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).
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