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# Rectangular Ducts - Hydraulic Diameter

Hydraulic diameter for rectangular ducts can be calculated as

dh = 2 a b / (a + b)         (1)

where

dh = hydraulic diameter (mm, in)

a = width of the duct (mm, in)

b = height of the duct (mm, in)

Hydraulic diameter of some common rectangular ducts:

Rectangular Ducts - Hydraulic Diameters
Width of Duct
(mm)
Hydraulic Diameter - dh - (mm)
Height of Duct (mm)
10015020025030040050060080010001200
200 133 171 200
250 143 188 122 250
300 150 200 222 273 300
400 160 218 240 308 343 400
500 231 267 333 375 444 500
600 240 286 353 400 480 545 600
800 300 381 436 533 615 686 800
1000 320 400 462 571 667 750 889 1000
1200 480 600 706 800 960 1091 1200
1400 622 737 840 1018 1167 1292
1600 640 762 873 1067 1231 1371
1800 783 900 1108 1286 1440
2000 800 923 1143 1333 1500

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