DN to NPS Pipe Size Conversion

Complete conversion table for DN (nominal diameter, metric) to NPS (nominal pipe size, imperial) pipe and valve sizes. Includes outside diameter (OD), nominal bore (NB) designation used in India, and typical schedule availability.

DN	NPS	OD (mm)	OD (in)	India (NB)	Schedules
DN 15	½"	21.3	0.840	15 NB	Sch 40/80/160
DN 20	¾"	26.7	1.050	20 NB	Sch 40/80/160
DN 25	1"	33.4	1.315	25 NB	Sch 40/80/160
DN 32	1¼"	42.2	1.660	32 NB	Sch 40/80/160
DN 40	1½"	48.3	1.900	40 NB	Sch 40/80/160
DN 50	2"	60.3	2.375	50 NB	Sch 40/80/160
DN 65	2½"	73.0	2.875	65 NB	Sch 40/80/160
DN 80	3"	88.9	3.500	80 NB	Sch 40/80/160
DN 100	4"	114.3	4.500	100 NB	Sch 40/80/160
DN 125	5"	141.3	5.563	125 NB	Sch 40/80/120
DN 150	6"	168.3	6.625	150 NB	Sch 40/80/120
DN 200	8"	219.1	8.625	200 NB	Sch 20/40/80
DN 250	10"	273.0	10.750	250 NB	Sch 20/40/60
DN 300	12"	323.9	12.750	300 NB	Sch 20/40/60
DN 350	14"	355.6	14.000	350 NB	Sch 10/20/40
DN 400	16"	406.4	16.000	400 NB	Sch 10/20/40
DN 450	18"	457.0	18.000	450 NB	Sch 10/20/40
DN 500	20"	508.0	20.000	500 NB	Sch 10/20/40
DN 600	24"	609.6	24.000	600 NB	Sch 10/20/30

OD per ASME B36.10M. NPS = nominal pipe size (no units). DN = diamètre nominal (mm).

What is DN vs NPS?

NPS (Nominal Pipe Size) is the American/Imperial system for pipe and valve sizing, defined in ASME B36.10M. NPS is expressed in inches but is not the actual outer diameter — it is a nominal (approximate) designator. For example, NPS 4" pipe has an OD of 4.500" (114.3mm), not 4.000".

DN (Diamètre Nominal) is the ISO/European equivalent — a dimensionless number approximating the pipe OD in millimetres. DN50 ≈ 2" NPS, DN100 ≈ 4" NPS. The conversion is not a simple multiply/divide — DN50 maps to NPS 2" (OD 60.3mm, not 50mm). Use the table above for exact mapping. In India, the same sizes are often called "NB" (Nominal Bore), e.g. 100 NB = DN100 = NPS 4".

Key Rules for DN / NPS Sizing

The OD (outside diameter) of a given NPS/DN is fixed regardless of schedule — only the wall thickness (and therefore bore) changes with schedule.

Schedule numbers (Sch 40, 80, 160, XXS) define wall thickness, not pipe OD. Two pipes of the same NPS but different schedules have the same OD but different IDs.

Valve face-to-face and flange bolt circle dimensions are defined by ASME B16.5 (flanged) and B16.10 (face-to-face) — not directly by OD.

DN / NPS sizes above NPS 14" (DN350) have OD equal to nominal size in inches (NPS 16" = 16.000" OD). Below NPS 14", OD ≠ nominal.

For instrument and small-bore lines in India, sizes like 15 NB (½" NPS), 25 NB (1" NPS), and 50 NB (2" NPS) are the most common valve sizes.

FAQ — DN to NPS Conversion

What is DN100 in NPS?

DN100 = NPS 4". The outside diameter is 114.3mm (4.500").

What is NPS 6" in DN?

NPS 6" = DN150. OD is 168.3mm (6.625").

What is 150 NB in NPS?

150 NB (India designation) = NPS 6" = DN150. OD = 168.3mm.

Is DN the same as NPS?

No. DN and NPS are separate systems that map to the same set of pipe sizes but with different numbers. DN25 ≈ NPS 1". The OD is the same, but the designation differs.

What schedule is DN100 NPS 4" pipe?

Schedule applies to the wall thickness, not the DN/NPS designation. DN100/NPS 4" pipe is available in Schedule 10, 20, 40, 80, 120, 160, and XXS. The OD (114.3mm) is fixed; only the wall thickness changes.

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