Two Nominal Systems, One Pipe
NPS (Nominal Pipe Size, inches) is used in ASME-based markets; DN (Diamètre Nominal, mm) is the metric ISO/EN designation. Both are nominal — neither equals the actual OD exactly, except from NPS 14 upward, where NPS happens to equal the OD in inches.
| NPS (in) | DN (mm) | Actual OD (mm) |
|---|---|---|
| 1/2 | 15 | 21.3 |
| 3/4 | 20 | 26.7 |
| 1 | 25 | 33.4 |
| 1-1/4 | 32 | 42.4 |
| 1-1/2 | 40 | 48.3 |
| 2 | 50 | 60.3 |
| 2-1/2 | 65 | 73.0 |
| 3 | 80 | 88.9 |
| 4 | 100 | 114.3 |
| 5 | 125 | 141.3 |
| 6 | 150 | 168.3 |
| 8 | 200 | 219.1 |
| 10 | 250 | 273.0 |
| 12 | 300 | 323.9 |
| 14 | 350 | 355.6 |
| 16 | 400 | 406.4 |
| 18 | 450 | 457.0 |
| 20 | 500 | 508.0 |
| 24 | 600 | 610.0 |
Conversion Rules You Can Trust
For NPS 14 and larger, DN ≈ NPS × 25 (14 in → DN350, 24 in → DN600). Below NPS 14, memorize the table — NPS 12 is 323.9 mm OD (DN300), not 12 × 25.4 = 304.8. Wall thickness is never carried by NPS/DN: it is defined by schedule (Sch 10/40/80, STD, XS) under ASME B36.10M or by wall class under EN standards.
On drawings and POs, pair the nominal size with the execution standard: “6 in NPS, Sch 40, ASTM A106 B” or “DN150, SDR, EN 10216”. A nominal size alone is not a purchase specification.
Schedule Numbers: Where Wall Thickness Lives
NPS and DN carry only the nominal size. Wall thickness is selected by schedule (Sch 10, 40, 80…) under ASME B36.10M, or by wall class in EN practice. For a given NPS the OD is fixed and the ID shrinks as the schedule rises: NPS 2 exists as Sch 10 (2.77 mm), Sch 40 (3.91 mm) and Sch 80 (5.54 mm), all with a 60.3 mm OD.
| Designation | Meaning | Watch Out |
|---|---|---|
| STD / XS / XXS | Standard, Extra Strong, Double Extra Strong | Legacy wall classes; above NPS 10 STD wall is thinner than Sch 40 |
| Sch 10 / 40 / 80 | Modern schedule series | Same OD across schedules — fittings must match the wall, not the OD |
| SDR | Standard Dimension Ratio (OD ÷ WT) | Used for plastic and some EN steel specs — do not mix with schedule |
Three Mistakes That Cause Claims
1) Ordering “DN200 pipe” without wall — DN200 could be 6.35 mm or 18 mm of wall; the quote you get back is a guess. 2) Assuming NPS equals OD in inches below NPS 14 — NPS 12 OD is 12.75 in (323.9 mm). 3) Mixing schedule systems across standards — an EN wall class never maps 1:1 onto an ASME schedule; check the wall number itself.
Frequently Asked Questions
Is NPS 14 OD really 14 inches?
Yes — from NPS 14 upward the nominal number equals the OD in inches (NPS 14 = 355.6 mm). Below NPS 14 it does not, for historical reasons.
Why is it called “nominal”?
Because early pipe was sized by approximate bore. Wall standards evolved separately, so the name stuck while the actual dimensions moved — the number is a label, not a measurement.
How do I write size on a purchase order?
Pair nominal size with wall and standard: “6 in NPS × Sch 40, ASTM A106 Gr B, SMLS, BE” or “DN150 × wall 7.11 mm, EN 10216-1 P265GH”. That sentence is unambiguous anywhere in the world.
If your supplier answers a size inquiry without asking for wall thickness or schedule, you are talking to an order-taker, not a pipe specialist.
Need a quotation for steel pipe? CREATEEL supplies carbon steel, alloy and stainless pipes to API, ASTM and EN standards with EN 10204 3.1/3.2 certification. Contact us for a fast offer.
Standards & references: ASME (B36.10M) · ISO 6708
