SCH10S Pipe vs OD Tube: Sizes Explained
Ask a supplier for "1.5 inch stainless" and you could get two different things. 1.5" exhaust tube is 38.1 mm across the outside. 1½" NPS pipe is 48.3 mm. That 10 mm gap is why flanges don't fit, jigs don't sit right and mockups don't match the material that turns up. Below: how each is sized, SCH10S and OD tube charts in mm and inches, and the few places the two systems genuinely line up.
The short answer
- Tube is named by its actual outside diameter (OD). 1.5" tube measures 1.5" (38.1 mm) across.
- Pipe is named by Nominal Pipe Size (NPS). Each NPS has a fixed OD that doesn't match the name. 1½" NPS (often written 1.5" NPS) is 48.3 mm (1.900") OD.
- Schedule sets the pipe's wall thickness. The OD stays the same, so a heavier schedule means a smaller bore.
- Fittings, flanges and jigs go by the OD. Match the OD and you're right.
Tube: the size is the OD
Exhaust and header tube is sized by what you can measure. 2.5" tube is 2.5" (63.5 mm) across the outside, whatever the wall. The wall is quoted separately, and for stainless exhaust tube it's typically around 1.2 to 2 mm. Mandrel bends and slip joints for tube are sold by the same OD.
Pipe: the size is a name
NPS is the North American pipe sizing system, defined in ASME B36.10M for carbon steel and ASME B36.19M for stainless. The names come from old iron pipe sizes, when the number roughly described the bore of standard-wall pipe. The OD was then held fixed so threads and fittings kept fitting, and as wall thicknesses changed, the bore drifted away from the name. Only from NPS 14 upwards does the name equal the OD in inches.
So 1½" NPS is just a label for a 1.900" (48.3 mm) OD, the same OD as UK scaffold tube. In Europe it's DN 40, and many UK suppliers list it as 48.3 × 2.77 mm: real OD, real wall.
What "schedule" and the S in SCH10S mean
Schedule is a wall thickness designation. For any NPS the OD is fixed; a higher schedule number means a thicker wall and a smaller bore. The common stainless schedules are 5S, 10S, 40S and 80S.
The S marks the stainless schedules in ASME B36.19M. For every size in this guide, SCH10 and SCH10S have the same wall; the two standards only differ in some larger sizes. Only two wall figures matter here: 2.77 mm (0.109") from NPS 1 to 2, and 3.05 mm (0.120") from NPS 2½ to 4.
SCH10S pipe sizes in mm and inches
| NPS | DN | OD (mm) | OD (in) | Wall (mm) | ID (mm) |
|---|---|---|---|---|---|
| 1 | 25 | 33.4 | 1.315 | 2.77 | 27.9 |
| 1¼ | 32 | 42.2 | 1.660 | 2.77 | 36.7 |
| 1½ | 40 | 48.3 | 1.900 | 2.77 | 42.8 |
| 2 | 50 | 60.3 | 2.375 | 2.77 | 54.8 |
| 2½ | 65 | 73.0 | 2.875 | 3.05 | 66.9 |
| 3 | 80 | 88.9 | 3.500 | 3.05 | 82.8 |
| 3½ | 90 | 101.6 | 4.000 | 3.05 | 95.5 |
| 4 | 100 | 114.3 | 4.500 | 3.05 | 108.2 |
ID = OD − 2 × wall, rounded. Charts worked in inches may differ by 0.1 mm, and real pipe varies within mill tolerance.
OD tube sizes, with the closest pipe
| Tube size | OD (mm) | Closest SCH10S pipe | Pipe OD (mm) | Difference |
|---|---|---|---|---|
| 1.5" | 38.1 | NPS 1¼ | 42.2 | pipe 4.1 mm bigger |
| 1.75" | 44.45 | NPS 1¼ | 42.2 | pipe 2.3 mm smaller |
| 2" | 50.8 | NPS 1½ | 48.3 | pipe 2.5 mm smaller |
| 2.25" | 57.15 | NPS 2 | 60.3 | pipe 3.2 mm bigger |
| 2.5" | 63.5 | NPS 2 | 60.3 | pipe 3.2 mm smaller |
| 2.75" | 69.85 | NPS 2½ | 73.0 | pipe 3.2 mm bigger |
| 3" | 76.2 | NPS 2½ | 73.0 | pipe 3.2 mm smaller |
| 3.5" | 88.9 | NPS 3 | 88.9 | exact match |
| 4" | 101.6 | NPS 3½ | 101.6 | exact match |
| 4.5" | 114.3 | NPS 4 | 114.3 | exact match |
Where pipe and tube match, and where they nearly do
The exact matches. 3" NPS pipe and 3.5" tube are both 88.9 mm OD. 3½" NPS and 4" tube are both 101.6 mm, and 4" NPS and 4.5" tube are both 114.3 mm. In this range the pipe OD is exactly half an inch more than its name, and tube comes in half-inch steps, so they line up. The OD matches; the wall usually doesn't. SCH10S is 3.05 mm here, thicker than most exhaust tube.
The near misses that catch people out:
- Same name, different size. 1.5" tube and 1½" NPS are 10.2 mm apart. At 2" the gap is 9.5 mm, and at 3" it's 12.7 mm.
- 1.75" tube and 1¼" NPS. Both are popular primary sizes, only 2.3 mm apart. They look alike on the bench but aren't interchangeable.
- 1⅞" tube and 1½" NPS. 1⅞" is a common US header size at 47.6 mm, only about 0.6 mm (0.025") under 1½" NPS. That's close enough to fool your eye and far enough to spoil a snug flange or slip joint.
- 1⅝" tube and 1¼" NPS. 41.3 mm against 42.2 mm. Same trap, 0.9 mm apart.
- NPS 2 and 2½. Each sits exactly ⅛" (3.2 mm) between two tube sizes, so there's no tube equivalent at all.
Why turbo manifolds use SCH10S and headers use tube
Turbo manifolds are often built in SCH10S. A turbo manifold runs very hot, goes through repeated heat cycles and usually carries the weight of the turbo; the thicker wall copes better with that. SCH10S butt-weld elbows are also widely stocked, made to ASME B16.9 in long radius (1.5D) and short radius (1D), so tight, consistent bends come off the shelf.
On weld elbows, D is the nominal size, not the OD. A 1½" long radius elbow has a 57.2 mm centreline radius (1.5 × 1.5"), not 1.5 × 48.3 mm. Our bend radius guide has more on this.
Naturally aspirated headers and exhaust systems are usually thin-wall mandrel tube. It's lighter, mandrel bends are easy to buy in the common OD sizes, and tube comes in finer size steps, which helps when choosing a primary diameter. Plenty of turbo manifolds are built in tube too. Pick one system and buy everything to match; how to mock up a turbo manifold covers the design stage.
How to tell what you have
Measure the OD, never the ID
- Measure across the outside with calipers, a little back from the cut end so burrs or a squashed end don't throw the reading.
- Take a second reading at 90° to the first. Pipe and tube are rarely perfectly round, so use the average.
- Compare with the charts above. Near 48.3 mm is 1½" NPS; near 38.1 mm is 1.5" tube.
- No calipers? Wrap a strip of paper round it, mark the overlap, measure the length and divide by 3.1416. 1½" NPS gives about 151.7 mm round; 1.5" tube about 119.7 mm.
Don't identify pipe by its bore. Every schedule of the same NPS shares one OD and has a different ID, so the bore tells you the schedule, not the size.
Check the wall
Measure the wall at a clean, square cut end. SCH10S reads 2.77 mm (NPS 1 to 2) or 3.05 mm (NPS 2½ to 4); exhaust tube is usually 1.2 to 2 mm. When the OD alone is borderline, as with 1⅞" tube and 1½" NPS, the wall settles it.
It also matters when you weld:
- Thicker wall takes more heat, but it's more forgiving of heat input and less likely to blow through.
- Joining thick pipe to thin tube, keep the arc on the thicker side and let the puddle wash onto the thin one.
- Where matching ODs meet (3" NPS to 3.5" tube, for example), expect a step of roughly 1 to 2 mm all the way round the bore.
- Back purge stainless with argon for full-penetration joints, whichever you use. Use fume extraction when welding stainless.
Why it matters when you buy
Everything that fits over or onto the outside is made for one OD:
- Butt-weld elbows, reducers and tees
- Weld-on flanges (head, turbo, wastegate, V-band), where the bore must suit the OD
- Slip joints and clamps
- Slip-on marking and cutting sleeves
- Mockup blocks
Because the OD is fixed for each NPS, anything that fits over the outside of 1½" SCH10S should also fit 1½" SCH40S, and that includes our SCH10S sleeves. Butt-weld fittings are the exception: order them in the same schedule so the walls line up.
Write every size as OD × wall in mm on cut lists and orders ("48.3 × 2.77", "38.1 × 1.5") and nothing can be misread.
Tools that help
Our pie cut and Y piece sleeves come in two separate versions, one for OD tube and one for SCH10S pipe. Other sizes are available on request.
- Pie cutting sleeves give 7.5° per cut, so 15° per joint, for building tight bends from straight material. The OD tube version comes in 1.5" to 4.5" tube, and the SCH10S version in 1" to 3½" NPS. The method is in how to pie cut exhaust tube.
- Y piece sleeves mark 60° Y merges. Again there's an OD tube version (1.5" to 4.5") and a SCH10S version (1" to 3½" NPS).
Our sleeves fit by outside diameter, so where the ODs match exactly, such as 3" NPS and 3.5" tube, the same sleeve size suits both. If you're unsure, ask us before you order.
- Mockup kits come in 1¼" NPS SCH10S (42.2 mm), 1½" NPS SCH10S (48.3 mm) and 1.75" OD tube (44.45 mm), each in Starter (73pc), Builder (136pc) and Workshop (219pc) sizes. Blocks from different sizes don't connect, so buy the kit that matches the pipe or tube you'll build in. On the NPS kits, D follows the nominal size, as it does on weld elbows. The 1½" NPS blocks are 38.1 mm (1D) and 57.2 mm (1.5D) CLR, the same as standard short and long radius elbows. On the tube kit, D is the actual OD.
FAQ
What is the OD of 1.5 inch schedule 10 pipe?
48.3 mm (1.900"). The wall is 2.77 mm (0.109") and the ID is about 42.8 mm. SCH10 and SCH10S are the same at this size.
Is 3 inch pipe the same as 3 inch tube?
No. 3" NPS pipe is 88.9 mm OD and 3" tube is 76.2 mm. 3" NPS has exactly the same OD as 3.5" tube.
What does the S in SCH10S mean?
It marks the stainless steel schedule from ASME B36.19M. From NPS 1 to 4 the wall is the same as SCH10.
Can I use tube fittings on schedule pipe?
Only where the OD matches exactly: 3" NPS with 3.5" tube, 3½" NPS with 4" tube, and 4" NPS with 4.5" tube. For butt-welded fittings, check the wall matches too.