Is a 2-inch pipe two inches? And is Schedule 40 one wall thickness?
Two questions that sound like beginner questions and are not, because the answers decide whether a line works.
A 2-inch pipe is 60.3 mm across
Not 50.8 mm, which is what two inches would be. The NPS figure is a designation — a name for a size, not a measurement of anything.
| NPS | DN | Actual outside diameter |
|---|---|---|
| 1/2 | 15 | 21.3 mm |
| 1 | 25 | 33.4 mm |
| 2 | 50 | 60.3 mm |
| 4 | 100 | 114.3 mm |
| 12 | 300 | 323.8 mm |
| 14 | 350 | 355.6 mm |
| 24 | 600 | 610.0 mm |
Below NPS 14 the name and the dimension simply disagree, and the gap is largest in the small sizes — NPS 1/2 pipe is a third wider than its name. From NPS 14 upward the designation becomes the actual outside diameter in inches: NPS 14 is 355.6 mm, which is exactly 14.000 inches. The convention changes halfway up the range, which is itself worth knowing.
DN is the metric designation and is likewise a name. DN 50 pipe is not 50 mm anything. It is the same pipe as NPS 2.
Schedule 40 is not a thickness
This is the one that causes real errors. Schedule 40 in a 1-inch line and Schedule 40 in a 12-inch line are not the same wall:
| NPS | SCH 40 wall | SCH 80 wall |
|---|---|---|
| 1/2 | 2.77 mm | 3.73 mm |
| 2 | 3.91 mm | 5.54 mm |
| 6 | 7.11 mm | 10.97 mm |
| 12 | 10.31 mm | 17.48 mm |
| 24 | 17.48 mm | 30.96 mm |
Schedule 40 spans 2.77 mm to 17.48 mm across the range — a factor of more than six. The schedule is a rule that scales wall thickness with diameter to keep roughly the same pressure capability, which is why it has to grow. The number itself comes from the old relationship of roughly 1000 × P/S — pressure over allowable stress — so a schedule number was, originally, a crude statement of pressure class.
Notice the 24-inch SCH 40 wall and the 12-inch SCH 80 wall are both 17.48 mm. The same steel, two different schedules, because they are doing different jobs at different diameters.
The consequence people miss
Because the OD is fixed, thickening the wall eats the bore.
Take NPS 2. At SCH 40 the bore is 60.3 − (2 × 3.91) = 52.5 mm. At SCH 80 it is 60.3 − (2 × 5.54) = 49.2 mm. That is only 3.3 mm smaller, which sounds like nothing, but flow area goes with the square: the area drops by about 12%, so for the same flow the velocity rises by about 14% and the frictional pressure drop rises by roughly 30%.
So the common site decision — “add a schedule for corrosion allowance” — is not a free safety margin. It is a hydraulic change made silently, usually without going back to the line sizing. In a long line, a pump near its duty point, or a gravity drain with little head to spare, that is enough to matter.
The same trap runs the other way in small bore. In a 1/2-inch line the wall jumps from 2.77 to 3.73 mm between SCH 40 and SCH 80, and the bore falls from 15.8 to 13.8 mm — an area loss of about 24%. Small bore is where schedule changes hurt most, and small bore is where they get made most casually.
Where to go next
The full table and the reasoning behind it are in pipe sizes and schedules. What the schedule has to be for a given duty comes out of line sizing and hydraulics, and the document that fixes it for a given service is the line class — see reading a line class.