Nominal Pipe Size (NPS) is a standardized labeling system used in North America to identify pipes by a convenient reference number rather than by their actual physical dimensions. The number on the label does not match the pipe’s true outside or inside diameter for most common sizes. Instead, it serves as a shorthand that lets engineers, plumbers, and suppliers all refer to the same product without confusion.
Why NPS Doesn’t Match the Actual Diameter
Early pipe manufacturers sized their products by internal diameter, since that’s what determines how much fluid can flow through. But wall thickness varied from one maker to the next, so two pipes labeled with the same internal diameter could have different outside diameters. They wouldn’t fit the same fittings, creating real problems on job sites.
As manufacturing improved, pipe walls could be made thinner while maintaining the same strength. That meant the internal diameter of a pipe could change even though its outside diameter stayed the same. The industry needed a system that kept the outside diameter constant for a given size so that fittings, flanges, and connectors would always match up. NPS became that system: a reference number that points you to a specific outside diameter, regardless of how thick the walls are.
The result is a quirk that confuses people the first time they encounter it. A pipe labeled NPS 1 does not have a 1-inch outside diameter. Its actual outside diameter is 1.315 inches. A 1-inch copper tube (sized under a completely different system called CTS) has an outside diameter of 1.125 inches. The two are not interchangeable, and mixing them up means fittings won’t connect without special adapters.
The NPS 14 Dividing Line
There is one clean breakpoint in the system. For NPS 1/8 through NPS 12, the nominal size and the actual outside diameter are different numbers. For NPS 14 and larger, the nominal size equals the outside diameter. An NPS 14 pipe really does measure 14 inches (360 mm) across the outside.
This means that if you’re working with pipes smaller than 14 inches, you need a reference table to convert between the nominal label and the real outside diameter. For pipes 14 inches and up, the label tells you the measurement directly.
How Pipe Schedule Affects Wall Thickness
NPS tells you the size category. A second number, called the schedule, tells you how thick the pipe wall is. Common schedules include 5, 10, 40, 80, and 160, along with designations like STD (standard), XS (extra strong), and XXS (double extra strong).
The key rule: for any given NPS, the outside diameter stays the same no matter what schedule you choose. What changes is the wall thickness, and therefore the inside diameter. A higher schedule number means a thicker wall, a smaller internal opening, and a pipe rated for higher pressure. A lower schedule number means a thinner wall, a larger internal opening, and a lighter pipe suited for lower-pressure applications.
For example, an NPS 10 pipe has an outside diameter of 273 mm regardless of schedule. At Schedule 5, the wall is just 3.40 mm thick. At Schedule 40 (which is also STD for this size), the wall is 9.27 mm. At Schedule 160, it jumps to 23.01 mm. That’s a massive difference in internal space and weight, all within the same nominal size.
Carbon Steel vs. Stainless Steel Standards
Two ASME standards govern NPS dimensions. B36.10 covers carbon steel and alloy steel pipes. B36.19 covers stainless steel pipes. You’ll notice stainless steel schedules often carry an “S” suffix (Schedule 5S, 10S, 40S, 80S) to distinguish them from their carbon steel counterparts.
For many sizes, the wall thicknesses are identical between the two standards. But they diverge at certain points, particularly as pipes get larger. At NPS 10 and above, extra strong (XS) stays at 12.70 mm in both standards, but “true” Schedule 80 in B36.10 gets progressively thicker while Schedule 80S in B36.19 stays at 12.70 mm. The B36.19 standard stops covering sizes above 12 inches (300 mm nominal bore). For stainless steel pipes NPS 14 and larger, specifications typically default to the schedule system in B36.10.
This distinction matters when ordering materials. Specifying the wrong standard can result in pipes and fittings with mismatched wall thicknesses.
The Metric Equivalent: DN
Outside North America, the equivalent system is called DN, for Diamètre Nominal (or Diameter Nominal). DN uses millimeters as its reference unit. An NPS 2 pipe corresponds to DN 50, for instance. Like NPS, DN is a label rather than a direct measurement. The two systems are designed to be interchangeable: an NPS 4 pipe and a DN 100 pipe have the same outside diameter and will accept the same fittings.
How to Determine a Pipe’s NPS
If you have a pipe in hand and need to identify its nominal size, start by measuring the outside diameter. Wrap a flexible measuring tape around the pipe to get the circumference, then divide by pi (3.14159). A circumference of 12.57 inches, for example, gives you an outside diameter of about 4 inches.
If that outside diameter is 14 inches or more, you already have the NPS. The nominal size equals the outside diameter. If it’s under 14 inches, you need to look up the measurement in a reference table, because the nominal size won’t match. A measured outside diameter of 4.500 inches, for instance, corresponds to NPS 4, not NPS 4.5.
NPS vs. Other Sizing Systems
NPS applies specifically to steel and iron pipe. Copper tubing uses a separate system called Copper Tube Size (CTS), where the outside diameter is always 1/8 inch larger than the nominal label. A 1/2-inch CTS tube has an outside diameter of 0.625 inches, while a 1/2-inch NPS pipe has a completely different outside diameter.
The two systems are not compatible. IPS (Iron Pipe Size, essentially the same as NPS) fittings will not work on CTS tubing, and vice versa. If you need to join copper tubing to steel pipe, you’ll need transition adapters specifically designed for that purpose. Getting this wrong is one of the most common sizing mistakes in plumbing and piping work.