NPS vs DN is calculated by, 25 x NPS = DN. This is however not true for all cases, especially for pipe sizes below NPS 5. In the table below, NPS vs DN values are tabulated for quick reference.
Nominal Pipe Size (NPS) is based on inches and is listed throughout ASME piping standards. EX: ASME B36.10M “Welded and Seamless Wrought Steel Pipe”
Diametre Nominal (DN) is based on millimetres and is standardized in ISO 6708 “Pipework components – Definition and selection of DN (nominal size)”
A general rule of thumb for NPS vs DN is to divide DN by 25 for NPS when not using reference table. This formula breaks at lower sizes and sizes with fractions.
NPS = DN / 25
[note] This formula does not hold true for sizes below NPS 5 for all cases.
Pipe Size Callout vs Actual Size
All pipes have an inside diameter, and an outside diameter, of which the differences is wall thickness. As a result, there are often confusion of which diameter they are referring to. When people call out a size, they are typically referring to a “nominal” size. For example, an NPS 2 pipe does not have an outside diameter or inside diameter which matches 2 inch exactly. A more in-depth discussion on piping size and corresponding butt welding ends is on a separate page. Typically, the outside diameter of a pipe in a certain size stays the same throughout the thickness classes and the inside diameter changes. This means heavier class pipe have reduced opening.
For further reading on the detail of pipe and pipe thickness and sizes, this wikipedia article can be a good starting place.