The nominal 1-1/2-inch steel pipe actually measures 1.900 inches across; at Schedule 40 the wall is 0.145 inch, so the opening is 1.610 inches.
For the full breakdown, see our best 1 1/2 Inch Steel Pipe guide.
The number that anchors 1 1/2 inch steel pipe dimensions is the outside diameter: 1.900 inches, not 1.500. That fixed OD applies to every wall schedule of this nominal size, and changing the schedule only changes wall thickness and inside diameter. This matters whether you are pricing a supplier quote, choosing a fitting, or confirming what will actually fit the pipe you already have.
The label “1 1/2 inch” is a Nominal Pipe Size, shortened to NPS. Pipe got its inch name from early sizing conventions, and the name stayed even as manufacturing changed. Modern steel pipe is built to OD-based standards, so you order it by NPS and schedule, never by measuring the OD and assuming it will match the name.
Why “1 1/2 Inch” Doesn’t Mean 1.500 Inches
The outside diameter of NPS 1 1/2 pipe is fixed at 1.900 inches under the ASME B36.10M sizing convention used across US steel pipe charts. In metric listings the same pipe is called DN 40, with an outside diameter of 48.3 mm. Both names describe the same physical product.
Nominal sizes were set generations ago, when the number roughly described the pipe’s internal flow opening. A modern 1 1/2 inch pipe still carries that historical name, but no part of it measures exactly 1.500 inches. Relying on the inch name as if it were the OD is the most common mistake in pipe ordering, and it is why fittings and pipe are both labeled with the NPS number rather than a tape-measure dimension.
What Are The Schedule 40 Dimensions For 1 1/2 Inch Pipe?
For NPS 1 1/2, Schedule 40 specifies a wall thickness of 0.145 inch. Subtract two walls from the 1.900-inch OD, and the inside diameter works out to 1.610 inches, with a nominal weight of 2.72 pounds per foot. In metric rounded terms that is a 48.3 mm OD, a 3.68 mm wall, an inside diameter around 40.9 mm, and roughly 4.04–4.05 kg per meter.
Schedule 40 is the default specification for most black and galvanized steel pipe, and it is often marked “STD” on charts because its wall matches the old standard-weight class. Engineering Toolbox’s ANSI steel pipe schedule table confirms 1 1/2 inch pipe at 1.900-inch OD with a 1.61-inch inside diameter for Schedule 40.
Here is how the same 1.900-inch OD pipe changes at other common wall classes:
| Wall Class | Wall Thickness | Inside Diameter |
|---|---|---|
| Schedule 5S | 0.065 in | 1.770 in |
| Schedule 10S | 0.109 in | 1.682 in |
| Schedule 40 (STD) | 0.145 in | 1.610 in |
| Schedule 80 (XS) | 0.200 in | 1.500 in |
| Schedule 160 | 0.281 in | 1.338 in |
| Double Extra Strong (XXS) | 0.400 in | 1.100 in |
Match Schedule And Standard Before You Order
Buying steel pipe is not just a matter of asking for “1 1/2 inch.” The order should name the NPS size, the schedule, the material standard, and the grade, since ASTM A53 carbon steel pipe, for example, uses the same NPS 1 1/2 label with a 48.3 mm OD and 3.68 mm Schedule 40 wall.
Threaded fittings are sized around the 1.900-inch OD and the NPT thread form, which is why common Schedule 40 and Schedule 80 threaded fittings can both be matched to this pipe. Weld-type and socket-type fittings are another story: their bores are machined for a specific wall thickness, so the schedule must match exactly. Always confirm the fitting’s NPS and wall class rather than judging by the inch name alone.
FAQs
What does a 1 1/2 inch steel pipe actually measure across the outside?
NPS 1 1/2 steel pipe measures 1.900 inches across the outside, no matter which wall schedule you choose. That is about 48.3 mm in metric terms. The “1 1/2 inch” part of the name is a nominal, historical label rather than a physical dimension of the finished pipe.
What is the inside diameter of Schedule 40 pipe in this size?
Subtract the 0.145-inch wall twice from the 1.900-inch outside diameter, and Schedule 40 leaves an inside diameter of 1.610 inches, or about 40.9 mm. Heavier schedules like Schedule 80 use the same outside diameter but a thicker wall, so their inside diameter is smaller.
Are NPS 1 1/2 and DN 40 the same pipe?
Yes. NPS 1 1/2 is the US name for nominal 1.5-inch steel pipe, and DN 40 is the metric designation used on the same ASME B36.10M charts. Both refer to pipe with a 1.900-inch outside diameter, so they are not two different sizes that happen to be close.
References & Sources
- Engineering Toolbox. “ANSI Schedule 40 Steel Pipes.” Provides the 1.900-inch OD and 1.61-inch inside diameter figures for 1 1/2 inch Schedule 40 steel pipe.
