Nominal Pipe Size (NPS) vs. DN: When to Use Each and How to Convert

Nominal Pipe Size (NPS) vs. DN: When to Use Each and How to ConvertPiping dimensions are a routine but critical part of engineering, construction, plumbing, and process industries. Two common systems used to identify pipe sizes are Nominal Pipe Size (NPS) and Diameter Nominal (DN). They look similar at a glance, but they come from different standards and serve different needs. This article explains what NPS and DN mean, when to use each, how they relate to actual pipe dimensions, and practical methods for converting between them.

What is Nominal Pipe Size (NPS)?

Nominal Pipe Size (NPS) is an American designation for pipe diameter used primarily in the United States and Canada. It is a North American standard historically managed by ANSI/ASME (for example, ASME B36.10M/B36.19M for steel pipe).

• Definition: NPS is a nominal — not exact — diameter used to categorize pipe. For NPS ⁄₈ through NPS 12, the NPS number does not always equal the pipe’s actual outside diameter (OD). For NPS 14 and larger, NPS equals the OD in inches.
• Notation: Commonly written as NPS followed by a number (e.g., NPS 2, NPS ⁄₂). The standard abbreviation NPS is sometimes used interchangeably with “Pipe Size” in U.S. pipe schedules and fittings.
• Associated details: NPS is combined with a pipe schedule (Schedule 40, Sch 80, etc.) to determine the wall thickness and therefore the internal (nominal) diameter (ID). For example, NPS 2, Sch 40 has a different ID than NPS 2, Sch 80.

Why NPS is “nominal”: Early pipe manufacturing used different wall thicknesses and practices, so the nominal system served as a practical label matching historical tube sizes. Over time the OD for small NPS sizes stayed fixed for compatibility, while wall thicknesses changed with schedules.

What is DN (Diameter Nominal)?

DN stands for Diamètre Nominal (French) or Diameter Nominal — a metric-based, international designation governed by ISO standards (for instance ISO 6708). DN is widely used outside North America, especially in Europe, Asia, and most international specifications.

• Definition: DN is a dimensionless number that approximates the pipe’s nominal internal or nominal size in millimeters, but it is not an exact measurement of either OD or ID. DN values are rounded and standardized to simple integer values (e.g., DN 15, DN 50, DN 100).
• Notation: Written as DN followed by a number (e.g., DN15, DN50). The number is typically the nominal size in millimeters, but with the same “nominal” caveat — it doesn’t always equal exact ID or OD.
• Associated details: DN is often used with PN (Pressure Nominal, e.g., PN16) in metric standards to indicate pressure rating rather than the wall thickness-based schedule system used with NPS.

DN’s advantage is unification across fittings and components in the metric world — DN100 means the same nominal category in most international standards, simplifying global procurement.

Key differences and practical implications

• Units and origin:
  • NPS uses inches and is North American-centric.
  • DN uses a dimensionless number tied to millimeters and is international/metric.
• Actual dimensions:
  • For many sizes NPS number ≠ OD in inches (except NPS 14+ where NPS = OD).
  • DN number ≈ nominal millimeter size but is not exact OD or ID.
• Wall-thickness systems:
  • NPS is paired with pipe schedules (Sch 10, 40, 80, etc.) that control wall thickness.
  • DN is typically paired with pressure ratings like PN (e.g., PN10, PN16) or with specific standard wall thicknesses in metric norms.
• Compatibility:
  • Do not assume NPS and DN are directly interchangeable even if the DN number equals the inch-to-mm conversion of the NPS. Fittings, flanges, and valves must be specified to match the standard (ASME vs. ISO/EN/BS) to ensure correct mating.

When to use NPS vs. DN

• Use NPS when:

  • Working in North American projects, codes, or equipment specified in ASME/ANSI standards.
  • Specifying pipe and fittings by schedule (Sch) and using inches for dimensions.
  • Ordering components from suppliers that list sizes as NPS.

• Use DN when:

  • Working on international or metric projects, or to comply with ISO/EN/BS standards.
  • Specifying pipe in millimeter-based systems and pairing with PN pressure classes.
  • Coordinating procurement across countries that use DN as the default.

In projects with global suppliers, it’s common to include both designations in specifications (for example, “DN50 / NPS 2”) and to state the standard (ASME B36.10M or ISO 4200) and flange standard (ASME B16.5, EN 1092-1) to avoid mismatches.

How to convert between NPS and DN

There is no exact universal conversion because NPS is not a single physical dimension and DN is nominal. However, approximate cross-reference tables are widely used for practical matching. Below are common equivalences used in procurement and piping engineering.

Common conversions (approximate):

• NPS ⁄₈ ≈ DN 6
• NPS ⁄₄ ≈ DN 8
• NPS ⁄₈ ≈ DN 10
• NPS ⁄₂ ≈ DN 15
• NPS ⁄₄ ≈ DN 20
• NPS 1 ≈ DN 25
• NPS 1-⁄₄ ≈ DN 32
• NPS 1-⁄₂ ≈ DN 40
• NPS 2 ≈ DN 50
• NPS 2-⁄₂ ≈ DN 65
• NPS 3 ≈ DN 80
• NPS 4 ≈ DN 100
• NPS 6 ≈ DN 150
• NPS 8 ≈ DN 200
• NPS 10 ≈ DN 250
• NPS 12 ≈ DN 300
• For NPS 14 and larger, NPS in inches equals OD in inches; convert OD to mm and pick the DN closest to that OD in mm.

If you need precise mating (flanges, threaded fittings, valves), always check the actual OD, ID, and flange specifications. Use manufacturer datasheets and relevant standards (ASME B36.10M/B36.19M for NPS steel pipe, ISO 6708 and EN standards for DN).

Example: converting NPS 2, Schedule 40 to DN

1. Look up NPS 2, Sch 40 dimensions: OD = 2.375 in (60.33 mm); ID depends on schedule (Sch 40 ID ≈ 2.067 in / 52.5 mm).
2. The nearest DN value is DN 50 (nominal 50 mm). For flange or valve matching, you would normally specify DN50 and ensure connecting flange or valve OD matches 60.33 mm per the chosen flange standard.

Tips for avoiding mismatches

• Always specify the standard alongside size: e.g., NPS 2, Sch 40, ASME B36.10M or DN50, PN16, EN 1092-1.
• For flanges, match flange standard (ASME B16.5 vs EN 1092-1) — these define bolt circle diameters, raised face dimensions, and bolt sizes.
• When retrofitting or replacing parts, measure OD and bolt patterns rather than relying on nominal labels alone.
• Keep a conversion table or chart handy during procurement; many piping handbooks include full cross-reference tables with exact ODs and IDs by schedule.

Quick reference conversion table (partial)

Summary

• NPS is the North American nominal sizing system (inches) commonly paired with pipe schedules.
• DN is the international/metric nominal designation (dimensionless number approximating mm).
• Use NPS for ASME/ANSI-based projects and DN for ISO/EN/metric-based projects.
• Conversions are approximate; for fittings and flanges always verify actual OD, ID, and standard specifications before purchasing or installing.

If you want, I can provide a full conversion table including OD and ID for common schedules (Sch 10, Sch 40, Sch 80) or generate printable charts for your project.