mm to Inch Pipe Size Conversion

Pipe size conversion between millimeters and inches is a routine task on international piping projects. The conversion applies to outside diameter (OD), wall thickness, and internal diameter. Since pipe dimensions are standardized per ASME B36.10, the conversion is a direct lookup rather than a simple unit multiplication.

Conversion Table: OD and Common Wall Thicknesses

NPS	DN	OD (mm)	OD (in)	SCH 40 Wall (mm)	SCH 40 Wall (in)	SCH 80 Wall (mm)	SCH 80 Wall (in)
1/2	15	21.3	0.840	2.77	0.109	3.73	0.147
3/4	20	26.7	1.050	2.87	0.113	3.91	0.154
1	25	33.4	1.315	3.38	0.133	4.55	0.179
1-1/4	32	42.2	1.660	3.56	0.140	4.85	0.191
1-1/2	40	48.3	1.900	3.68	0.145	5.08	0.200
2	50	60.3	2.375	3.91	0.154	5.54	0.218
2-1/2	65	73.0	2.875	5.16	0.203	7.01	0.276
3	80	88.9	3.500	5.49	0.216	7.62	0.300
4	100	114.3	4.500	6.02	0.237	8.56	0.337
5	125	141.3	5.563	6.55	0.258	9.53	0.375
6	150	168.3	6.625	7.11	0.280	10.97	0.432
8	200	219.1	8.625	8.18	0.322	12.70	0.500
10	250	273.1	10.750	9.27	0.365	15.09	0.594
12	300	323.9	12.750	10.31	0.406	17.48	0.688
14	350	355.6	14.000	11.13	0.438	19.05	0.750
16	400	406.4	16.000	12.70	0.500	21.44	0.844
18	450	457.2	18.000	14.27	0.562	23.83	0.938
20	500	508.0	20.000	15.09	0.594	26.19	1.031
24	600	610.0	24.000	17.48	0.688	30.96	1.219
30	750	762.0	30.000	19.05	0.750	-	-
36	900	914.4	36.000	19.05	0.750	-	-
42	1050	1066.8	42.000	19.05	0.750	-	-
48	1200	1219.2	48.000	19.05	0.750	-	-

Values per ASME B36.10. For NPS 30 and above, SCH 40 and SCH 80 are not always defined; STD (standard) wall is listed.

Unit Conversion Factors

Conversion	Factor
mm to inches	Divide by 25.4
Inches to mm	Multiply by 25.4
kg/m to lb/ft	Multiply by 0.672
lb/ft to kg/m	Multiply by 1.488
bar to psi	Multiply by 14.504
psi to bar	Multiply by 0.06895
MPa to psi	Multiply by 145.04

Key Points

Pipe OD is not a simple conversion: NPS 4 pipe has an OD of 4.500 inches (114.3 mm), not 4.000 inches (101.6 mm). The OD values are standardized and must be looked up from ASME B36.10, not calculated by simple unit multiplication.

From NPS 14 upward: The NPS value equals the OD in inches. NPS 14 = 14.000 inches = 355.6 mm. This is the only range where NPS directly corresponds to a physical dimension.

European pipe sizes: EN 10220 defines additional metric OD sizes (e.g., 76.1 mm, 219.1 mm) that may or may not align with ASME NPS values. Always verify which standard applies.

Common Mistake

Converting NPS to millimeters by simply multiplying by 25.4. This only works for NPS 14 and above where NPS equals OD in inches. For smaller sizes, NPS is a nominal label, not a measurement. NPS 2 is not 2 x 25.4 = 50.8 mm; the actual OD is 60.3 mm. Always use the ASME pipe size tables.

Read the full pipe sizes guide