NPS Inch to DN (Nominal Diameter) Table and Conversions on Valves

05 / 28, 2025

There are many types of industrial valve interfaces, commonly used forms of flange connection, clamping connection, butt welding connection, internal threaded connection, external threaded connection, ferrule connection, etc., used to indicate the size of the caliber of the unit is generally NPS inch ” or metric DN units are millimeters.

But in the industrial valve and pipeline often encountered in the NPS, DN of these codes, specifically what is the relationship and how to correspond to the well-known valve manufacturer ZZJG comprehensive ASME, ISO and industry standards to organize the most comprehensive NPS inches and DN (nominal diameter) corresponds to the table, covering the common and some special sizes from 1/8 inch to 48 inches:

NPS（inch）	DN（mm）	Correspondence Description
1/8	6	Smallest size, commonly used in instrument pipelines
1/4	8	Suitable for small fluid control
3/8	10	Common in compressed air pipelines
1/2	15	Typical household pipelines (such as DN15=4 branches)
3/4	20	6 branches, suitable for medium flow requirements
1	25	1-inch pipe, widely used in industry and civil
1-1/4	32	1.2-inch pipe, used in slightly larger flow scenarios
1-1/2	40	1.5-inch pipe, commonly used in fire protection or irrigation systems
2	50	2-inch pipe, suitable for water supply and drainage main pipelines
2-1/2	65	2.5-inch pipe, corresponding to DN65 (outer diameter 73mm)
3	80	3-inch pipe, commonly used in ventilation or chemical pipelines
4	100	4-inch pipe, mainstream specification for industrial pipelines
5	125	5-inch pipe, suitable for larger diameter fluid transportation
6	150	6-inch pipe, commonly used in municipal water supply or heating systems
8	200	8-inch pipe, core specification for large pipeline systems
10	250	10-inch pipe, suitable for high pressure or large flow scenarios
12	300	12-inch pipe, key size for industrial and municipal engineering
14	350	14-inch pipe, corresponding to DN350 (outer diameter 355.6mm)
16	400	16-inch pipe, commonly used in oil and gas pipelines
18	450	18 inch pipe, customized specifications for special projects
20	500	20 inch pipe, dedicated to large infrastructure
24	600	24 inch pipe, extreme specifications for ultra-large pipeline systems
26	650	26 inch pipe, special industrial applications
28	700	28 inch pipe, corresponding to DN700 (OD 711.2mm)
30	750	30 inch pipe, rare oversized pipe
32	800	32 inch pipe, dedicated to ultra-large projects
34	850	34 inch pipe, special customized requirements
36	900	36 inch pipe, corresponding to DN900 (OD 914.4mm)
40	1000	40 inch pipe, extreme engineering scenarios
42	1050	42 inch pipe, special applications with ultra-wide diameters
48	1200	48 inch pipe, one of the largest standard specifications in the industry

According to unit conversion, 1 inch is equal to 25.4 mm, but in actual working conditions, it is necessary to base it on the above table.

This table can be used as a basic reference for pipeline design, equipment selection, practical application is recommended to combine with specific product manuals or industry standards for further verification, for large specifications (such as DN1200 or more) need to refer to specific engineering design specifications.

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