Direct conversion: 1.5 inches (1 1/2 in) = 38.1 mm
According to the international standard conversion formula (1 inch = 25.4 mm), 1.5 inches is exactly 38.1 mm.
How many mm is 1 1/4 inches? – The answer is 31.75 mm.
But please note: These two values are the pure mathematical conversion results of “Nominal Pipe Size” (NPS), not the actual outside diameters of pipes or elbows.
✔️If you are selecting or machining elbows, tees, or other fittings for a piping system, directly using 38.1 mm or 31.75 mm to match the physical part is very likely to cause errors.
This article will thoroughly explain:
-
The real dimensional logic in the elbow industry
-
Why converting inches to millimeters cannot be used directly for selection
-
Key manufacturing data for 1.5‑inch and 1¼‑inch elbows
-
A ready‑to‑save quick reference table for fitting dimensions
Core Knowledge – Why Can’t Pipe Sizes Be Simply Converted?
In the pipe fitting industry (especially elbows, flanges, reducers), we use nominal diameter to designate a size, but it represents neither the inside diameter nor the outside diameter.
1. Nominal Diameter: NPS (inch) vs DN (mm)
-
NPS (Nominal Pipe Size): US standard, unit in inches, e.g., 1½″, 2″, 4″.
-
DN (Diameter Nominal): International standard (ISO), unit in mm, e.g., DN40, DN50, DN100.
There is a standard correlation between the two:
| NPS (inch) | DN (mm) |
|---|---|
| 1 ¼″ | 32 |
| 1 ½″ | 40 |
| 2″ | 50 |
| 3″ | 80 |
1½″ corresponds to DN40, and 1¼″ corresponds to DN32 – this is the actual size identification used in elbow manufacturing.
2. Actual Outside Diameter (OD) is the Fitting Dimension
✔️Pipes and elbows are welded or connected by their Outside Diameter (OD).
And the OD is not equal to the converted value of the nominal diameter.
✔️The following table shows the actual outside diameters for common sizes according to the ASME B36.10 standard (essential knowledge for every purchaser and engineer):
| NPS (inch) | DN (mm) | Mathematical conversion (mm) | Actual OD (mm) | Actual OD (inch) |
|---|---|---|---|---|
| 1 ¼″ | 32 | 31.75 | 42.2 mm | 1.660″ |
| 1 ½″ | 40 | 38.1 | 48.3 mm | 1.900″ |
| 2″ | 50 | 50.8 | 60.3 mm | 2.375″ |
| 3″ | 80 | 76.2 | 88.9 mm | 3.500″ |
Conclusion at a glance:
-
The actual outside diameter of a 1.5‑inch elbow is 48.3 mm, not 38.1 mm.
-
The actual outside diameter of a 1¼‑inch elbow is 42.2 mm, not 31.75 mm.
👉 If you use 38.1 mm to machine or purchase a part, it will never match the 48.3 mm pipe.
