Ordering the wrong one is easier than you'd think. Pipes and tubing look similar on a rack. They're often made from the same materials, and people use the terms interchangeably in conversation. But the measurement systems are entirely different — and if you're working from a spec sheet, that difference matters before you pick up the phone. Whether you're sourcing round steel tube for a structural frame or black pipe for a fluid system, knowing which measurement system applies keeps the order right the first time.
Why Pipe Is Measured the Way It Is
Pipes exist to move things—fluids, gases, and steam. The part that matters most is the inside, because that's where the flow happens. That's why pipe sizing is built around the interior, not the exterior.
The system is called Nominal Pipe Size, or NPS. "Nominal" means in name only—the number assigned to a pipe doesn't directly
match any real measurement. A 2" pipe, for example, has an actual outside diameter of 2.375". The "2-inch" label is an approximation of its flow capacity, not a ruler measurement.
The second variable is wall thickness, which is set by an ANSI schedule number. The schedule tells you how thick the walls are, and that determines how much space is left on the inside. Using that same 2" pipe as an example:
- Schedule 5 walls are .065" thick, leaving an interior diameter of 2.245"
- Schedule 160 walls are .344" thick, leaving an interior diameter of just 1.687"
Same nominal size, same outside diameter, but those are unique pipes for very different jobs. A higher schedule means thicker walls, which handle higher pressure but reduce flow capacity and add weight. ANSI schedules also vary by pipe size, so a Schedule 40 wall is not the same thickness across every pipe on the rack.
When you're specifying pipe, you need both numbers: the nominal size and the schedule.
Why Tubing Is Measured Differently
Tubing is built for structure, not flow. Steel square tubing, steel round tubing, and rectangular steel tubing go into frames, supports, enclosures, railings, and mechanical assemblies—places where the outside dimension has to be exact so parts fit together as designed. That's why tubing is specified by its actual outside diameter.
What you're selecting when you order tubing is the exact OD you need, then the wall thickness that works for your load or application. Because the tolerances are tighter and the sizing is exact, tubing is more predictable for fabrication work. You know what you're getting. That's especially true for thick-wall steel tube applications where dimensional consistency affects structural performance directly.
Boyd stocks carbon steel tubing in round, square, and rectangular profiles, as well as stainless steel tubing — including stainless square tubing and rectangular stainless steel tubing — in multiple configurations. See the full specs and details of Carbon Steel & Stainless Steel here.
The One Exception Worth Knowing
Copper tubing doesn't follow the exact OD rule. Its nominal size runs .125" smaller than the actual outside diameter, a legacy of old plumbing standards that never got updated. If you're specifying copper for a new application and you haven't worked with it before, that offset will catch you off guard. Boyd stocks copper tubing and copper pipe, and our team can help you confirm what you need before the order goes in.
The Mistake That Costs Projects Time
The most common ordering error with pipe and tubing is treating the two systems as interchangeable. Specifying a "2-inch tube" when you need pipe, or ordering pipe dimensions for a structural application, creates the kind of problem that doesn't surface until the material shows up on site, and by then, the timeline is already affected.
The quickest way to avoid it is to confirm two things before ordering:
-
What is the application? If something is moving through it—liquid, gas, or steam—you're ordering pipe, and you need nominal size plus schedule. If it's structural, you're ordering tubing, and you need outside diameter plus wall thickness.
-
What material do you need? Carbon steel, stainless steel, and red metals like copper and brass all have their own pipe and tubing options. The environment the product is going into—corrosion exposure, temperature, and sanitary requirements—should drive that decision. If you're not certain, that's a question worth asking before the order goes in, not after.
Boyd's team works from spec sheets every day. If you have a spec sheet and don't know how to order, we can help.
What Boyd Stocks
As a metals service center serving industrial buyers across the region, Boyd carries pipe and tubing across carbon steel, stainless steel, and red metals. The PVF line—pipe, valves, and fittings—rounds out the inventory for projects that need flanges, fittings, and ancillary pipe products from the same source. Full product details and spec information are available on the Boyd Metals product pages linked below.
