Round HSS and steel pipe can look identical, and some sizes even share the same outside diameter and wall, but they are different products. Round HSS is structural tube, sized by outside diameter and wall thickness and made to structural grades such as CSA G40.21 350W (350 MPa minimum yield). Pipe is sized by nominal pipe size (NPS) and schedule, made mainly for carrying fluids, and commonly supplied to ASTM A53 Grade B, with a 240 MPa minimum yield. For columns, posts and frames, specify HSS unless the engineer has designed for pipe.
Two naming systems for the same kind of shape
Round HSS is named by what you can measure. A Canadian drawing might show HSS 168x6.4: a 168 mm (6.625 in) outside diameter with a 6.4 mm wall. The imperial form is HSS6.625x0.250. Our HSS size chart lists common round sizes, and how to read HSS designations explains the format.
Pipe uses an older system from the piping world:
- NPS (nominal pipe size) is a family name, not a measurement. For NPS 1/8 to NPS 12, the actual outside diameter is larger than the nominal size. NPS 6 pipe is 168.3 mm (6.625 in) outside, not 6 in. From NPS 14 up, the outside diameter equals the nominal size.
- Schedule or weight class sets the wall. For each NPS the outside diameter is fixed, so a heavier wall means a smaller inside diameter. The common classes are Standard (STD), Extra Strong (XS) and Double Extra Strong (XXS), and numbered schedules such as 40 and 80. Up to NPS 10, Schedule 40 matches STD; up to NPS 8, Schedule 80 matches XS.
So "6 inch pipe, schedule 40" and an HSS6.625x0.280 (168.3 mm outside, 7.1 mm wall) describe tubes with the same outside diameter and wall. They are still bought, certified and designed differently.
Common pipe sizes in metric
Dimensions follow the standard pipe dimension tables (ASME B36.10M); mass is for the STD wall, converted from the AISC Shapes Database and rounded.
| NPS | Outside diameter (mm) | STD wall (mm) | XS wall (mm) | STD mass (kg/m) |
|---|---|---|---|---|
| 2 | 60.3 | 3.91 | 5.54 | 5.4 |
| 3 | 88.9 | 5.49 | 7.62 | 11.3 |
| 3-1/2 | 101.6 | 5.74 | 8.08 | 13.6 |
| 4 | 114.3 | 6.02 | 8.56 | 16.1 |
| 5 | 141.3 | 6.55 | 9.53 | 21.7 |
| 6 | 168.3 | 7.11 | 10.97 | 28.3 |
| 8 | 219.1 | 8.18 | 12.70 | 42.6 |
| 10 | 273.1 | 9.27 | 12.70 | 60.3 |
| 12 | 323.9 | 9.53 | 12.70 | 73.8 |
Most round HSS diameters used in Canada (88.9, 114.3, 168.3, 219.1, 273.1 and 323.9 mm, for example) match these pipe diameters. That is convenient for fittings and caps, and it is also why pipe and HSS get mixed up in the yard.
Grades and strength
| Product | Common specification | Minimum yield |
|---|---|---|
| Round HSS | CSA G40.21 350W, Class C or Class H | 350 MPa |
| Round HSS | ASTM A500 Grade C | 315 MPa (46 ksi) |
| Round HSS | ASTM A500 Grade B | 290 MPa (42 ksi) |
| Pipe | ASTM A53 Grade B | 240 MPa (35 ksi) |
For the same outside diameter and wall, 350W HSS has a minimum yield about 45% higher than A53 Grade B pipe, and A500 Grade C round tube about 30% higher. In compression, the advantage is smaller for long, slender members, where stiffness rather than yield strength governs, but it is real for typical posts and columns.
Two more differences matter to the designer:
- Design wall thickness. In US practice (AISC), both A500 tube and pipe are designed with a wall taken as 0.93 times the nominal thickness, to allow for mill tolerance. Canadian tables for CSA G40.21 HSS use the full nominal wall. Our guide to HSS steel grades explains this in detail.
- What the specification tests. Pipe standards focus on pressure service: chemistry, tensile properties and leak or hydrostatic testing. HSS standards focus on structural properties and tolerances, and some (such as A1085 or toughness-tested G40.21 grades) add impact testing for demanding work.
When pipe makes sense, and when it doesn't
Pipe is used structurally all the time, as long as the designer knows that is what is being supplied:
- Bollards and guard posts. Thick-walled pipe filled with concrete is a common bollard.
- Handrails and light frames. Small pipe sizes are easy to bend and fit.
- Pipe racks, supports and mechanical frames. Where the pipe trade already works with pipe sizes and fittings.
- Basement columns. Some adjustable and fixed basement columns are built from pipe; see lally column sizes.
Be cautious with:
- Substituting pipe for specified HSS. Same diameter does not mean same capacity. The engineer has to approve it and check it with pipe properties.
- Used or reclaimed pipe. Its grade, remaining wall and corrosion are unknown without testing. It should not go into a structure on looks alone.
- Mechanical tubing. Light round tube sold for mechanical and furniture uses (for example to ASTM A513) is another product again, specified by diameter and gauge. It isn't structural HSS either.
Fabrication is much the same
In the shop, round HSS and pipe are handled the same way. Both are cut square or mitred, coped to fit other round members, and welded to base plates, cap plates and knife plates. A few points apply to both:
- Ends are usually closed. A welded cap plate keeps water out of exterior posts, or the tube is vented for galvanizing.
- Fish-mouth joints need care. Where one round member meets another, the cope has to be cut to the curve for a sound weld.
- Bolting through the wall needs sleeves or plates, so the round wall isn't crushed when the bolt is tightened.
- Grade affects welding procedures. The material on the mill certificate has to match what the welding procedure covers.
HSS vs pipe: what to put on the order
For HSS, give the outside diameter and wall exactly as on the drawing, plus the standard, grade and class, for example "HSS 168x6.4, CSA G40.21 350W Class C". Add cut lengths, cap and base plates, holes and the finish.
For pipe, give the NPS, schedule or weight class and specification, for example "NPS 6 STD, ASTM A53 Grade B", plus the same fabrication details.
If the drawing only says "6 inch pipe column" or "round post", ask the designer which product and grade they designed for before ordering. Mill test certificates, where the project needs them, show what was actually supplied. Galvanized round posts need vent and drain holes, just like square tube; see galvanized vs painted structural steel.
Round posts and columns, fabricated
We supply HSS posts and columns cut to length with base and cap plates welded in our CWB certified shop, prime or galvanize to suit, and deliver across the GTA and Ontario. Tell us the size, grade and finish on your drawings, and we'll confirm what we can supply. New to tube shapes? Start with what is HSS steel.
Send your drawings for a quote, or browse our steel products, including posts, beams, plates and channels.
Frequently asked questions
What is the difference between HSS and pipe?
Round HSS is structural tube, sized by outside diameter and wall thickness and made to structural specifications such as CSA G40.21 350W or ASTM A500. Steel pipe is sized by nominal pipe size and schedule and made mainly for carrying fluids, commonly to ASTM A53, which has a lower minimum yield strength.
Is round HSS the same as pipe?
They can share the same outside diameter and even the same wall, but they are different products with different specifications, tolerances and minimum strengths. A drawing that calls for HSS needs HSS, unless the engineer approves pipe as a substitute.
Is HSS stronger than pipe?
For the same size, usually yes, because of the grade. CSA G40.21 350W HSS has a minimum yield of 350 MPa and ASTM A500 Grade C round tube 315 MPa, compared with 240 MPa for common ASTM A53 Grade B pipe.
What does Schedule 40 mean?
Schedule is a wall thickness class for pipe. For each nominal pipe size the outside diameter is fixed, and a higher schedule means a thicker wall and a smaller inside diameter. For sizes up to NPS 10, Schedule 40 matches the standard (STD) wall.
