Round hollow steel sections come in two fundamentally different sizing systems: pipe (sized by Nominal Pipe Size and schedule) and tube (sized by actual outside diameter and wall thickness). These are not interchangeable. Using the wrong one in a design leads to assemblies that don't fit and fittings that won't connect.
The Fundamental Distinction
| Pipe | Tube | |
|---|---|---|
| Sized by | NPS (nominal) + schedule | Actual OD + wall thickness |
| Primary purpose | Fluid/gas conveyance | Structural, mechanical, decorative |
| Fittings | NPS-based threaded/flanged | Welded, machined, or clamped |
| OD fixed? | Yes — OD fixed per NPS regardless of schedule | Yes — OD is the specified dimension |
| Call-out example | "2" Sch 40 pipe" | "2.000 OD × 0.120 wall tube" |
The rule: Use pipe when you need to connect to standard fittings (NPT threads, flanges, Victaulic). Use tube when you need a specific geometry for welding, bending, or machining.
Why NPS Doesn't Equal the Actual Diameter
Nominal Pipe Size (NPS) originated when iron fittings were sized by approximate inside diameter. As pipe manufacturing evolved through more schedules and heavier walls, the "nominal" number became detached from any actual dimension.
Today's rule:
- NPS 1/8 through NPS 12: The NPS number is neither OD nor ID. The OD is a fixed value specific to each NPS. The ID shrinks as schedule increases.
- NPS 14 and above: The NPS number equals the OD in inches.
This means a "2-inch pipe" has an outside diameter of 2.375", not 2.000". All pipe fittings are manufactured to match the fixed OD for each NPS — you cannot substitute a 2.000" OD tube and expect fittings to connect.
Pipe Schedule Chart
Schedule controls wall thickness. Higher schedule = thicker wall = smaller bore = higher pressure rating. The OD never changes within a given NPS.
| NPS | OD (in) | Sch 10 Wall | Sch 40 Wall | Sch 80 Wall | Sch 160 Wall | XXS Wall |
|---|---|---|---|---|---|---|
| 1/2 | 0.840 | 0.083 | 0.109 | 0.147 | 0.187 | 0.294 |
| 3/4 | 1.050 | 0.083 | 0.113 | 0.154 | 0.219 | 0.308 |
| 1 | 1.315 | 0.109 | 0.133 | 0.179 | 0.250 | 0.358 |
| 1-1/2 | 1.900 | 0.109 | 0.145 | 0.200 | 0.281 | 0.400 |
| 2 | 2.375 | 0.109 | 0.154 | 0.218 | 0.344 | 0.436 |
| 2-1/2 | 2.875 | 0.120 | 0.203 | 0.276 | 0.375 | 0.552 |
| 3 | 3.500 | 0.120 | 0.216 | 0.300 | 0.438 | 0.600 |
| 4 | 4.500 | 0.120 | 0.237 | 0.337 | 0.438 | 0.674 |
| 6 | 6.625 | 0.134 | 0.280 | 0.432 | 0.562 | 0.864 |
| 8 | 8.625 | 0.148 | 0.322 | 0.500 | 0.719 | 0.875 |
| 10 | 10.750 | 0.165 | 0.365 | 0.500 | 0.844 | 1.000 |
| 12 | 12.750 | 0.180 | 0.406 | 0.500 | 1.000 | 1.000 |
| 14 | 14.000 | 0.188 | 0.438 | 0.594 | — | — |
| 16 | 16.000 | 0.188 | 0.500 | 0.656 | — | — |
| 18 | 18.000 | 0.188 | 0.562 | 0.750 | — | — |
| 20 | 20.000 | 0.218 | 0.594 | 0.812 | — | — |
| 24 | 24.000 | 0.218 | 0.688 | 0.969 | — | — |
Common schedule guidance:
- Sch 5/10 — light wall, low-pressure process and fire suppression
- Sch 40 (STD) — default for most plumbing and process piping
- Sch 80 (XS) — heavy pressure service; also used where pipe threads are cut
- Sch 160 / XXS — high-pressure hydraulic and steam
Tube Manufacturing Methods
ERW — Electric Resistance Welded
Strip steel is roll-formed into a tube and seam-welded by electrical resistance. The OD is sized by roll tooling; the ID has a visible longitudinal weld seam.
| Attribute | Value |
|---|---|
| OD tolerance | ±0.015" (typical) |
| Wall tolerance | ±12.5% |
| Internal bore | Seam visible |
| Cost | Baseline |
| Best for | Structural frames, guards, general fabrication |
DOM — Drawn Over Mandrel
ERW tube that has been cold-drawn over a mandrel after welding. Drawing improves OD/ID concentricity, eliminates internal weld flash, tightens tolerances, and work-hardens the material.
| Attribute | Value |
|---|---|
| OD tolerance | ±0.005" (typical) |
| Internal bore | Smooth, no seam |
| Cost | 2–3× ERW |
| Best for | Hydraulic cylinders, precision bearing fits, shafts |
Seamless
Extruded or pierced from a solid billet — no weld seam. Required where seam-related failure cannot be tolerated.
| Attribute | Value |
|---|---|
| OD tolerance | ±1% or ±0.020" |
| Internal bore | Smooth, no seam |
| Cost | 1.5–2.5× ERW |
| Best for | High-pressure hydraulics, critical pressure vessels |
Common Mechanical Tube Sizes
| OD (in) | Common Wall Thicknesses (in) |
|---|---|
| 3/4" | 0.035, 0.049, 0.065, 0.083, 0.120 |
| 1" | 0.049, 0.065, 0.083, 0.120, 0.156, 0.188 |
| 1-1/2" | 0.065, 0.083, 0.120, 0.156, 0.188, 0.250 |
| 2" | 0.083, 0.120, 0.156, 0.188, 0.250, 0.313 |
| 2-1/2" | 0.083, 0.120, 0.156, 0.188, 0.250, 0.313 |
| 3" | 0.083, 0.120, 0.156, 0.188, 0.250, 0.375 |
| 4" | 0.120, 0.156, 0.188, 0.250, 0.375, 0.500 |
The "2-Inch Round Tube" Trap
If someone asks for "2-inch round tube," there are two very different objects:
- 2.000" OD mechanical tube — actual 2.000" outside diameter
- NPS 2 pipe — 2.375" actual outside diameter
These do not fit the same connection points, the same bender tooling, or the same end caps. Always specify either the actual OD or the NPS explicitly on drawings and purchase orders.
Pipe callout: 2" NPS Sch 40 ASTM A53 Gr. B ERW
Tube callout: 2.000 OD × 0.120 Wall ERW ASTM A513 Type 5
Fitting Compatibility
| Fitting Type | Matches | Does Not Match |
|---|---|---|
| NPT threaded fittings | NPS pipe of matching NPS | Mechanical tube |
| Flanged fittings (ANSI 150#, 300#) | NPS pipe | Mechanical tube |
| Victaulic grooved couplings | Grooved pipe (NPS-based OD) | Mechanical tube |
| Compression fittings (Swagelok) | Actual OD specification | NPS pipe |
| Weld fittings (butt-weld elbows) | NPS pipe | Mechanical tube |