Engineering ToolBox - Resources, Tools and Basic Information for Engineering and Design of Technical Applications!

Equivalent Diameter - Rectangular vs. Circular HVAC Ducts

Rectangular equivalent diameter for air flows between 100 - 50000 cfm.

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.

The table below indicates maximum air flow in low and medium pressure loss ducts.

  • maximum friction < 0.1 - 0.2 inches W.G./100 ft
  • air velocity < 1500 - 2000 ft/min (8 - 10 m/s)

Customized values can be calculated in the top row.

HVAC Ducts - Rectangular vs. Circular Equivalent Diameters
Air flow
- q -
(Cubic Feet per Minute, cfm)
(m3/s)		Rectangular Duct Sizes
(inches)
(mm) x (mm)		Equivalent Diameter
Round Duct Sizes
- de -
(inches)
(mm)		Velocity in equivalent round Duct
- v -
(ft/min)
(m/s)		Friction Loss
(inH2O / 100 ft duct)
(Pa/100m)

()		 x
() x ()
()
()
()
80
(0.04)		 3 x 7
4 x 5		 4.9
4.9		 611
(3.1)		 0.15
100
(0.045)		 4 x 6 5.33 646
(3.28)		 0.16
120
(0.05)		 4 x 7
5 x 6		 5.7
6.0		 670
(3.57)		 0.15
140
(0.063)		 4 x 8 6.09 691
(3.5)		 0.15
150
(0.068)		 3.5 x 10 6.26 702
(3.6)		 0.15
160
(0.07)		 4 x 9
5 x 7
6 x 6		 6.4
6.4
6.6		 710
(3.6)		 0.15
180
(0.08)		 4 x 10 6.74 726
(3.69)		 0.15
200
(0.09)		 6 x 7 7.1 732
(3.7)		 0.14
230
(0.12)		 4 x 12 7.31 790
(4.01)		 0.15
250
(0.11)		 6 x 8 7.55 803
(4.08)		 0.15
270
(0.1)		 4 x 14 7.81 811
(4.12)		 0.15
300
(0.14)		 5 x 12
6 x 10
7 x 8		 8.3
8.4
8.2		 804
(4.1)		 0.14
400
(0.18)		 7 x 10
8 x 9		 9.1
9.3		 884
(4.5)		 0.15

480
(0.22)

 8 x 10
9 x 9		 9.8
9.8		 923
(4.7)		 0.15
600
(0.27)		 8 x 12
10 x 10		 10.7
10.9		 909
(4.9)		 0.14
750
(0.34)		 8 x 14
9 x 12
10 x 11		 11.5
11.3
11.5		 1047
(5.3)		 0.15
800
(0.36)		 8 x 15
10 x 12		 11.8
12.0		 1048
(5.3)		 0.15
1000
(0.45)		 10 x 14
12 x 12		 12.9
13.1		 1104
(5.6)		 0.15
1300
(0.59)		 12 x 14 14.1 1189
(6.0)		 0.15
1400
(0.63)		 12 x 15 14.6 1197
(6.1)		 0.15
1700
(0.77)		 10 x 22
14 x 15		 15.9
15.8		 1231
(6.3)		 0.14
1900
(0.86)		 12 x 19
14 x 16		 16.4
16.4		 1295
(6.6)		 0.15
2000
(0.9)		 10 x 25
12 x 20
15 x 16		 16.9
16.8
16.9		 1291
(6.6)		 0.14
2500
(1.13)		 14 x 20
15 x 18		 18.2
17.9		 1381
(7.0)		 0.15
2800
(1.26)		 12 x 26
16 x 20		 19.0
19.5		 1428
(7.3)		 0.15
3300
(1.49)		 12 x 30
14 x 25		 20.2
20.2		 1480
(7.5)		 0.15
3800
(1.71)		 12 x 34
15 x 25		 21.4
21.0		 1525
(7.8)		 0.15
4100
(1.84)		 12 x 36
16 x 25
20 x 20		 21.9
21.7
21.9		 1565
(8.0)		 0.15
5400
(2.43)		 12 x 45
16 x 30
20 x 24		 24.1
23.7
23.9		 1699
(8.6)		 0.15
6200
(2.79)		 16 x 36
18 x 30
23 x 25		 24.7
25.2
26.2		 1719
(8.7)		 0.15
7200
(3.24)		 16 x 40
20 x 32
25 x 25		 27.0
27.5
27.3		 1816
(9.2)		 0.15
8400
(3.78)		 20 x 35
25 x 28		 28.6
28.9		 1879
(9.5)		 0.15
10300
(4.64)		 16 x 55
20 x 43
25 x 38		 31.0
31.5
33.5		 1964
(10.0)		 0.15
13000
(5.85)		 20 x 50
30 x 32		 33.7
33.9		 2098
(10.7)		 0.15
13500
(6.08)		 20 x 55
30 x 35		 35.2
35.4		 2002
(10.2)		 0.13
15000
(6.75)		 25 x 48
30 x 40		 37.4
37.8		 1969
(10)		 0.12
17000
(7.65)		 32 x 40 39.1 2044
(10.4)		 0.12
19000
(8.55)		 32 x 45
35 x 40		 41.3
40.9		 2039
(10.4)		 0.11
  • 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 - a circular duct is always more efficient than a rectangular duct.

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).

Related Topics

  • Air Conditioning Systems

    Design of Air Conditioning systems - heating, cooling and dehumidification of indoor air for thermal comfort.

  • Ventilation Systems

    Design of systems for ventilation and air handling - air change rates, ducts and pressure drops, charts and diagrams and more.

Related Documents

Search

Search is the most efficient way to navigate the Engineering ToolBox.

Engineering ToolBox - SketchUp Extension - Online 3D modeling!

3D Engineering ToolBox Extension to SketchUp - add parametric components to your SketchUp model

Add standard and customized parametric components - like flange beams, lumbers, piping, stairs and more - to your Sketchup model with the Engineering ToolBox - SketchUp Extension - enabled for use with older versions of the amazing SketchUp Make and the newer "up to date" SketchUp Pro . Add the Engineering ToolBox extension to your SketchUp Make/Pro from the Extension Warehouse !

Translate this Page

Translate this page to Your Own Language .

About the Engineering ToolBox!

Privacy Policy

We don't collect information from our users. More about

We use a third-party to provide monetization technologies for our site. You can review their privacy and cookie policy here.

You can change your privacy settings by clicking the following button: .

Citation

This page can be cited as

  • The Engineering ToolBox (2003). Equivalent Diameter - Rectangular vs. Circular HVAC Ducts . [online] Available at: https://www.engineeringtoolbox.com/equivalent-diameter-d_443.html [Accessed Day Month Year].

Modify the access date according your visit.

3D Engineering ToolBox - draw and model technical applications! 2D Engineering ToolBox - create and share online diagram drawing templates! Engineering ToolBox Apps - mobile online and offline engineering applications!

Unit Converter

















































4.19.9

.