The table below can be used to compare equivalent diameters for rectangular and round circular ducts. The table is based on the ducts friction loss formula.

equivalent rectangular diameter

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 0.08 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 in equivalent round Duct
v
(ft/min)
(m/s)
Friction Loss
(Inches water gauge per 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
 Note that a due to larger circumference and more friction area due to transport volume Рa rectangular duct is always more inefficient than a round duct. For the same friction Рlarger volume and smaller velocity for the rectangular duct.

Credits: Engineering Toolbox

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