An HSS designation lists the outside dimensions first and the wall thickness last. HSS 152x102x6.4 is a rectangular hollow structural section 152 mm by 102 mm across the outside with a 6.4 mm wall; in imperial it's HSS 6x4x1/4. Square tubes repeat the side (HSS 102x102x6.4), and round tubes give the outside diameter and wall (HSS 168x6.4). What the name doesn't tell you is the material standard, the class or the corner radius, and those can matter as much as the size.
This guide explains HSS nomenclature in detail: how metric and imperial names relate, what "tube steel" and "gauge" mean, how corner radius affects fabrication, and how to measure a tube you already have. For the size and weight tables, see our HSS sizes chart.
Reading the designation
| Callout | Shape | What the numbers are |
|---|---|---|
| HSS 102x102x6.4 | Square | 102 mm x 102 mm outside, 6.4 mm wall |
| HSS 152x102x6.4 | Rectangular | 152 mm x 102 mm outside (larger first), 6.4 mm wall |
| HSS 168x6.4 | Round | 168 mm outside diameter (168.3 mm actual), 6.4 mm wall |
| HSS 4x4x1/4 | Square, imperial | 4 in x 4 in outside, 1/4 in wall |
A few rules of thumb:
- All dimensions are outside dimensions. The inside is the outside minus two wall thicknesses, less the rounded corners.
- The larger side comes first on rectangular sections. Which way the tube is oriented in the structure is shown on the drawing, not in the name.
- "HSS" may be dropped on some drawings and supplier lists, or older drawings may use "TS" (structural tubing). Same idea, same numbers.
Metric and imperial HSS names
Most HSS used in Canada is made to imperial dimensions, so the metric names are converted inches rounded to a whole millimetre. Once you know the pattern, you can switch between the two quickly.
Outside dimensions:
| Imperial (in) | Metric name (mm) |
|---|---|
| 2 | 51 |
| 3 | 76 |
| 4 | 102 |
| 5 | 127 |
| 6 | 152 |
| 8 | 203 |
| 10 | 254 |
Wall thicknesses:
| Imperial (in) | Metric name (mm) |
|---|---|
| 1/8 | 3.2 |
| 3/16 | 4.8 |
| 1/4 | 6.4 |
| 5/16 | 7.9 |
| 3/8 | 9.5 |
| 1/2 | 13 |
Round HSS is different: many round sizes follow pipe outside diameters, so the metric name rounds an odd decimal (168.3 mm is a 6.625 in outside diameter). Our post on HSS vs pipe explains why round tube and pipe of the same nominal size can still differ.
HSS vs tube steel vs mechanical tubing
People call HSS "tube steel", "structural tube", "box section" or "square tubing". On a construction site those usually mean the same product. The important distinction is between structural and non-structural tube:
- HSS (structural tube) is made to a structural specification. In Canada that's commonly CSA G40.20/G40.21 grade 350W, Class C or Class H; ASTM A500 and A1085 tube is also common. These standards control strength and dimensional tolerances, and section tables for design are based on them.
- Mechanical tubing is sold for furniture, racks and machinery, often by outside size and gauge. It may look identical, but it isn't made or certified to a structural specification.
If a drawing calls for HSS, the mill certificate should show the structural specification. Our guide to HSS steel grades covers the standards, including the reduced design wall thickness that US tables use for A500 tube.
Why HSS isn't sold by gauge
"What gauge is HSS?" is a common question, especially from people who work with sheet metal or light tubing. Structural HSS walls are specified in millimetres or fractions of an inch, never gauge. Gauge numbers come from sheet steel, where a bigger number means thinner material.
For comparison, using the standard sheet steel gauge:
| Sheet gauge | Approximate thickness | Nearest HSS wall |
|---|---|---|
| 14 gauge | about 1.9 mm | Lighter than standard structural HSS walls |
| 11 gauge | about 3.0 mm | 3.2 mm (1/8 in), slightly thicker |
| 7 gauge | about 4.6 mm | 4.8 mm (3/16 in), slightly thicker |
Because the numbers are close but not equal, "11 gauge tube" and "1/8 inch wall HSS" are not interchangeable on a structural drawing. If a supplier quotes tube by gauge, ask which specification it is made to.
Corner radius: the dimension not in the name
Square and rectangular HSS are formed from strip steel, usually by shaping a welded round tube, so the corners are rounded, not sharp. The corner radius isn't part of the designation, but it affects several things:
- Flat width. The flat part of each face is narrower than the outside dimension. That flat width limits where bolt holes, plates and clips can go. Design standards and the CISC tables allow for the corners when working out section properties and flat widths.
- Welding at the corners. A plate welded along the edge of a tube meets the curved corner, which changes the weld detail. Fillet welds that work on a flat face may need to become a different weld type at the corner. Our post on welding HSS tube covers that.
- Fit-up. Cap plates sized to the outside dimension, tubes nesting inside other tubes, and cladding or glass fitted against the tube all depend on the real corner shape.
- Weight. The rounded corners mean a tube weighs a little less than a sharp-cornered box would. The formula in our HSS weight calculator includes a correction for that.
The radius grows with wall thickness and varies with the forming process and the mill. Class C and Class H tube from different producers won't necessarily have identical corners. Where a detail depends on the radius, check the producer's data or measure the actual material.
How to measure an existing tube
To identify an HSS member on an existing building:
- Measure the outside dimensions with a tape or caliper. Match them to the nearest standard size: 101.6 mm reads as a 102 (4 in) tube.
- Measure the wall. At a cut end this is easy with a caliper. On a member in place, a contractor can use an ultrasonic thickness gauge, which reads the wall from one side. Paint and rust add to or subtract from the reading.
- Look for the seam. A faint weld line along one face, often visible inside, confirms a welded tube.
- Note the corners. Sharp corners and a thin wall can mean a light mechanical or cold-formed section, not structural HSS.
A measured size tells you the shape; it doesn't tell you the grade. For anything carrying load, the designer decides what to assume about an unknown tube.
Ordering HSS by the right name
Give the full designation, the material specification and class if the drawings show one, lengths and quantities, and any plates, holes or slots. We cut, drill and weld HSS posts and frames with base and cap plates to your shop drawings, with CWB certified welding, and can prime or galvanize to suit. Send your drawings through our quote form or see our steel products.
Frequently asked questions
How do you read an HSS size?
An HSS designation lists the outside dimensions and then the wall thickness. HSS 152x102x6.4 is a rectangular tube 152 mm by 102 mm outside with a 6.4 mm wall, the same as the imperial HSS 6x4x1/4. Round HSS gives the outside diameter and wall, such as HSS 168x6.4.
Is HSS the same as tube steel?
In construction, tube steel and structural tubing usually mean HSS. Strictly, HSS is tube made to a structural specification such as CSA G40.21 or ASTM A500. Mechanical tubing sold by diameter and gauge is a different product and should not be substituted for HSS on a structural drawing.
What gauge is HSS steel?
HSS is not specified by gauge. Its wall is given in millimetres or fractions of an inch. Gauge numbers come from sheet steel and light tubing; for comparison, 11 gauge is about 3.0 mm, slightly under the 3.2 mm (1/8 inch) HSS wall.
What is the corner radius of HSS?
It isn't part of the designation. The outside corner radius grows with wall thickness and varies with the forming process and the mill. Where a detail depends on it, check the producer's data or measure the actual tube.
