Half-inch steel pipe, listed as NPS 1/2, has a 0.840 in outside diameter — not 0.500 in — and its inside diameter changes with schedule.
For the full breakdown, see our best 1/2 Inch Steel Pipe guide.
Grab a tape measure and check a pipe sold as “1/2 inch” and it reads 0.840 inches, not 0.500 — and both labels are correct. The size is a nominal trade designation that follows NPS (Nominal Pipe Size), so it isn’t a direct measurement of the pipe’s opening or its outer shell. Here are the 1/2 inch steel pipe dimensions that apply to standard NPS 1/2 (DN 15) carbon steel pipe, plus why the schedule rating matters just as much as the name on the tag.
What Does 1/2 Inch Steel Pipe Actually Measure?
For NPS 1/2 steel pipe, the outside diameter stays fixed at 0.840 inches (21.3 mm) no matter which schedule you pick. The schedule sets the wall thickness, and the wall thickness is what changes the inside diameter and the weight.
Schedule 40 is the default for water, gas, and structural work, and these are its standard published values:
- Outside diameter: 0.840 in (21.3 mm)
- Wall thickness: 0.109 in (2.77 mm)
- Inside diameter: 0.622 in (15.8 mm)
- Empty weight: about 0.85 lb/ft (1.27 kg/m)
The dimension tables trace back to ASME B36.10M, and Engineering Toolbox’s ANSI steel pipe chart lists the same figures. Engineers Edge and United Pipe publish matching values, which matters because 0.622 in is the opening you actually plan flow around.
The Schedule Rating: What Changes And What Stays Fixed
Schedules 10, 40, and 80 all share the same 0.840 in outside diameter. A higher schedule number means a thicker wall, a smaller inside opening, and a heavier pipe.
| Schedule | Wall Thickness | Inside Diameter |
|---|---|---|
| 10 | 0.083 in | about 0.674 in |
| 40 | 0.109 in | 0.622 in (15.8 mm) |
| 80 | 0.147 in | about 0.546 in |
Weight climbs with the wall too: roughly 0.67 lb/ft for Schedule 10, 0.85 lb/ft for Schedule 40, and 1.09 lb/ft for Schedule 80. Manufacturing tolerances also allow a Schedule 40 pipe’s measured OD to fall between about 0.809 and 0.856 in, so a pipe that reads 0.838 in at the end is within spec, not defective.
Three Sizing Mistakes That Cause Fit Problems
Nearly every fit failure with this pipe comes from one of three mix-ups.
- Treating NPS as a tape-measure number. NPS 1/2 describes a 0.840 in outside diameter, so the pipe never measures 0.500 in. Cut threads and fittings are built around that fixed OD.
- Assuming every 1/2 inch pipe is Schedule 40. Order Schedule 80 and the outside diameter stays the same but the opening shrinks to roughly 0.546 in, which changes flow and the fittings that reach inside the bore.
- Mixing pipe standards with tubing standards. Structural tubing is usually specified by actual outside diameter, so a 0.500 in OD tube is not a substitute for NPS 1/2 pipe even though the names sound alike.
On international charts the same pipe is called DN 15, and both names point to the same 21.3 mm outside diameter. When you order, the spec that removes all doubt is NPS 1/2 plus an explicit schedule. When you’re ready to buy rather than work from a chart, our tested picks for 1/2 inch steel pipe cover suppliers who stock the schedules above.
FAQs
Is 1/2 inch pipe measured by the inside or outside diameter?
Neither, exactly. The “1/2 inch” label is a nominal trade size, not a direct measurement of either opening. NPS 1/2 pipe always has a 0.840 in outside diameter, while the inside diameter depends on the schedule — 0.622 in for Schedule 40 and roughly 0.546 in for Schedule 80.
What is the inside diameter of 1/2 inch Schedule 40 steel pipe?
0.622 inches (15.8 mm). That figure comes from a 0.109 in wall thickness wrapped around the standard 0.840 in outside diameter. Since the ID changes with schedule, always confirm the schedule before calculating flow or choosing fittings that reach into the bore.
Is DN 15 the same as 1/2 inch steel pipe?
Yes, in practical terms. DN 15 is the metric designation used on international charts for the same NPS 1/2 pipe. Both names refer to the same 21.3 mm (0.840 in) outside diameter and the same schedule-based wall thicknesses, so a DN 15 Schedule 40 pipe matches an NPS 1/2 Schedule 40 pipe dimensionally.
References & Sources
- Engineering Toolbox. “ANSI Schedule 40 Steel Pipes.” Source for the NPS 1/2 outside diameter, wall thickness, inside diameter, and weight values in this article.
