News - Steel Pipe Basics: How to Read NPS, OD and Schedule Like a Buyer

If you've ever ordered steel pipe, you know the sizing system can feel like a language of its own. NPS, OD, ID, WT, SCH 40, SCH 80... Some of these terms seem interchangeable, and for certain sizes they are. For others, they're not. Get one wrong and you could end up with pipe that doesn't fit the fitting, the pressure rating, or the project.

This guide breaks it down in plain English: what the measurements mean, how to read a pipe spec, and — most importantly — what to tell us when you're ready for a quote.

What is a Steel Pipe, Anyway?

A steel pipe is a hollow cylindrical section of steel, used mainly to carry gas, liquid or other fluids from one place to another. It's also one of the most widely produced products in the steel industry.

The easiest way to tell a pipe from a tube: pipes are specified by nominal size and wall schedule, while tubes are specified by exact outside diameter and wall thickness. Pipes are almost always round, and they're defined by the standards they're made to — think ASTM A53, A106, API 5L, or EN 10219. That distinction matters when you're comparing quotes, because a "2-inch pipe" and a "2-inch tube" are not the same thing.

Where Steel Pipes are Used

The most obvious application is underground transport of oil, natural gas and water. But pipes also carry sewage, protect electrical wiring in construction, and — thanks to steel's strength-to-weight ratio — show up in handrails, scaffolding, flagpoles, street lamps and bike frames.

At Royal Steel Group, we supply pipe for all of these: line pipe for oil and gas projects, and structural pipe for construction and fabrication, in ASTM, API 5L, EN and GB standards.

The Four Measurements That Matter

Length — self-explanatory: how long each piece is, usually in standard increments like 6 m (20 ft) or 12 m (40 ft).

Outside diameter (OD) — the distance from one outside edge of the pipe to the opposite edge. Think of it as the total height of the pipe.

Inside diameter (ID) — the distance between the inside walls, i.e., how wide the opening is. You can always work it out: ID = OD − 2 × wall thickness.

Wall thickness (WT) — the thickness of the pipe wall. This is where things get interesting: wall thickness is usually expressed as a "schedule" (SCH), a non-dimensional number you convert to a real thickness using a chart. Common schedules run from SCH 5 up to SCH 160 and beyond. The higher the schedule, the thicker the wall and the higher the pressure the pipe can handle.

NPS: The "Size" That isn't a Measurement

Nominal pipe size (NPS) is how the industry refers to pipe size — but here's the catch most buyers trip on. For NPS 1/8" through 12", the NPS is neither the inside diameter nor the outside diameter. It's a designation, a way of talking about a pipe, not a measurement of it.

From NPS 14 and up, the rule changes: the nominal size and the outside diameter become the same. A 14" NPS pipe has an OD of exactly 14".

Here's a quick reference table for common sizes:

NPS	Outside Diameter (OD)
1/2"	0.840"
3/4"	1.050"
1"	1.315"
1-1/4"	1.660"
1-1/2"	1.900"
2"	2.375"
2-1/2"	2.875"
3"	3.500"
3-1/2"	4.000"
4"	4.500"
5"	5.563"
6"	6.625"
8"	8.625"
10"	10.750"
12"	12.750"
14"	14.000"

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How to Read a Pipe Spec

Most pipe specifications are written in one line. Take this example:

1" SCH 40 x 20'

1" is the nominal pipe size. The actual outside diameter is 1.315".
SCH 40 is the wall schedule. For a 1" pipe, that works out to a wall thickness of 0.133".
20' is the length: 20 feet.

So the spec means: 1.315" OD, 0.133" wall thickness, 20 feet long. And if you need the inside diameter, it's ID = OD − 2 × WT = 1.315 − 0.266 = 1.049".

What to Include in Your Inquiry (and Why it Saves You Time)

The fastest way to get an accurate quote is to give us a complete picture on the first email. When you send an inquiry, include as many of these as you can:

Size: NPS or exact outside diameter

Wall thickness: schedule (e.g., SCH 40) or the exact WT in mm or inches

Length and quantity: per-piece length and total tonnage or pieces

Standard and grade: e.g., ASTM A53 Gr. B, API 5L X52, EN 10219 S355JRH, or GB/T 3091

End finish: plain end, beveled, or threaded

Surface: black, galvanized, or coated

Destination port: this lets us quote freight properly

Not sure about the schedule you need? Tell us what the pipe will carry, the working pressure, and the temperature — our technical team will recommend the right wall thickness. And if you only have a drawing or an old spec sheet, send that over; we can work from it.

Get the Right Pipe for Your Project

No matter the size, standard, or quantity, Royal Steel Group can supply it — and we'll help you decide what's best for your project so you don't need to become a pipe terminology expert overnight. We export to buyers across North America, the Middle East, Africa and Europe, and every order ships with the documents you need for customs and quality control.

Ready to get started? Send us your inquiry with your sizes and quantities today, and our team will come back with a quote and a delivery timeline.

ROYAL GROUP

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Kangsheng development industry zone,
Wuqing district, Tianjin city, China.

E-mail

admin@royalsteelgroup.com

Phone

Cherry Yang - CEO : +8613652091506
Wendy Wu -Business Director : +8615222747108

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Post time: Sep-07-2026

Steel Pipe Sizes Explained: NPS, OD, Schedule, and How to Spec the Right Pipe