Customized values can be calculated in the top row.
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 | 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 |
Note! - due to larger circumference and more friction area related to air volume - a circular duct is always more efficient than a rectangular 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).
Air Conditioning systems - heating, cooling and dehumidification of indoor air for thermal comfort.
Systems for ventilation and air handling - air change rates, ducts and pressure drops, charts and diagrams and more.
A major friction loss diagram for air ducts - Imperial units ranging 10 - 100 000 cfm.
A major friction loss diagram for air ducts - SI units.
Head loss or major loss in air ducts - equations and online calculator for rectangular and circular ducts - Imperial and SI units.
The equal friction method for sizing air ducts is easy and straightforward to use.
Circular ducts sross-sectional areas.
Thickness of sheet metal used in ductwork.
Maximum sizes of ducts or pipes through K series joists.
The velocity reduction method can be used when sizing air ducts.
Ductwork air leakage classes.
Calculate minor pressure loss in piping systems with the Equivalent Pipe Length Method.
Convert rectangular and oval duct geometry to a equivalent circular diameter - online calculator with imperial and SI-units.
Velocity diagram for rectangular air ducts - metric units.
Commonly used metric dimensions for rectangular air ducts in ventilation systems.
Standard dimensions of spiral ducts - Imperial units.
Recommended air velocities in ventilation ducts
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