Computed pipe diameters should be increased to a larger nominal dimension to avoid pressure flow. Standard English and metric nominal sizes used for storm drains are given in the table below.

Nominal Pipe Diameters
English metric
inch ft mm
12 1.00 300
15 1.25 375
18 1.50 450
21 1.75 525
24 2.00 600
30 2.50 750
36 3.00 900
42 3.50 1050
48 4.00 1200
54 4.50 1350
60 5.00 1500
66 5.50 1650
72 6.00 1800
78 6.50 1950
84 7.00 2100
90 7.50 2250
96 8.00 2400
102 8.50 2550
108 9.00 2700
114 9.50 2850
120 10.00 3000
126 10.50 3150
132 11.00 3300
138 11.50 3450
144 12.00 3600

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