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Equivalent Diameter - Rectangular and Circular HVAC Ducts

Equivalent diameter for rectangular and circular ducts - air flows between 100 - 50000 cfm

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The equivalent diameter is the diameter of a circular duct or pipe that gives the same pressure loss as an equivalent rectangular duct or pipe.

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 0.8 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 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
  • 1 in = 25.4 mm
  • 1 inH2O / 100 ft = 84 mmH2O /100 m

Note! - due to larger circumference and more friction area related to air volume - circular ducts are always more efficient than rectangular ducts.

Example - Air Flow in Rectangular vs. Circular Duct

2000 cfm (0.94 m3/s) of air flows through a 10 x 12 inches rectangular duct.

The friction loss according the table above is 0.81 inches water gauge per 100 ft duct (aprox. 6.6 Pa/m).

The velocity in the rectangular duct is

v = (2000 cfm) / (((10 inches) (1/12 ft/inches) ((12 inches)(1/12 ft/inches)))

  = 2400 ft/min  (12.2 m/s)

The velocity in the the equivalent 12 inch circular duct according the table above is 2589 ft/min (13.2 m/s).

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