Steel tubing is hollow steel formed into round, square, or rectangular shapes, used for mechanical or structural applications depending on its ASTM specification.
For the full breakdown, see our best 2″ Steel Tubing guide.
Steel tubing shows up everywhere—from the frame of your truck to the handrails in an office building—but “tubing” isn’t a single product. It’s a category defined by shape, dimensions, and the standard it’s made to. Whether you’re fabricating a part or building a structure, the spec that governs the tubing is the key to choosing correctly.
The Two Main Types: Mechanical vs. Structural
The first question to answer is what the tubing will do. That choice splits the category in two: mechanical tubing and structural tubing. They’re made to different standards and aren’t interchangeable without checking the governing spec.
Mechanical tubing is selected for precision dimensions, machinability, and mechanical performance. The common spec is ASTM A513, which covers electric-resistance-welded (ERW) carbon and alloy steel tubing in round, square, rectangular, and special shapes. Its listed size ranges include hot-rolled tubing with wall thicknesses from 0.065 to 0.650 inches and cold-rolled tubing with walls from 0.022 to 0.134 inches.
Structural tubing, on the other hand, is the backbone of construction. ASTM A500 is the standard for cold-formed welded and seamless carbon steel structural tubing in rounds and shapes, and it’s the common reference for hollow structural sections (HSS). This is the spec you’d call out for columns, beams, and frames where load-bearing strength is the priority.
How Tubing Is Measured: OD, Wall, and Gauge
Unlike pipe, which is ordered by nominal pipe size, tube is ordered by its outside diameter (OD) and wall thickness. Getting this distinction right is the single most common point of confusion.
Standard reference tables from manufacturers list steel tubing by OD × wall thickness. One industry chart shows welded steel tubing dimensions ranging from 0.500 in. to 5.000 in. for square tubing, rectangles up to 4 in. × 6 in., and round OD from 1.000 in. to 3.500 in., with wall thicknesses from 0.035 in. to 0.250 in. and mill lengths of 5 to 40 feet. Gauge can also be used to specify wall thickness, but it’s not a universal measure—one gauge number can correspond to different actual thicknesses depending on the diameter and chart used.
| Type | Common Standard | Typical Use |
|---|---|---|
| Mechanical | ASTM A513 | Precision parts, machined components |
| Structural | ASTM A500 | Columns, beams, frames |
| Seamless Mechanical | ASTM A519 | High-pressure, critical applications |
When it’s time to buy, the Steel Tube Institute’s side-by-side reference table is a solid starting point for comparing specs. Before any fabrication or installation, you’ll want to confirm the OD, wall thickness, material grade, shape, and governing standard with your supplier.
Common Mistakes To Avoid
The most frequent errors come from assuming all steel tubing is the same. Confusing tube with pipe is the big one—pipe is ordered by nominal size, tube by OD and wall. Assuming one standard covers everything is another; A513, A500, and A519 govern different products with different performance characteristics. And relying on gauge alone without verifying the actual wall thickness can leave you with material that’s too thin for the job.
References & Sources
- Steel Tube Institute. “Steel Tube Specifications Side-by-Side Reference Table.” Identifies ASTM A500 as the standard for cold-formed structural tubing.
- ASTM International. “ASTM A513/A513M-20a Standard Specification.” Official document for electric-resistance-welded carbon and alloy steel mechanical tubing.