Pipe Schedules
Authority: ASTM D1785 / ASME B36.10M
Schedule 40 and Schedule 80 pipes share identical outer diameters (OD) for a given nominal size, but Schedule 80 features thicker internal walls, reducing inner diameter (ID) to handle higher operating pressures.
| Nominal Pipe Size (NPS) | Outer Diameter (in) | Sched 40 Wall (in) | Sched 40 Max PSI | Sched 80 Wall (in) | Sched 80 Max PSI |
|---|---|---|---|---|---|
| 1/2" NPS | 0.840 in | 0.109 in | 600 PSI | 0.147 in | 850 PSI |
| 3/4" NPS | 1.050 in | 0.113 in | 480 PSI | 0.154 in | 690 PSI |
| 1" NPS | 1.315 in | 0.133 in | 450 PSI | 0.179 in | 630 PSI |
| 1-1/2" NPS | 1.900 in | 0.145 in | 330 PSI | 0.200 in | 470 PSI |
| 2" NPS | 2.375 in | 0.154 in | 280 PSI | 0.218 in | 400 PSI |
From Iron Pipe Size to Schedules
The nominal size of a pipe hasn't matched any of its actual dimensions for more than a century. The system traces back to Robert Briggs, an engineer at Philadelphia's Pascal Iron Works, who developed gauges for wrought-iron pipe threads starting in the 1830s and published what became known as the Briggs Standard in 1862. Under the original Iron Pipe Size (IPS) system, the nominal number referred loosely to the pipe's inside diameter. Most manufacturers had adopted the standard by 1886, and the American Standards Association formalized it as National Pipe Taper (NPT) thread in 1919.
As pipe manufacturing diversified into multiple wall thicknesses for different pressure ratings, the industry fixed the outside diameter (OD) for each nominal size instead, so fittings and threads would still interchange regardless of wall thickness. That's why, as the table above shows, 1/2" NPS pipe has an OD of 0.840 inches, not 0.5 inches -- the nominal number is a trade size inherited from the original inside-diameter system, decoupled from the pipe's actual measurements once schedules were introduced.
Why Schedule Numbers Matter
Because OD stays fixed within a nominal size, increasing a pipe's schedule number adds wall thickness entirely at the expense of inside diameter. A Schedule 80 pipe in a given nominal size uses the same fittings and threads as Schedule 40 in that size, but its thicker wall narrows the bore and raises its safe operating pressure -- useful when a system needs a higher pressure rating without changing the surrounding fittings, flanges, or valves.