Pipe Size Chart: DN, NPS and Outside Diameter (ASME B36.10M & EN 10220)

Pipe size cross-reference: nominal bore DN, nominal pipe size NPS and pipe outside diameter under both ASME B36.10M and EN 10220 / ISO 4200, from DN15 (NPS 1/2) to DN600 (NPS 24). The starting point for selecting or matching the four flange references (EN 1092-1 PN16 / PN100 and ASME B16.5 Class 150 / Class 300). Lists where the two OD series diverge — most visibly DN65 at 73.0 mm (ASME) vs 76.1 mm (EN) — plus DN20, DN25, DN32, DN125 and DN250.

Pipe size chart: DN, NPS and outside diameter

DN (nominal diameter) and NPS (nominal pipe size) are size labels, not measurements — they name the bore class, not the actual outside diameter. For a given pipe size the outside diameter is fixed by the standard so that all wall thicknesses (schedules) share one OD and one set of fittings, with the wall growing inward as the schedule rises. DN and NPS run in step (DN15 is NPS 1/2, DN100 is NPS 4), but the outside diameter they fix differs between the inch-based ASME series and the metric EN series at several sizes — most visibly at DN65, where ASME B36.10M sets 73.0 mm and EN 10220 sets 76.1 mm. The two are not interchangeable at those sizes. The table lists both so a DN/NPS label can be turned into the right outside diameter for the standard in use.

Where the two OD columns read the same the size is harmonised across both standards; where they differ (DN20, DN25, DN32, DN65, DN125, DN250) the pipe, fittings and flange drilling must be sized to one standard throughout. The inch OD column is exact for the ASME series; the millimetre value is 1 in = 25.4 mm of that figure.

Reading the chart

• DN — the metric size label (nominal diameter). A round number near the bore in millimetres, not the measured bore or outside diameter.
• NPS — the inch-based size label used by ASME. NPS 1/2 to NPS 12 are nominal; from NPS 14 up the NPS number equals the outside diameter in inches exactly.
• Outside diameter — the measured outside diameter, fixed by the standard for each size and shared by every schedule. The figure to measure a pipe against to identify its size, and the figure a flange bore and fitting socket are cut to.
• OD (in) — the outside diameter in inches for the ASME B36.10M series; multiply by 25.4 to convert to millimetres.

Using it to size and match

To identify an unmarked pipe, measure the outside diameter and read across to the DN/NPS label for the standard in use; the wall thickness then gives the schedule. To match a flange, gasket or fitting, confirm the DN/NPS and the OD standard agree on both parts — a DN65 EN pipe at 76.1 mm will not seat in an ASME NPS 2-1/2 socket cut for 73.0 mm. DN and NPS describe size only; the pressure class (PN or Class) and the flange standard (EN 1092-1 or ASME B16.5) are separate selections that fix the bolt pattern. The outside diameters above are nominal pipe dimensions; the wall thickness for a given schedule is read from ASME B36.10M (carbon and alloy steel) or ASME B36.19M / EN 10217 (stainless and welded pipe).

Dimensions are nominal ASME B36.10M and EN 10220 / ISO 4200 values and provided for identification and cross-reference. Confirm against the controlling standard and the specific pipe or flange data sheet before manufacture or procurement.

Reference compiled for identification and cross-reference. Dimensions are nominal; confirm against the controlling ASME / EN ASME B36.10M / EN 10220 revision and the specific component data sheets before manufacture or procurement. All trademarks are the property of their respective owners.