DN to NPS Pipe Size Conversion

DN (Diametre Nominal) is the metric designation for pipe sizes used in international standards such as ISO 6708, while NPS (Nominal Pipe Size) is the imperial designation used primarily in North American standards like ASME B36.10M and ASME B36.19M. Neither DN nor NPS directly refers to a physical dimension of the pipe; rather, they are dimensionless designators that correspond to a standardized outside diameter (OD).

DN to NPS Conversion Chart

DN	NPS (inch)	Actual OD (mm)	Actual OD (inch)
DN 6	1/8	10.3	0.405
DN 8	1/4	13.7	0.540
DN 10	3/8	17.1	0.675
DN 15	1/2	21.3	0.840
DN 20	3/4	26.7	1.050
DN 25	1	33.4	1.315
DN 32	1-1/4	42.2	1.660
DN 40	1-1/2	48.3	1.900
DN 50	2	60.3	2.375
DN 65	2-1/2	73.0	2.875
DN 80	3	88.9	3.500
DN 90	3-1/2	101.6	4.000
DN 100	4	114.3	4.500
DN 125	5	141.3	5.563
DN 150	6	168.3	6.625
DN 200	8	219.1	8.625
DN 250	10	273.1	10.750
DN 300	12	323.9	12.750
DN 350	14	355.6	14.000
DN 400	16	406.4	16.000
DN 450	18	457.2	18.000
DN 500	20	508.0	20.000
DN 550	22	559.0	22.000
DN 600	24	610.0	24.000

Notes

• DN is a metric designation defined by ISO 6708. It is a rounded number used for reference purposes and does not correspond exactly to the manufactured outside diameter or bore of the pipe.
• NPS is the North American designation per ASME standards. For NPS 14 and above, the NPS value in inches equals the actual outside diameter. For sizes below NPS 14, the NPS value is a nominal designator only and does not match the OD or ID.
• The actual outside diameter (OD) is the same for both DN and NPS systems; only the naming convention differs. A DN 100 pipe and an NPS 4 pipe have the same OD of 114.3 mm (4.500 inches).
• Wall thickness, and therefore internal diameter, varies according to the pipe schedule or pressure class selected for the given size.
• European standards (EN 10220, EN 10216, EN 10217) use DN, while American standards (ASME B36.10M, ASME B36.19M, ASTM A106, API 5L) use NPS.