HSS Sizes and Wall Thickness
How hollow structural section callouts work, why the wall thickness is nominal rather than actual, and what the 495 standard sizes cover.
Hollow structural sections are the second most common family on a modern steel job after wide flanges. They show up as columns, braces, truss members, railings and anything that needs to look clean from all four sides.
There are 495 standard HSS sizes, which is more than any other family — more than all 273 wide flanges, and nearly four times the 127 angles. That count exists because every outside dimension is offered in a range of wall thicknesses.
Reading the callout
HSS6X6X3/8 is a square section, 6 inches by 6 inches outside, with a 3/8 inch nominal wall.
Unlike a wide flange, none of those numbers is weight. They are all dimensions, so the weight has to be looked up:
| Callout | lb/ft |
|---|---|
| HSS4X4X1/4 | 12.21 |
| HSS6X6X3/8 | 27.48 |
| HSS8X8X1/2 | 48.85 |
| HSS12X8X1/2 | 62.46 |
| HSS20X12X5/8 | 127.37 |
Rectangular sections list the larger dimension first: HSS12X8X1/2 is 12 inches by 8. Round sections are called out by outside diameter and decimal wall — HSS5.563X0.258 — which looks unlike the rest of the family and reads more like pipe.
The range is wide. The lightest standard section is an HSS2X1X1/8 at 2.2 lb/ft; the heaviest are HSS20X12X5/8 and HSS16X16X5/8, both at 127.37 lb/ft.
Why the same outside size has so many weights
The outside dimension stays fixed and the wall thickens inward. That is the whole design of the family, and it is what makes HSS useful: a connection detail, a base plate and a hole pattern all stay valid while the member gets stronger.
A single 6×6 square is offered in seven walls:
1/8, 3/16, 1/4, 5/16, 3/8, 1/2, 5/8
Same footprint every time, from about 10 lb/ft up to nearly 45. An engineer can step up capacity without a detailer redrawing the connection — which is exactly why substitutions get proposed late, and exactly why they are usually cheap to accept.
Nominal wall is not actual wall
This is the HSS equivalent of nominal depth on a wide flange, and it surprises people more.
The actual wall of an HSS is thinner than the nominal wall in the callout. ASTM A500, the usual specification for cold-formed HSS, permits a wall thickness tolerance, and design values account for it by using a reduced design wall thickness — conventionally 0.93 times nominal for A500 sections.
So a nominal 1/2 inch wall is designed as roughly 0.465 inches. That factor is already baked into published section properties, so it is not something to apply twice — but it matters when someone measures a section in the yard, finds it under, and assumes the material is wrong. It is not. It is within specification.
Corner radii are real, and they matter
An HSS is formed, not rolled to shape, so the corners are radiused rather than square. The radius is a function of wall thickness — thicker wall, larger radius.
That has practical consequences:
- A plate cannot sit flush into the corner
- A member framing tight to the corner needs the radius accounted for
- Cope and fit-up dimensions taken from a square-cornered model will be wrong
Modelling software knows this. A sketch on a napkin does not.
The connection problem
HSS look simple and detail hard. The wall is thin relative to a wide flange flange, and there is no back side to reach — you cannot get a wrench inside a closed tube.
That drives most HSS connections toward welding, slotted gusset plates, or through-plates that pass entirely through the section. It is why an HSS brace often costs more to connect than a comparable wide flange, even though the member itself is lighter and cheaper.
Worth knowing at estimating time rather than at detailing time.
Look it up
- Structural shape weights — all 495 HSS sizes with weight per foot and area, plus piece weight for any length and count
- Structural steel connections — gusset plates, base plates and the rest, with what goes wrong in each
- Wide flange beam sizes explained — the other big family, and why nominal depth misleads there too