Overview of NPS vs DN Nominal Sizes
Valve and pipe diameters are designated by NPS in inches and DN in millimetres, two nominal size systems that describe the same physical products; understanding the relationship avoids ordering errors.
NPS, nominal pipe size, is the North American designation in inches: NPS 1/2, 3/4, 1, 2 and so on. DN, diameter nominal, is the international designation in millimetres, where DN 15, DN 20, DN 25 and DN 50 are the metric names for the same sizes. For sizes from NPS 1/2 to NPS 12, the DN value is approximately the NPS value multiplied by 25, so NPS 2 corresponds to DN 50, and for NPS 14 and above, the DN value is close to the outside diameter in millimetres, so NPS 24 corresponds to DN 600.
Specification and Grades
The key point is that both systems are nominal: they name a size family, not an exact dimension. A valve and a pipe of the same NPS and DN connect properly because the flange or connection standard fixes the matching face dimensions, bore and pressure class. When valves and pipe are bought from different systems, the NPS, DN, pressure class and end connection standard must all be checked together, because the nominal size alone does not define the physical dimensions.
Material Properties
The mechanical properties that matter for this product are defined by the governing specification and grade: minimum yield strength, tensile strength, elongation and, where applicable, hardness and impact toughness. These values are verified by testing of each heat and reported on the mill test certificate, and the buyer should confirm that the guaranteed values meet the design requirements before ordering.
Typical Applications
The comparison applies to valves, flanges, fittings and pipe in water, oil, gas, steam and process systems, wherever North American and metric designs meet, such as plants built to mixed standards or replacement valves ordered for existing lines.
Manufacturing, Testing and Quality Assurance
Before ordering, confirm the NPS or DN, the pressure class or rating, the end connection standard (flange, threaded, welded) and the material standard with the supplier, and compare the connection dimensions against the existing line to guarantee a match.
Selection and Ordering Guidance
When ordering NPS vs DN Nominal Sizes, the governing standard, grade or material designation, outside diameter, wall thickness, length, end finish and any supplementary test or certification requirements must be stated on the purchase order. Ask the supplier to confirm availability, heat treatment condition and delivery documentation before production, and request the mill test certificate so that the chemical analysis, mechanical test results and traceability can be checked against the order.
FAQ
Q1. What does NPS mean?
Nominal pipe size, the North American designation of pipe and valve sizes in inches, such as NPS 1, NPS 2 and NPS 6.
Q2. What does DN mean?
Diameter nominal, the international metric designation such as DN 25, DN 50 and DN 150, used for pipe, valves and fittings worldwide.
Q3. How do NPS and DN compare?
From NPS 1/2 to NPS 12, DN is about 25 times the NPS number; above NPS 12, DN follows the outside diameter in millimetres, so NPS 24 is DN 600.
Q4. Can NPS and DN be used interchangeably?
They name the same size families, but the physical dimensions are fixed by the product standard and pressure class, so always confirm the connection standard when mixing systems.
Q5. Why is the pressure class important?
Two valves of the same NPS or DN can have different flange dimensions and ratings at different pressure classes, so the size and class must be specified together.
