Schedules, NB and OD: how pipe is actually specified
Nominal bore is not the bore. Schedule is not a thickness. Getting this wrong is the most common ordering error there is.
Pipe is specified by nominal bore and schedule. Tube is specified by outside diameter and wall thickness. Confusing the two conventions is the single most common cause of an incorrect order.
Nominal bore is a label, not a measurement
A 2-inch NB pipe does not have a 2-inch bore, and its outside diameter is not 2 inches either — it is 60.3 mm. NB is a historical designation that no longer corresponds to any physical dimension. What it does guarantee is that outside diameter is fixed for a given NB regardless of schedule, so fittings and flanges interchange.
Schedule is a wall thickness series
Because OD is fixed, increasing the schedule increases the wall thickness inward, reducing the bore. A higher schedule therefore means a thicker wall, a higher pressure rating, and a smaller internal flow area.
| NB | OD mm | SCH 10 | SCH 40 | SCH 80 | SCH 160 |
|---|---|---|---|---|---|
| 1/2″ | 21.3 | 2.11 | 2.77 | 3.73 | 4.78 |
| 1″ | 33.4 | 2.77 | 3.38 | 4.55 | 6.35 |
| 2″ | 60.3 | 2.77 | 3.91 | 5.54 | 8.74 |
| 4″ | 114.3 | 3.05 | 6.02 | 8.56 | 13.49 |
| 6″ | 168.3 | 3.40 | 7.11 | 10.97 | 18.26 |
| 8″ | 219.1 | 3.76 | 8.18 | 12.70 | 23.01 |
Scroll the table horizontally to see all columns.
The S suffix
Stainless pipe is frequently supplied to 5S, 10S and 40S rather than 5, 10 and 40. The S series is defined in ASME B36.19 specifically for stainless steel and uses thinner walls in the smaller sizes, taking advantage of the higher allowable stress. 40S and 40 are identical up to 10 inches; below that they differ. Always confirm which is meant.
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