Rectangular and Circular HVAC Ducts - Equivalent Diameter

Equivalent diameters for rectangular and circular ducts - air flows between 100 - 50,000 cfm

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 - a rectangular duct is always more inefficient than a round duct. For the same friction - larger volume and smaller velocity for the rectangular duct. 

Related Topics

  • Air Conditioning - Air Conditioning systems - heating, cooling and dehumidification of indoor air for thermal comfort
  • Ventilation - Systems for ventilation and air handling - air change rates, ducts and pressure drops, charts and diagrams and more

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