Structural pipe (ASTM A53) and round HSS (ASTM A500/A1085) are both round hollow steel sections, but they are different products. Pipe is named by nominal diameter and schedule (PIPE6STD has an actual OD of 6.625 in., not 6 in.), comes in three wall classes (Std, XS, XXS), and has a yield strength of 35 ksi. Round HSS is named by its actual outside diameter and nominal wall (HSS6.625X0.280), comes in far more wall options, and A500 Grade C gives you 46 ksi. At the same OD and wall the two weigh essentially the same — PIPE6STD and HSS6.625X0.280 are both 19.0 lb/ft — so the choice comes down to strength, availability, and what the engineer specified, not weight.
If you estimate steel, you need both families in your head, because drawings mix them freely and detailers sometimes call out one when they mean the other. Here is the full breakdown with the AISC weights for all 51 pipe sections and the round HSS sizes that overlap them.
What is the difference between pipe and round HSS?
Four things separate them: the material spec, the naming convention, the available wall thicknesses, and the yield strength.
| Structural pipe | Round HSS | |
|---|---|---|
| Material spec | ASTM A53 Grade B | ASTM A500 Gr. B/C, or A1085 |
| Yield strength | 35 ksi | 42 ksi (A500 Gr. B), 46 ksi (Gr. C), 50 ksi (A1085) |
| Naming | Nominal diameter + class (PIPE6STD) | Actual OD × nominal wall (HSS6.625X0.280) |
| Wall options per size | 2–3 (Std, XS, XXS) | Typically 3–9 walls per OD |
| Size range (AISC) | 1/2 in. to 26 in. nominal | 1.660 in. to 28.000 in. OD |
| AISC catalog count | 51 sections | 189 round sections |
| Typical use | Posts, bollards, handrail, misc. metals | Designed compression members, bracing, trusses, AESS |
The naming trap is the one that bites estimators. A "6-inch pipe" is 6.625 in. across. An HSS6.000 is exactly 6.000 in. across. If you're nesting sleeves, checking clearances, or matching a base plate detail, that 5/8 in. matters. Below 14 in. nominal, pipe OD is always bigger than the name; from 14 in. up, nominal and actual OD finally agree.
Design wall is the other quiet difference. For both A53 pipe and A500 HSS, AISC design uses 0.93 times the nominal wall to account for manufacturing tolerance. A1085 HSS is the exception — its tighter tolerances let you design with the full nominal wall, which is one reason engineers spec it on efficiency-critical members.
For how these fit into the broader AISC catalog, see the AISC shape database guide.
Structural pipe size chart: Std, XS, XXS
All 51 pipe sections in the AISC database, weight in lb/ft. Std corresponds roughly to Schedule 40 (through 10 in.) and XS to Schedule 80 (through 8 in.); above those sizes the schedules diverge from the weight classes, so don't treat "Sch 40" and "Std" as interchangeable on big pipe.
| Nominal size | OD (in.) | Std (lb/ft) | XS (lb/ft) | XXS (lb/ft) |
|---|---|---|---|---|
| 1/2 | 0.840 | 0.85 | 1.09 | — |
| 3/4 | 1.050 | 1.13 | 1.48 | — |
| 1 | 1.315 | 1.68 | 2.17 | — |
| 1-1/4 | 1.660 | 2.27 | 3.00 | — |
| 1-1/2 | 1.900 | 2.72 | 3.63 | — |
| 2 | 2.375 | 3.66 | 5.03 | 9.04 |
| 2-1/2 | 2.875 | 5.80 | 7.67 | 13.7 |
| 3 | 3.500 | 7.58 | 10.3 | 18.6 |
| 3-1/2 | 4.000 | 9.12 | 12.5 | — |
| 4 | 4.500 | 10.8 | 15.0 | 27.6 |
| 5 | 5.563 | 14.6 | 20.8 | 38.6 |
| 6 | 6.625 | 19.0 | 28.6 | 53.2 |
| 8 | 8.625 | 28.6 | 43.4 | 72.5 |
| 10 | 10.750 | 40.5 | 54.8 | 104.0 |
| 12 | 12.750 | 49.6 | 65.5 | 126.0 |
| 14 | 14.000 | 54.6 | 72.2 | — |
| 16 | 16.000 | 62.6 | 82.9 | — |
| 18 | 18.000 | 70.7 | 93.5 | — |
| 20 | 20.000 | 78.7 | 104.0 | — |
| 24 | 24.000 | 94.7 | 126.0 | — |
| 26 | 26.000 | 103.0 | 136.0 | — |
Notes worth remembering at the takeoff desk:
- XXS exists only from 2 in. through 12 in. (and skips 3-1/2). If a drawing calls out "PIPE 16 XXS," someone made it up — query it.
- XXS is not "double XS" in weight, but it's close: PIPE6XXS at 53.2 lb/ft is nearly double PIPE6XS at 28.6. Miss the second X on a long run and you've cut your tonnage nearly in half.
- PIPE10XS and PIPE20XS both weigh 104.0 lb/ft, and PIPE12XXS and PIPE24XS both weigh 126.0. Weight alone won't tell you which section you're looking at.
Round HSS size chart: highlights and where the families overlap
The AISC round HSS catalog runs 189 sections across 33 outside diameters, from HSS1.660X0.140 up to HSS28.000. Thirteen of those ODs are exactly the pipe ODs — 1.660, 1.900, 2.375, 2.875, 3.500, 4.000, 4.500, 5.563, 6.625, 8.625, 10.750, 12.750, plus the whole-inch sizes from 14 up. The rest (2.500, 3.000, 5.000, 6.000, 7.000, 9.625, 10.000, and so on) are HSS-only ODs with no pipe equivalent.
Where the ODs match, there is usually an HSS wall that lands on or near the pipe wall, and the weights track within rounding:
| Pipe section | lb/ft | Round HSS equivalent | lb/ft |
|---|---|---|---|
| PIPE2STD | 3.66 | HSS2.375X0.154 | 3.66 |
| PIPE2-1/2STD | 5.80 | HSS2.875X0.203 | 5.80 |
| PIPE3STD | 7.58 | HSS3.500X0.216 | 7.58 |
| PIPE3-1/2STD | 9.12 | HSS4.000X0.226 | 9.12 |
| PIPE4STD | 10.8 | HSS4.500X0.237 | 10.8 |
| PIPE5STD | 14.6 | HSS5.563X0.258 | 14.6 |
| PIPE6STD | 19.0 | HSS6.625X0.280 | 19.0 |
| PIPE8STD | 28.6 | HSS8.625X0.322 | 28.6 |
| PIPE12STD | 49.6 | HSS12.750X0.375 | 49.6 |
| PIPE6XS | 28.6 | HSS6.625X0.432 | 28.6 |
| PIPE8XS | 43.4 | HSS8.625X0.500 | 43.4 |
| PIPE10XS | 54.8 | HSS10.750X0.500 | 54.8 |
| PIPE12XS | 65.5 | HSS12.750X0.500 | 65.5 |
Same geometry, same weight — different steel. The HSS version carries 46 ksi yield (A500 Gr. C) against the pipe's 35 ksi, which is why an engineer can swap HSS6.625X0.280 for PIPE6STD but not the other way around without checking the numbers.
Where HSS pulls away is wall selection. At 6.625 OD, pipe gives you three choices (0.280, 0.432, 0.864 nominal). HSS gives you eight walls from 0.125 to 0.500. At 14.000 and 16.000 OD there are nine HSS walls each, up to a full 1.000 in. — pipe tops out at 0.500 XS in those sizes. Light walls are HSS-only territory too: nothing in the pipe family gets you a 6.625 OD at 0.125 wall (8.69 lb/ft) for a canopy strut where 19.0 lb/ft of PIPE6STD is triple the steel you need.
For square and rectangular HSS, and the full round chart, see the HSS size reference.
Which is cheaper: pipe or round HSS?
Per pound, A53 pipe is usually cheaper and easier to source — every pipe distributor in the country stocks Std and XS in the common sizes, while an odd HSS round OD/wall combo may be a mill order with a real lead time. That's why pipe still owns the miscellaneous-metals scope: bollards, handrail posts, guardrail, sleeves, dunnage.
Per member, HSS often wins on designed structural work. The 31% yield-strength advantage (46 vs 35 ksi) means a lighter section can do the same job, and the wider wall selection means the engineer isn't forced to jump from 0.280 to 0.432 wall when 0.312 would do. On a truss with forty round compression members, that granularity is real tonnage.
The estimating takeaway: price what's specified. If the drawing says PIPE6STD, don't quote HSS6.625X0.280 because your service center has it on the floor — the substitution usually goes pipe→HSS fine (stronger material, same geometry) but it still needs engineer approval, and the reverse substitution is a strength downgrade that can get rejected in shop drawing review.
How is each called out on drawings?
Pipe callouts you'll see, all meaning the same member:
- PIPE 6 STD or PIPE6STD — the AISC designation
- P6 or P6 STD — detailer shorthand, common on older drawings
- 6" SCH 40 or 6" φ SCH 40 — plumbing-style schedule callout that migrated onto structural drawings; treat Sch 40 as Std through 10 in.
- 6" O.D. PIPE — ambiguous and wrong-ish; 6 in. nominal pipe is 6.625 OD, so query whether they mean PIPE6 or a true 6.000 OD section (which would be HSS6.000)
Round HSS callouts:
- HSS6.625X0.280 — the AISC designation, OD × wall in decimal inches
- HSS6.625X1/4 — mixed decimal/fraction, same section as HSS6.625X0.250
- 6.625 DIA X .280 WALL — spelled out on some fabricator standards
- TS6.625X0.280 — the pre-2003 "TS" (tube steel) prefix still shows up on renovation drawings and old details; read it as HSS
The single-X format is also your visual cue on a busy sheet: round HSS has one X (OD × wall), square and rectangular HSS have two (HSS6X6X1/4). If a callout reads HSS with two dimensions and the first looks like a pipe OD — HSS6.625X0.280 — it's a round.
Takeoff tip: shorthand callouts that confuse pipe and HSS
The label errors that actually cost money on round sections:
- P6 vs HSS6.000. "P6" means PIPE6 (6.625 OD, 19.0 lb/ft Std). HSS6.000X0.250 is a true 6-inch section at 15.4 lb/ft. Confuse them across a hundred handrail posts and your weight, your sleeves, and your caps are all wrong.
- Dropped schedule. A bare "PIPE 6" with no class defaults to Std in most shops — but if the general notes say "all pipe XS UNO," that same callout is 28.6 lb/ft, a 51% swing.
- XS vs XXS at a glance. One character, roughly double the weight. PIPE8XS is 43.4; PIPE8XXS is 72.5. On faxed or scanned drawings the third X is exactly the kind of mark that disappears.
- TS legacy callouts. TS4X0.237-style callouts on old details are round HSS/pipe-family members, not square tube. The single X tells you it's round.
These shorthand traps are the same class of problem covered in steel section shorthand decoded — degraded prints and detailer abbreviations are where manual takeoffs quietly lose accuracy. It's also why SteelFlo's AI takeoff draws a bounding box on every pipe and HSS label it finds and shows you the box for verification, rather than making you trust a count you can't see; the detection-plus-human-review workflow runs 95-99% accurate across 4,500+ sections in 6 standards, pipe schedules and round HSS included.
FAQ quick hits
Is round HSS the same as pipe? No. Same shape, different products: different ASTM specs (A500 vs A53), different naming (actual OD vs nominal diameter), different yield strength (46 vs 35 ksi for the common grades).
What does STD, XS, and XXS mean? Standard, extra strong, and double-extra strong — the three pipe weight classes. Each nominal size keeps the same OD across all three; only the wall (and therefore the ID and weight) changes.
Can I substitute HSS for pipe? Geometrically yes where the OD and wall match (see the overlap table above), and the HSS is the stronger material — but any substitution still needs the engineer's sign-off. Never substitute pipe for a specified HSS round without approval; that's a downgrade.
How many structural pipe sizes are there? 51 in the AISC database: 21 nominal sizes from 1/2 in. to 26 in., each in Std and XS, plus XXS in nine sizes from 2 in. through 12 in.
Need a weight without opening the manual? Every one of these sections is in the free steel weight calculator — punch in PIPE6STD or HSS6.625X0.280 and a length, get pounds and cost in a couple of seconds. Individual sections also have dedicated pages with dimensions and example pricing in the shapes reference. And if you'd rather not hunt round-section callouts across a 40-sheet set by hand, SteelFlo's first 3 takeoffs are free.