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.
| 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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