Pipe sizing can get confusing when a project involves more than one material. Terms like nominal pipe size, outside diameter, and schedule may appear across stainless steel, carbon steel, alloy, and plastic piping, but they don’t always tell you everything you need to know about the pipe’s actual dimensions.
Understanding how those measurements relate makes it easier to read specifications, compare pipe sizes, and avoid incorrect assumptions when working across different materials. Continue reading as we break down the fundamentals of pipe sizing and look at where the similarities and differences matter.
What Do You Need to Understand About Pipe Sizing?
A pipe’s stated size doesn’t necessarily describe its actual dimensions. A 2-inch nominal pipe, for example, does not simply measure 2 inches across. Changing the pipe schedule can also change the wall thickness without changing its outside diameter.
To understand what a pipe size actually represents, it helps to look at four pieces of information together:
- Nominal Pipe Size (NPS): The standardized designation used to identify the pipe size.
- Outside Diameter (OD): The actual measurement across the exterior of the pipe.
- Wall Thickness: The thickness of the pipe wall.
- Schedule: A designation used to identify wall thickness for a given nominal pipe size.
These measurements work together to describe the pipe’s dimensions. Understanding their relationship provides the foundation for reading a pipe wall thickness chart or comparing pipe across different materials.
What Is the Difference Between Nominal Pipe Size vs. Outside Diameter?
Nominal pipe size vs. outside diameter is an important distinction because the two aren’t necessarily the same measurement.
Nominal pipe size (NPS) is a standardized way of identifying pipe size. Outside diameter, on the other hand, is the physical measurement across the outside of the pipe.
For example, a 2-inch NPS pipe has an outside diameter of 2.375 inches. That outside diameter remains consistent across different schedules within the same dimensional standard, while the wall thickness and inside diameter can change.
This is why NPS and OD shouldn’t be used interchangeably. NPS identifies the standardized size you’re working with, while OD tells you the pipe’s actual exterior dimension.
What Does Pipe Schedule Tell You?
Pipe schedule identifies the wall thickness associated with a particular nominal pipe size. As the schedule increases, the pipe wall generally becomes thicker.
When comparing schedules for the same nominal pipe size:
- Outside diameter stays the same within the same dimensional standard.
- Wall thickness changes as the schedule changes.
- Inside diameter changes because a thicker wall leaves less open space inside the pipe.
Schedule provides an important piece of dimensional information, but it isn’t a complete pipe specification. Material, grade, and other application requirements still need to be considered.
What Is the Difference Between Schedule 40 vs. Schedule 80 Pipe?
The main difference between Schedule 40 vs. Schedule 80 pipe is wall thickness. For the same nominal pipe size, Schedule 80 has a thicker wall than Schedule 40 while maintaining the same outside diameter.
A pipe wall thickness chart makes that relationship easier to see:
| Nominal Pipe Size | Outside Diameter | Schedule 40 Wall Thickness | Schedule 80 Wall Thickness |
| 1″ | 1.315″ | 0.133″ | 0.179″ |
| 2″ | 2.375″ | 0.154″ | 0.218″ |
| 4″ | 4.500″ | 0.237″ | 0.337″ |
| 6″ | 6.625″ | 0.280″ | 0.432″ |
Because the outside diameter stays the same, the additional wall thickness extends inward and reduces the available inside diameter.
A higher schedule isn’t automatically the better choice. The appropriate schedule depends on the material, operating conditions, and specifications established for the application.
Does Pipe Sizing Work the Same Across Steel, Alloy, and Plastic?
Many piping materials share sizing terminology, but not every material or piping product follows exactly the same dimensional conventions. That’s why the applicable material and specification should always be confirmed before relying on a sizing chart.
Here’s how some common materials compare:
- Stainless and carbon steel: A stainless steel pipe size chart and a carbon steel pipe size chart may share NPS, OD, and schedule dimensions when the pipe is manufactured to the same dimensional standard. Matching dimensions, however, don’t make the materials interchangeable.
- Alloy pipe: Alloy piping may also use familiar NPS and schedule designations. The material grade and applicable specification remain important parts of identifying the required pipe.
- PVC and CPVC: A PVC pipe size chart or plastic pipe size chart may also use nominal sizes and schedules such as Schedule 40 and Schedule 80. Similar terminology doesn’t mean plastic and steel pipe have the same performance characteristics.
- Copper: A copper pipe size chart can look different because copper tube is commonly identified by nominal size and type, such as Type K, L, or M, rather than relying primarily on pipe schedules.
- Aluminum: An aluminum pipe size chart should likewise be interpreted according to the particular product and dimensional specification being used.
The important point isn’t to memorize a separate sizing system for every material. It’s to recognize which sizing convention applies before using the dimensions on a chart or specification.
How Do You Read a Pipe Wall Thickness Chart?
Once you understand the terminology, reading a pipe wall thickness chart becomes much more straightforward.
Work through the specification in this order:
- Identify the material. Confirm whether you’re working with stainless steel, carbon steel, alloy, plastic, or another material.
- Find the nominal pipe size. This establishes the standardized size designation.
- Confirm the schedule. This identifies the corresponding wall thickness.
- Check the outside diameter. OD gives you the actual exterior dimension.
- Find the wall thickness. This tells you the thickness of the pipe wall.
- Determine the inside diameter if needed.
When OD and wall thickness are known, inside diameter can be calculated as:
Inside Diameter = Outside Diameter − (2 × Wall Thickness)
Following those measurements in order makes it easier to understand what the sizing chart is telling you, rather than relying on nominal size alone.
What Should You Confirm Before Ordering Pipe?
Once you’ve determined the required dimensions, make sure the complete pipe specification is clear before placing an order.
Confirm:
- Material and grade
- Nominal pipe size
- Schedule or wall thickness
- Applicable specification
- Quantity and length
- End or connection requirements
Providing the complete specification reduces ambiguity and makes it easier for your supplier to identify the material your job requires.
Pipe Sizing FAQ
Is nominal pipe size the same as outside diameter?
Nominal pipe size vs. outside diameter refers to two different things. NPS is a standardized size designation, while OD is the pipe’s actual exterior measurement. For example, a 2-inch NPS pipe has an outside diameter of 2.375 inches.
Does Schedule 80 pipe have a larger outside diameter than Schedule 40?
No. For the same nominal pipe size within the same dimensional standard, Schedule 40 and Schedule 80 have the same outside diameter. The difference in Schedule 40 vs. Schedule 80 is wall thickness, with Schedule 80 having the thicker wall and therefore a smaller inside diameter.
What is the difference between Schedule 10 and Schedule 40 stainless steel pipe?
When comparing Schedule 10 vs. 40 stainless, Schedule 10 has a thinner wall at the same nominal pipe size. The appropriate schedule depends on the design and operating requirements established for the piping application.
Can I use the same pipe size chart for stainless steel, carbon steel, and PVC?
Not as a universal reference. A stainless steel pipe size chart, a carbon steel pipe size chart, and a PVC pipe size chart may share some terminology and dimensions, but the applicable dimensional and material specifications still need to be considered.
Understanding the Complete Pipe Specification
Pipe sizing becomes easier to navigate once you understand how the individual measurements fit together. Nominal pipe size identifies the standardized size, schedule establishes wall thickness, and outside and inside diameters describe the pipe’s physical dimensions.
Those fundamentals also make it easier to work across different materials without assuming that every sizing convention or specification is the same.
American Stainless & Supply carries stainless steel, carbon steel, alloy, and plastic pipe and PVF for industrial applications. When you have an established material and pipe specification, our team can source what you need and get your order underway.
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