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.
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.
Air flow  q  (Cubic Feet per Minute, cfm) (m^{3}/s)  Rectangular Duct Sizes (inches) (mm) x (mm)  Equivalent Diameter Round Duct Sizes  d_{e } (inches) (mm)  Velocity in equivalent round Duct  v  (ft/min) (m/s)  Friction Loss (inH_{2}O / 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 
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 inH_{2}O / 100 ft = 84 mmH_{2}O /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 m^{3}/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

Air Ducts  Friction Loss Diagram
A major friction loss diagram for air ducts  Imperial units ranging 10  100 000 cfm. 
Air Ducts  Friction Loss Diagram
A major friction loss diagram for air ducts  SI units. 
Air Ducts  Major Friction Head Loss, Online Calculator
Head loss or major loss in air ducts  equations and online calculator for rectangular and circular ducts  Imperial and SI units. 
Duct Sizing  the Equal Friction Method
The equal friction method for sizing air ducts is easy and straightforward to use. 
Ducts  CrossSectional Area vs. Diameter
Circular ducts srosssectional areas. 
Ducts  Sheet Metal Gauges
Thickness of sheet metal used in ductwork. 
Ducts or Pipes through Steel Joists  Maximum Sizes
Maximum sizes of ducts or pipes through K series joists. 
Ducts Sizing  the Velocity Reduction Method
The velocity reduction method can be used when sizing air ducts. 
Ductwork  Seal Classes
Ductwork air leakage classes. 
Equivalent Length Method  Minor Pressure Loss in Piping Systems
Calculate minor pressure loss in piping systems with the Equivalent Pipe Length Method. 
Fluid Flow  Equivalent Diameter
Convert rectangular and oval duct geometry to a equivalent circular diameter  online calculator with imperial and SIunits. 
Rectangular Air Ducts  Velocity Diagram
Velocity diagram for rectangular air ducts  metric units. 
Rectangular Ducts  Dimensions
Commonly used metric dimensions for rectangular air ducts in ventilation systems. 
Spiral Ducts  Dimensions
Standard dimensions of spiral ducts  Imperial units. 
Velocity Classification of Ventilation Ducts
Recommended air velocities in ventilation ducts