Nominal Pipe Size Chart | NPS to Inches, MM & DN Conversion

This conversion chart compares Nominal Pipe Size (NPS) values with their fractional, decimal, metric (mm), and DN (diamètre nominal) equivalents. Use it as a quick reference across imperial and metric piping standards.

Nominal Pipe Size Chart: NPS to Inches, MM & DN

For NPS 12 and smaller, nominal size does not equal actual outside diameter. Example: NPS 2 has an outside diameter of 2.3750 in (60.3249 mm), listed as DN 50.

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How to read it: find your Nominal (NPS) value in the first column, then read across for the fractional, decimal, metric, and DN equivalents. DN and metric values are rounded reference labels, not precision measurements. For fabrication tolerances, use ASME or ISO dimension tables for the specific schedule.

Fraction to Decimal to Metric Conversion Reference

Common inch fraction, decimal inch, and millimeter conversions from the original printable chart.

Show full table (1/16″ to 48″)

Inches Fraction	Inches Decimal	Metric (mm)
1/16 in	0.0625	1.5875
1/8 in	0.1250	3.1750
3/16 in	0.1875	4.7625
1/4 in	0.2500	6.3500
5/16 in	0.3125	7.9375
3/8 in	0.3750	9.5250
7/16 in	0.4375	11.1125
1/2 in	0.5000	12.7000
9/16 in	0.5625	14.2875
5/8 in	0.6250	15.8750
11/16 in	0.6875	17.4625
3/4 in	0.7500	19.0500
13/16 in	0.8125	20.6375
7/8 in	0.8750	22.2250
15/16 in	0.9375	23.8125
1 in	1.0000	25.4000
1-1/4 in	1.2500	31.7500
1-1/2 in	1.5000	38.1000
1-3/4 in	1.7500	44.4500
2 in	2.0000	50.8000
2-1/4 in	2.2500	57.1500
2-1/2 in	2.5000	63.5000
2-3/4 in	2.7500	69.8500
3 in	3.0000	76.2000
3-1/2 in	3.5000	88.9000
4 in	4.0000	101.6000
4-1/2 in	4.5000	114.3000
5 in	5.0000	127.0000
6 in	6.0000	152.4000
8 in	8.0000	203.2000
10 in	10.0000	254.0000
14 in	14.0000	355.6000
16 in	16.0000	406.4000
18 in	18.0000	457.2000
20 in	20.0000	508.0000
24 in	24.0000	609.6000
30 in	30.0000	762.0000
36 in	36.0000	914.4000
48 in	48.0000	1219.2000

Millimeter to Decimal Inch Conversion Reference

Millimeter-to-inch conversions from the original printable chart.

Show full table (1mm to 250mm)

Metric Size	Decimal Inches	Metric (mm)
1 mm	0.0394	1.0000 mm
2 mm	0.0787	2.0000 mm
3 mm	0.1181	3.0000 mm
4 mm	0.1575	4.0000 mm
5 mm	0.1969	5.0000 mm
6 mm	0.2362	6.0000 mm
7 mm	0.2756	7.0000 mm
8 mm	0.3150	8.0000 mm
9 mm	0.3543	9.0000 mm
10 mm	0.3937	10.0000 mm
11 mm	0.4331	11.0000 mm
12 mm	0.4724	12.0000 mm
13 mm	0.5118	13.0000 mm
14 mm	0.5512	14.0000 mm
15 mm	0.5906	15.0000 mm
16 mm	0.6299	16.0000 mm
17 mm	0.6693	17.0000 mm
18 mm	0.7087	18.0000 mm
19 mm	0.7480	19.0000 mm
20 mm	0.7874	20.0000 mm
21 mm	0.8268	21.0000 mm
22 mm	0.8661	22.0000 mm
23 mm	0.9055	23.0000 mm
24 mm	0.9449	24.0000 mm
25 mm	0.9843	25.0000 mm
26 mm	1.0236	26.0000 mm
27 mm	1.0630	27.0000 mm
28 mm	1.1024	28.0000 mm
29 mm	1.1417	29.0000 mm
30 mm	1.1811	30.0000 mm
31 mm	1.2205	31.0000 mm
32 mm	1.2598	32.0000 mm
33 mm	1.2992	33.0000 mm
34 mm	1.3386	34.0000 mm
35 mm	1.3780	35.0000 mm
36 mm	1.4173	36.0000 mm
37 mm	1.4567	37.0000 mm
38 mm	1.4961	38.0000 mm
39 mm	1.5354	39.0000 mm
40 mm	1.5748	40.0000 mm
41 mm	1.6142	41.0000 mm
42 mm	1.6535	42.0000 mm
43 mm	1.6929	43.0000 mm
44 mm	1.7323	44.0000 mm
45 mm	1.7717	45.0000 mm
46 mm	1.8110	46.0000 mm
47 mm	1.8504	47.0000 mm
48 mm	1.8898	48.0000 mm
49 mm	1.9291	49.0000 mm
50 mm	1.9685	50.0000 mm
51 mm	2.0079	51.0000 mm
52 mm	2.0472	52.0000 mm
53 mm	2.0866	53.0000 mm
54 mm	2.1260	54.0000 mm
55 mm	2.1654	55.0000 mm
56 mm	2.2047	56.0000 mm
57 mm	2.2441	57.0000 mm
58 mm	2.2835	58.0000 mm
59 mm	2.3228	59.0000 mm
60 mm	2.3622	60.0000 mm
61 mm	2.4016	61.0000 mm
62 mm	2.4409	62.0000 mm
63 mm	2.4803	63.0000 mm
64 mm	2.5197	64.0000 mm
65 mm	2.5591	65.0000 mm
66 mm	2.5984	66.0000 mm
67 mm	2.6378	67.0000 mm
68 mm	2.6772	68.0000 mm
69 mm	2.7165	69.0000 mm
70 mm	2.7559	70.0000 mm
71 mm	2.7953	71.0000 mm
72 mm	2.8346	72.0000 mm
73 mm	2.8740	73.0000 mm
74 mm	2.9134	74.0000 mm
75 mm	2.9528	75.0000 mm
76 mm	2.9921	76.0000 mm
77 mm	3.0315	77.0000 mm
78 mm	3.0709	78.0000 mm
79 mm	3.1102	79.0000 mm
80 mm	3.1496	80.0000 mm
81 mm	3.1890	81.0000 mm
82 mm	3.2283	82.0000 mm
83 mm	3.2677	83.0000 mm
84 mm	3.3071	84.0000 mm
85 mm	3.3465	85.0000 mm
86 mm	3.3858	86.0000 mm
87 mm	3.4252	87.0000 mm
88 mm	3.4646	88.0000 mm
89 mm	3.5039	89.0000 mm
90 mm	3.5433	90.0000 mm
91 mm	3.5827	91.0000 mm
92 mm	3.6220	92.0000 mm
93 mm	3.6614	93.0000 mm
94 mm	3.7008	94.0000 mm
95 mm	3.7402	95.0000 mm
96 mm	3.7795	96.0000 mm
97 mm	3.8189	97.0000 mm
98 mm	3.8583	98.0000 mm
99 mm	3.8976	99.0000 mm
100 mm	3.9370	100.0000 mm
110 mm	4.3307	110.0000 mm
120 mm	4.7244	120.0000 mm
130 mm	5.1181	130.0000 mm
140 mm	5.5118	140.0000 mm
150 mm	5.9055	150.0000 mm
160 mm	6.2992	160.0000 mm
170 mm	6.6929	170.0000 mm
180 mm	7.0866	180.0000 mm
190 mm	7.4803	190.0000 mm
200 mm	7.8740	200.0000 mm
250 mm	9.8425	250.0000 mm

About Nominal Pipe Size (NPS)

Nominal Pipe Size (NPS) is a standardized North American label for pipe diameter. It's a named designation, not a literal measurement. An NPS 2-inch pipe doesn't measure exactly 2.000 inches: its outside diameter (OD) is fixed at approximately 2.375 in (60.3 mm) for every schedule, while the inside diameter (ID) shrinks as wall thickness (schedule) increases. This convention traces back to early water pipe systems, where the nominal number approximated internal diameter before standardization.

Because OD stays fixed per NPS regardless of schedule, pipes of the same nominal size always fit the same fittings and flanges. That's exactly why accurate NPS-to-DN-to-metric conversion matters for procurement (avoiding ordering errors across suppliers who list dimensions differently), international projects (matching imperial specs to metric ones), and on-site quality control (verifying installations against metric measurements).

Common Mistakes & FAQs in Pipe Size Conversion

"Nominal 2 inch" vs. actual OD/ID

An NPS 2 pipe does not have a 2-inch OD. Its actual OD is 2.375 in (60.3 mm). Always check OD and wall thickness for precise fitting and performance calculations.

Metric vs. DN vs. NPS confusion

• Metric (mm): actual dimensional measurement.
• DN (Diamètre Nominal): rounded metric designation used as a reference label, not a measured value.
• NPS (Nominal Pipe Size): the imperial labeling system used primarily in North America.

What schedules do (and why they don't change OD)

"Schedule" refers to wall thickness. Increasing schedule (e.g., Sch 40 to Sch 80) increases wall thickness and reduces inside diameter, but the outside diameter stays the same for a given NPS, keeping fittings and flanges compatible across pressure ratings.