1-inch square tubing is hollow metal tube measuring 1 inch by 1 inch on the outside, sold in steel under ASTM A500 or in aluminum alloys like 6063-T5.
A tape measure alone won’t tell you the whole story. Nominal 1 x 1 square tubing is always described by its outside dimension and wall thickness, so two pieces that look identical on the shelf can behave nothing alike under load. The outside is the easy part: 1 inch across each flat side.
What changes everything is what the tube is made of and how thick its wall runs. Steel 1-inch square tube answers to a formal structural standard. Aluminum 1 x 1 tube is an extrusion catalog item, chosen by alloy, temper, and corner style. Those two facts decide where each material belongs.
What Does “1-Inch Square Tubing” Actually Measure?
The 1-inch figure is the outside dimension, and the inside opening is smaller by twice the wall thickness. Buyers who assume the 1-inch number describes the hole wind up with fittings that won’t slide in.
Wall thickness is the second number in any listing. Go up to a 0.125-inch wall and the opening narrows again. For a bracket, frame, or post that must accept another part, that inside measurement is the number worth writing down before you order.
The structure of the material matters too. Steel tube under ASTM A500 is produced as cold-formed welded or seamless carbon steel and is meant for welded, riveted, or bolted construction. Aluminum square tube is made by extrusion, and its corner geometry is sold as a feature: sharp corners on 6063-T5 stock, rounded corners on 6061-T6.
What Standard Governs Steel 1-Inch Square Tubing?
For structural steel work in the US, the governing specification is ASTM A500/A500M-23, which covers cold-formed welded and seamless carbon steel structural tubing in round, square, rectangular, and special shapes. Square and rectangular tubing under this standard is furnished in sizes from 1 to 10 inches across the flat sides, with wall thicknesses from 0.095 to 1.000 inches depending on size, so 1-inch square is the smallest standard structural size in the family.
The standard also fixes the mechanical floor for each grade. Grade B carries a minimum yield strength of 46,000 psi and a minimum tensile strength of 58,000 psi. Grade C raises those minimums to 50,000 psi yield and 62,000 psi tensile. When a fabricator says “A500 Grade B,” those are the numbers being promised.
Geometry gets its own tolerances. Adjacent sides of an A500 square tube must be square within plus or minus 2 degrees, and the outside corner radius may not exceed three times the specified wall thickness. Applications listed for the standard include bridges, buildings, and general structural work — construction duty, not furniture duty.
That distinction matters at the supply counter. A tube labeled only “1 inch square” tells you the shape, not whether it is engineered for a load. Because ASTM A500 sets minimums for yield, tensile, and corner squareness, it is the citation a structural design should carry rather than a generic tubing description. Steel Tube Institute’s overview of ASTM A500 lays out the scope and grades.
Steel vs Aluminum: Which One Do You Need?
Steel tubing is the pick for anything load-bearing or code-referenced, while aluminum tubing is the pick when weight and finish drive the decision.
Aluminum 1 x 1 tube is sold by alloy and temper, not by a structural specification. Two common examples: 6063-T5 aluminum square tubing with sharp corners and 6061-T6 aluminum square tubing with round corners, both offered in 1 x 1. Published weights for the common walls make the trade plain.
Compare that with steel of similar wall thickness and the aluminum is dramatically lighter per foot — which is exactly why it shows up on racks, trim, frames, and projects that move. The trade is stiffness and strength: aluminum’s modulus is lower, so a 1-inch square aluminum span flexes sooner than the same steel tube. If you are choosing between stock sizes for a build, this roundup of 1-inch square tubing options compares the available sizes and materials side by side.
| Material & Size | Weight Per Foot | Key Trait |
|---|---|---|
| 6063-T5, 1 x 1 x 0.065 in | 0.292 lb/ft | Lightest common wall, sharp corners |
| 6063-T5, 1 x 1 x 0.095 in | 0.413 lb/ft | Middle wall for stiffer frames |
| 6063-T5, 1 x 1 x 0.125 in | 0.512 lb/ft | Heaviest 6063-T5 wall listed |
| 6061-T6, 1 x 1 x 0.120 in | 0.474 lb/ft | Stronger alloy, rounded corners |
| A500 Grade B steel | Wall-dependent | 46,000 psi min yield, 58,000 psi min tensile |
| A500 Grade C steel | Wall-dependent | 50,000 psi min yield, 62,000 psi min tensile |
| A500 square range | 0.095–1.000 in walls | Structural, 1 to 10 in across flats |
One naming gap catches people out: aluminum stock is chosen by extruded alloy and finish rather than by a structural specification, so a 1-inch aluminum tube has no A500 equivalent to point at. Ask for the alloy and temper instead — 6063-T5 or 6061-T6 — because those names carry the actual mechanical properties.
The Three Mistakes That Cost the Most
Most bad purchases trace back to one of three mix-ups, and all three are avoidable with a spec sheet in hand.
Assuming all 1-inch square tube is structural steel. Aluminum 1 x 1 tubing is common stock, and its alloy and temper — 6063-T5 versus 6061-T6 — change the numbers. A listing that never names an alloy is not a structural listing.
Confusing nominal size with inside size. Tube is listed by outside dimension plus wall thickness. The inside opening is what remains, so a slip fit needs the inside figure, not the 1-inch label.
Borrowing an A500 assumption for an aluminum part. A500 applies to structural steel construction. Aluminum extrusion catalogs are a different world with different terminology, so a design that cites ASTM A500 should actually be built from A500 steel.
Get those right and the choice usually makes itself. Load, code, or welding contract on the drawing means steel under A500, quoted by grade. Weight, corrosion, or finish drives the job, and the aluminum alloy takes it. Either way, order by outside size, wall thickness, and material name — the three numbers that define the tube.
FAQs
Is 1-inch square tubing measured inside or outside?
Square tube is measured across the outside of the flat sides, so 1-inch square tubing is 1 inch from outside edge to outside edge. The inside opening is smaller because the wall thickness is subtracted on all four sides. A 1 x 1 x 0.120 in tube leaves roughly 0.76 inches of clear inside space.
Can aluminum 1-inch square tube be welded?
Yes, aluminum square tube is commonly welded, though it behaves differently than steel. Aluminum conducts heat away from the weld zone quickly and its oxide layer needs removing before welding. Alloys like 6061-T6 lose temper strength in the heat-affected zone, so a welded joint is usually weaker than the parent tube unless the design accounts for it.
What does the ASTM A500 grade letter mean?
The grade letter sets minimum mechanical properties for the steel tube. Grade B requires at least 46,000 psi yield strength and 58,000 psi tensile strength. Grade C requires at least 50,000 psi yield and 62,000 psi tensile. Higher grades cost more, so the drawing should specify the grade the structure actually needs.
References & Sources
- Steel Tube Institute. “ASTM A500” Scope of the standard, grade minimums, and size ranges for square and rectangular structural tubing.
- Steel Tube Institute. “ASTM Dimensions and Section Properties” Standard dimensions and section properties for A500 square tubing.
- ASTM International. “ASTM A500/A500M-23” Specification for cold-formed welded and seamless carbon steel structural tubing, including corner squareness and radius tolerances.
