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Pipe Diameter Converter (NPS/DN/Inch/mm)

Professional lookup tool for Nominal Pipe Sizes (NPS), DN (Metric), and Outer Diameters. Includes Schedule 40/80 wall thickness logic.

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Category Engineering Converters
Metric DN: -
Outer Diameter (OD): - mm / - in

Wall Thickness Lookup

Schedule 40 (Std)- mm wall
Schedule 80 (XH)- mm wall
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About

In industrial plumbing and HVAC engineering, pipe sizing is notoriously confusing. A "2-inch" pipe rarely measures exactly 2 inches. This is because pipes are categorized by "Nominal Pipe Size" (NPS) in North America or "Diamètre Nominal" (DN) in metric regions. These are standard identifiers, not literal measurements of the outer diameter (OD) or inner diameter (ID).

This tool is indispensable for site managers and procurement officers. It provides an immediate translation between NPS, DN, and the precise physical Outer Diameter in millimeters, compliant with ASME B36.10M and ISO 6708. Furthermore, it accounts for "Schedules" (wall thickness), allowing users to distinguish between standard (Sch 40) and high-pressure (Sch 80) variants, which dramatically affects the internal flow area.

pipe size NPS DN plumbing hvac schedule 40

Formulas

There is no single mathematical formula to convert NPS to OD directly; it is a standardized lookup defined by organizations like ASTM and ISO. However, the Inner Diameter (ID) is calculated mathematically based on the wall thickness defined by the Schedule.

ID = OD − (2 × twall)

Standard mapping (Approximate for NPS ≥ 14):

DN ≈ NPS × 25

Reference Data

NPS (Inch)DN (mm)Actual OD (mm)Actual OD (in)
1/8610.30.405
1/21521.30.840
3/42026.71.050
12533.41.315
1 1/24048.31.900
25060.32.375
38088.93.500
4100114.34.500
6150168.36.625

Frequently Asked Questions

Historically, pipe sizes were defined by the inner diameter. As manufacturing improved and walls became thinner, the outer diameter was kept constant to ensure fittings would still fit, while the inner diameter increased. Thus, "1 inch" became a name, not a measure.
Schedule refers to wall thickness. Sch 80 is thicker than Sch 40, allowing for higher pressure but reducing the internal cross-sectional area (flow capacity). The Outer Diameter (OD) remains identical for both.
No. DN (Diamètre Nominal) is a dimensionless designator. For example, DN 25 corresponds to 1 inch, but the actual OD is 33.4mm, not 25mm. It is a loose reference to the internal diameter.