If you've ever ordered "some pipe" or "a piece of tubing" and gotten the wrong thing, you're not alone. Tube and pipe look almost identical sitting on a rack, but they're sized differently, specced differently, and used for very different jobs. Choosing wrong means parts that don't fit, fittings that don't connect, or structural members that can't handle the load.

This guide breaks down the real differences between steel tube and pipe — how each one is measured, the most common sizes, and how to pick the right one for your project.

Tube vs Pipe: What's Actually Different?

The short answer: pipe is sized by its nominal bore (the inside), and tube is sized by its actual outside diameter (OD).

That single distinction changes everything about how you order, measure, and connect each one.

Pipe uses Nominal Pipe Size (NPS), which is based on the approximate inside diameter. A 1" NPS pipe doesn't actually measure 1" anywhere — its OD is 1.315". The NPS system dates back over a century and exists because pipe was originally designed for fluid flow, where the inside diameter determines capacity.

Tube is measured by its true outside diameter and wall thickness. A 1" OD tube measures exactly 1.000" on the outside. What you see on the spec sheet is what you measure with calipers.

Here's where it trips people up: a 1-1/4" pipe has an OD of 1.660", while a 1-1/4" tube has an OD of 1.250". Order the wrong one and your brackets, sleeves, or welded connections won't line up.

How Pipe Sizing Works: NPS and Schedules

Pipe wall thickness is designated by schedule numbers. The two you'll run into most often are Schedule 40 and Schedule 80.

Schedule 40 is the standard for most general-purpose applications — water lines, air lines, structural posts, and fence posts. It balances strength, weight, and cost.
Schedule 80 has a thicker wall for higher-pressure applications — hydraulic systems, steam lines, and industrial piping where pressure ratings matter.

Both schedules share the same OD for a given NPS. The difference is all in the wall: a 2" Schedule 80 pipe has a 0.218" wall versus 0.154" for Schedule 40. That extra wall thickness means higher pressure capacity but also more weight and cost (use our steel weight calculator to compare).

Common pipe sizes contractors order:

1/2" NPS (0.840" OD) — residential plumbing, air lines
3/4" NPS (1.050" OD) — gas lines, water supply
1" NPS (1.315" OD) — sprinkler systems, handrails
1-1/2" NPS (1.900" OD) — fence posts, structural supports
2" NPS (2.375" OD) — bollards, mechanical piping
3" NPS (3.500" OD) — drainage, industrial process lines
4" NPS (4.500" OD) — main water lines, structural columns

How Tube Sizing Works: OD and Wall Thickness

Tube is straightforward — you specify the outside diameter and wall thickness, and the inside diameter is whatever's left over. No schedules, no nominal sizes that don't match reality.

Tube wall thickness is often expressed in gauge (lower gauge = thicker wall) or in decimal inches. Common wall thicknesses range from 16 gauge (0.065") for light-duty work up to 1/4" (0.250") or heavier for structural applications.

Steel tube comes in three shapes:

Round tube — the most common. Used for handrails, roll cages, frames, and structural supports.
Square tube — popular for frames, brackets, trailer builds, and equipment stands. Flat surfaces make welding and bolting easier.
Rectangular tube — used where you need different strength in different directions. Common in trailer frames, equipment mounts, and structural framing.

Common tube sizes contractors and fabricators order:

1" x 1" square (16 ga–11 ga) — light frames, brackets
1-1/2" x 1-1/2" square (14 ga–3/16") — handrails, equipment frames
2" x 2" square (14 ga–1/4") — trailer frames, structural supports
2" x 3" rectangular (14 ga–3/16") — trailer tongues, heavy frames
1" OD round (16 ga–14 ga) — handrails, ornamental work
1-1/2" OD round (14 ga–11 ga) — roll bars, structural frames
2" OD round (14 ga–1/4") — columns, heavy structural work

When to Use Pipe vs Tube

Use pipe when:

You're carrying fluids — water, gas, oil, air, steam
The system uses threaded fittings, flanges, or standard pipe connections
You need to match existing plumbing or mechanical systems
Building codes call out pipe specifications (NPS/schedule)

Use tube when:

The application is structural or mechanical — frames, supports, roll cages, handrails
You need precise dimensions for welded or bolted connections
Appearance matters — tube has a cleaner finish, especially cold-drawn
You're fabricating custom parts where exact OD matters for fit-up

The gray area: Fence posts and bollards often use pipe (usually Schedule 40) because it's readily available and the NPS sizing doesn't matter for those uses. Handrails can go either way, but round tube gives a cleaner look and more consistent dimensions for brackets and fittings.

Material Options

At Ram Steelco, tube and pipe are stocked across multiple materials:

Hot rolled steel (see our steel grades guide) — the workhorse. Available in round, square, and rectangular tube plus standard pipe. Most affordable option for structural and general-purpose work.
Aluminum — lighter weight for applications where corrosion resistance matters or you need to reduce load. Available in round, square, and rectangular tube plus pipe.
Stainless steel — for food processing, marine environments, chemical exposure, or anywhere corrosion is a serious concern. Available in square and rectangular tube plus pipe.
Galvanized — hot rolled pipe with a zinc coating for outdoor applications where you want corrosion protection without the cost of stainless. Common for fence posts, handrails, and exposed structural work.

Processing Services That Save You Time

One advantage of buying tube and pipe from a full-service supplier: you can get it cut, formed, and processed before it ever hits your job site.

Saw cutting — cut to exact lengths so you're not hauling 20-foot sticks and cutting on site
Laser cutting — precision holes, slots, and profiles cut directly into tube and pipe. Ideal for brackets, mounting plates, and parts that need to fit together cleanly
Plasma cutting — heavier-gauge pipe and tube cut to shape for structural fabrication
Forming — bending tube and pipe to specific radii for handrails, frames, and custom shapes
Shearing — cutting flat stock that connects to your tube and pipe assemblies

Getting material processed before delivery means less labor on site, cleaner cuts, and tighter tolerances than most field tools can achieve.

Tips for Ordering the Right Material

Always specify whether you need tube or pipe. Don't assume your supplier knows which one you mean. "Two-inch round steel" could be either one — and they're very different sizes.
For pipe, state the NPS and schedule. Example: "2-inch Schedule 40 pipe." That removes all ambiguity.
For tube, state the OD and wall thickness (or gauge). Example: "2-inch OD round tube, 14 gauge." If square or rectangular, include both dimensions: "2x3 rectangular tube, 3/16 wall."
Specify the material. Hot rolled, aluminum, stainless, or galvanized — each has different availability and lead times.
Consider processing. If you need specific lengths, holes, bends, or copes, ask about getting it done before delivery. It's usually faster and more accurate than doing it in the field.
Order early when possible. Specialty sizes and stainless or aluminum tube can have longer lead times than standard hot rolled. Planning ahead keeps your project on schedule.

Get the Right Tube or Pipe for Your Next Project

Whether you need a bundle of Schedule 40 pipe for a plumbing run or precision-cut square tube for a custom fabrication, Ram Steelco stocks tube and pipe across four material types with in-house processing to get it job-ready.

Request a quote online or call our sales team at 503.588.1311. We offer next-day delivery across Oregon — so you can order today and have material on site tomorrow.

Ram Steelco is a locally owned steel and metals supplier in Portland, Oregon, serving contractors, fabricators, and builders since 1938. Learn more about us.