In the steel pipe industry—particularly regarding U.S. standards—purchasing professionals frequently encounter designations such as:
· SCH
· STD
· XS
· XXS
Although these specifications appear quite similar, they originate from different wall-thickness systems and carry distinct meanings; understanding the relationships between them is crucial for the selection, procurement, and design of piping systems.
Let’s first look at a table to explain the definitions of SCH STD, XS, and XXS:
| Designation | Full Name | Meaning |
| SCH | Schedule | Pipe Wall Class |
| STD | Standard Weight | Standard Wall Thickness |
| XS | Extra Strong | Reinforced |
| XXS | Double Extra Strong | Extra-Reinforced |
Note that SCH does not represent the actual wall thickness, but rather a wall thickness designation; for the same designation (e.g., SCH 40) but different NPS sizes, the actual wall thickness varies.
Another way to understand this is that SCH, XS, and XXS represent traditional U.S. standards; historically, these designations were used to specify steel pipe wall thickness. However, as manufacturing precision improved, more precise wall thickness designations—such as SCH 20, SCH 30, and SCH 40—were developed. The specific relationships between SCH STD, XS, and XXS are explained in detail below.
Note: All data in this article is derived from the ASME B36.10M standard; the data is reliable and suitable for use.
SCH STD Pipe- SCH 40
“STD” stands for “Standard Weight”; it indicates that the steel pipe is manufactured with a standard wall thickness and represents the baseline grade within the traditional pipe wall thickness system.
Relationship between STD and SCH 40
According to the ASME B36.10M standard, for pipe sizes ranging from NPS 1/8″ to NPS 10″, the STD wall thickness is identical to the SCH 40 wall thickness. Below is a comparison of dimensions between STD and SCH 40:
| NPS | STD (mm) | SCH40 (mm) |
| 1/2″ | 2.77 | 2.77 |
| 3/4″ | 2.87 | 2.87 |
| 1″ | 3.38 | 3.38 |
| 2″ | 3.91 | 3.91 |
| 4″ | 6.02 | 6.02 |
| 6″ | 7.11 | 7.11 |
| 8″ | 8.18 | 8.18 |
| 10″ | 9.27 | 9.27 |
The data in the table is derived from the ASME B36.10M standard and can be verified.
For NPS 12″ and larger: STD begins to deviate from SCH 40.
| NPS | STD (mm) | SCH40 (mm) |
| 12″ | 9.53 | 10.31 |
| 14″ | 9.53 | 11.13 |
| 16″ | 9.53 | 12.7 |
| 18″ | 9.53 | 14.27 |
| 20″ | 9.53 | 15.09 |
| 24″ | 9.53 | 17.48 |
It can be observed that:
- STD is fixed at 0.375″ (9.53 mm).
- SCH40 increases continuously as the pipe diameter increases.
The “STD” (Standard) designation originated early on, before large-diameter steel pipes existed; at that time, 3/8-inch piping sufficed for the vast majority of applications. It has since evolved into the “SCH” (Schedule) series, where wall thickness is directly linked to pressure-bearing capacity. Engineers determined that larger diameters required greater wall thicknesses; consequently, there is no “STD” reference standard for pipe sizes of NPS 12″ and above—instead, the required thickness is calculated by engineers based on specific requirements.
Why is “STD” still retained? It serves to bridge the past and the present; legacy pipelines required a standard to rely on, and current pipelines also need to interface with that history—hence the continued retention of STD.
The relationship between XS and SCH 80
The relationship between XS (Extra Strong) and SCH80 is similar.
NPS 1/8″ to 8″: XS = SCH80
| NPS | XS (mm) | SCH80 (mm) |
| 1/2″ | 3.73 | 3.73 |
| 1″ | 4.55 | 4.55 |
| 2″ | 5.54 | 5.54 |
| 4″ | 8.56 | 8.56 |
| 6″ | 10.97 | 10.97 |
| 8″ | 12.7 | 12.7 |
NPS 10″ and above: XS ≠ SCH80
| NPS | XS (mm) | SCH80 (mm) |
| 10″ | 12.7 | 15.09 |
| 12″ | 12.7 | 17.48 |
| 14″ | 12.7 | 19.05 |
| 16″ | 12.7 | 21.44 |
| 18″ | 12.7 | 23.83 |
| 20″ | 12.7 | 26.19 |
The relationship between XXS and SCH 160
XXS is not equivalent to SCH160; they share the same dimensions only for certain sizes. Many manufacturers lack expertise and often incorrectly equate “XXS” with “SCH160.” With years of deep experience in the steel pipe industry and extensive knowledge of standards and production, ALLLAND is ready to assist you with your piping requirements—please feel free to contact us.
| NPS | XXS (in.) | XXS (mm) | SCH160 (in.) |
| 1/8″ | 0.405 | 10.29 | — |
| 1/4″ | 0.54 | 13.72 | 0.119 |
| 3/8″ | 0.675 | 17.15 | 0.126 |
| 1/2″ | 0.294 | 7.47 | 0.294 |
| 3/4″ | 0.308 | 7.82 | 0.308 |
| 1″ | 0.358 | 9.09 | 0.406 |
| 1-1/4″ | 0.5 | 12.7 | 0.5 |
| 1-1/2″ | 0.625 | 15.88 | 0.562 |
| 2″ | 0.436 | 11.07 | 0.436 |
| 2-1/2″ | 0.552 | 14.02 | 0.5 |
| 3″ | 0.6 | 15.24 | 0.438 |
| 3-1/2″ | 0.625 | 15.88 | 0.5 |
| 4″ | 0.674 | 17.12 | 0.674 |
| 5″ | 0.75 | 19.05 | 0.75 |
| 6″ | 0.864 | 21.95 | 0.719 |
| 8″ | 0.875 | 22.23 | 0.906 |
| 10″ | 1 | 25.4 | 1.125 |
| 12″ | 1 | 25.4 | 1.312 |
In the steel pipe industry, the STD wall thickness designation corresponds to SCH 40, XS to SCH 80, and XXS to SCH 160; while there are similarities, there are also differences, requiring careful verification by purchasers.
The above outlines the transition between the old and new wall thickness standards. As times change and technology advances, the steel pipe industry continues to innovate. As an established manufacturer, ALLLAND embodies this evolution: it preserves the strengths of a traditional pipe maker while embracing modern concepts, thereby contributing to the global steel pipe industry.
