NPS 1-1/4 steel pipe measures 1.660 inches (42.2 mm) on the outside, not 1.25 inches — the nominal size is a label, not a measurement.
A tape measure across the end of a 1-1/4 inch steel pipe reads 1.660 inches, and that gap between the label and the metal surprises almost everyone the first time. The buyer who orders fittings expecting a 1.25-inch opening ends up with a part that will not thread. The confusion traces back to how pipe has been sized for over a century: the number stamped on the pipe describes a nominal pipe size, and for anything smaller than NPS 14, that number deliberately does not match the outside diameter.
Below is the exact outside diameter for NPS 1-1/4, the wall thicknesses that change with schedule while the OD stays fixed, and the one sizing rule that keeps you from ordering the wrong part again.
Why 1-1/4 Inch Pipe Measures 1.660 Inches
The outside diameter stays constant while the inside diameter shifts, because pipe was standardized so that a given nominal size threads into the same fittings no matter how thick the wall is. That single design decision explains the whole system. When pipe walls got thicker to handle higher pressure, the outside had to stay put so the threading tools, dies, and couplings on every job site would still work.
So the pipe grew inward. A Schedule 40 pipe and a Schedule 80 pipe of the same nominal size have identical outside diameters, but the Schedule 80 bore is noticeably smaller because the wall eats into it. The nominal number is a convenient label, not a dimension you can measure.
The rule holds firm across the small sizes. For NPS 1/8 through NPS 12, the nominal size never matches the outside diameter in inches. Only at NPS 14 and above does the number finally equal the OD. Arbtoolbox’s standard pipe dimension charts list NPS 1-1/4 at 1.660 in (42.2 mm), a figure unchanged across the common schedules in ASME B36.10-based tables.
Wall Thickness By Schedule: Same OD, Different Bore
Every common schedule of NPS 1-1/4 pipe shares the 1.660-inch outside diameter — only the wall thickness, and therefore the inside diameter, changes. That distinction matters when you are matching pipe to a pressure rating, a flow requirement, or an existing fitting.
| Schedule | Wall Thickness | What It’s Used For |
|---|---|---|
| Schedule 5 / 5S | 0.065 in | Low-pressure and light structural work |
| Schedule 10 / 10S | 0.109 in | General low-pressure piping and railing |
| Schedule 40 / STD / 40S | 0.140 in | The everyday default for water, air, and gas lines |
| Schedule 80 / XS / 80S | 0.191 in | Higher pressure and threaded connections |
| Schedule 160 | 0.250 in | High-pressure process piping |
| XX (Double Extra Strong) | 0.382 in | Extreme pressure and severe service |
Note the aliases. Schedule 40 is also called STD, or standard wall. Schedule 80 is XS, or extra strong. These are not separate products — they are the same wall thicknesses under trade names that predate the schedule system, which is why a fitting labeled STD threads onto pipe labeled Schedule 40 without issue.
Common Buying Mistakes With 1-1/4 Inch Pipe
The most frequent error is measuring the wrong thing: buyers check the inside diameter and then order pipe that is far too large. Because NPS 1-1/4 does not have a 1.25-inch outside diameter, and because the inside diameter varies by schedule, neither measurement alone tells you the nominal size. Measure the outside diameter to 1.660 inches and you have your answer.
The second mistake is treating pipe and tube as interchangeable. The 1.660-inch figure applies to steel pipe in the NPS system. Tube is sized by its actual outside diameter, so a 1-1/4 inch tube really does measure 1.25 inches across. If your project calls for tube and you buy pipe, the fittings will not cooperate.
Non-NPS pipe products muddy the water further. Copper tubing, PVC, and various specialty pipe follow their own sizing conventions, and none of them share the NPS 1-1/4 dimensions. If you are comparing parts across materials, confirm which sizing system each one uses before you commit. When you are ready to compare actual products, a tested roundup of 1 1 4 inch steel pipe options can save you a round of ordering errors.
Wermac’s pipe dimension reference confirms the NPS-to-OD relationship holds across the standard schedules, so a fitting rated for NPS 1-1/4 will match any of them. Wermac’s pipe dimension tables list the full OD and wall thickness values for NPS sizes.
What To Measure Before You Order
One number settles the question: 1.660 inches of outside diameter equals NPS 1-1/4 steel pipe, regardless of schedule. Here is the short version to carry to the supplier.
- Outside diameter: 1.660 in (42.2 mm) — the fixed dimension for NPS 1-1/4.
- Inside diameter: varies by schedule, so never use it to identify nominal size.
- Wall thickness: pick the schedule that matches your pressure and flow needs, not the OD.
- Material check: confirm it is steel pipe in the NPS system, not tube or non-NPS product.
- Fittings: any NPS 1-1/4 fitting threads onto any schedule of the same nominal size.
Measure the OD, confirm the schedule, and the rest falls into place.
