
23.9
24.7
25.2
26.2
27
27.5
27.3
28.6
28.9
31
31.5
33.5
33.7
33.9
35.2
35.4
37.4
37.8
39.1
41.3
40.9
1527
-7.8
1635
-8.3
1736
-8.8
 
1819
-9.2
1996
-10.1
 
2166
-11
2386
-12.1
2358
-11.9
2469
-12.5
 
2589
-13.2
2712
-13.8
2767
-14.1
3010
-15.3
2938
-14.9
3068
-15.6
3248
-16.5
 
3358
-17.1
3482
-17.7
3595
-18.3
3671
-18.6
3858
-19.6
 
4012
-20.4
 
4331
-22
 
4337
-22
 
4483
-22.8
4709
-23.9
 
4815
-24.5
5179
-26.3
5243
-26.6
5397
-27.4
5222
-26.5
0.88
0.82
0.8
0.78
0.77
0.79
0.88
0.77
0.78
0.81
0.8
0.75
0.84
0.73
0.76
0.82
0.8
0.8
0.8
0.78
0.83
0.8
0.87
0.79
0.79
0.78
0.74
0.81
0.77
0.77
0.66
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