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

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This conversion chart helps you easily compare Nominal Pipe Size (NPS) values with their fractional, decimal, metric (mm), and DN (diamètre nominal) equivalents. Use it as a quick reference to find accurate pipe size correlations across international standards.

What Is Nominal Pipe Size (NPS)? 

Nominal Pipe Size (NPS) is a standardized term used to describe the size of pipes in the imperial (inch-based) system. It’s important to understand that nominal size refers to a named designation, not the actual physical dimension of the pipe.

For example:

  • An NPS 2-inch pipe doesn’t measure exactly 2.000 inches in diameter. Instead, it has a fixed outside diameter (OD) of approximately 2.375 inches (60.3 mm), while the inside diameter (ID) varies depending on the pipe’s schedule (wall thickness).

This sizing convention originates from historical water pipe systems, where the nominal number represented the approximate internal diameter before modern standardization.

How the Conversion Chart Works

The conversion chart bridges the gap between imperial and metric pipe sizing systems. It lists the nominal size alongside its fractional, decimal (inch), millimeter, and DN equivalents for easy cross-reference.

Understanding Each Column: Nominal, Fraction, Decimal, Metric, DN

Column

Description

Nominal (NPS)

The standard U.S. designation for pipe size. It is a label, not a literal measurement.

Fraction (inches)

The traditional fractional representation of the nominal size (e.g., 1 ½″, 2 ¼″).

Decimal (inches)

The same size expressed as a decimal value for engineering or CAD use.

Metric (mm)

The approximate size in millimeters, based on conversion (1 in = 25.4 mm).

DN (Diamètre Nominal)

The international metric equivalent, used in ISO and EN standards. DN values are rounded and do not always match exact millimeter dimensions.

How to Read the Table: Tips & Examples

To find the correct size:

  1. Locate the Nominal (NPS) value in the first column.
  2. Read across to find its equivalent in fractional, decimal, metric, and DN units.
  3. Keep in mind that DN values and metric equivalents are nominal. They’re designed for reference, not precision measurement.
  4. If exact dimensions are needed for fabrication or fitting, refer to ASME or ISO pipe dimension tables for the specific schedule.

Example:

  • NPS 2 corresponds to an OD of 2.375 inches (60.3 mm) and is typically identified as DN 50 in metric systems.
  • This means that a “2-inch” pipe and a “DN 50” pipe are equivalent designations, though not exact dimensional matches.

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

Use the chart below to compare Nominal Pipe Size (NPS) with pipe outside diameter in inches, millimeters, and common DN equivalents. For NPS 12 and smaller, the nominal pipe size does not equal the actual outside diameter. For example, NPS 2 has an outside diameter of 2.3750 inches, or 60.3249 mm.

Nominal Pipe Size (NPS) Listed Inch Fraction / OD Decimal OD (in) Metric OD (mm) Common DN Equivalent
1/8 in - 0.405 10.287 DN 6
1/4 in - 0.540 13.7160 DN 8
3/8 in - 0.675 17.145 DN 10
1/2 in - 0.840 21.336 DN 15
3/4 in - 1.050 26.6700 DN 20
1 in - 1.315 33.4009 DN 25
1-1/4 in - 1.660 42.1639 DN 32
1-1/2 in - 1.900 48.2600 DN 40
2 in - 2.3750 60.3249 DN 50
2-1/2 in - 2.875 73.0249 DN 65
3 in 3-1/2 in 3.5000 88.9000 DN 80
3-1/2 in 4 in 4.0000 101.6000 DN 90
4 in 4-1/2 in 4.5000 114.3000 DN 100
5 in - 5.563 141.3002 DN 125
6 in - 6.625 168.2750 DN 150
8 in - 8.625 219.075 DN 200
10 in - 10.750 273.0500 DN 250
12 in - 12.750 323.850 DN 300
14 in 14 in 14.0000 355.600 DN 350
16 in 16 in 16.0000 406.400 DN 400
18 in 18 in 18.0000 457.200 DN 450
20 in 20 in 20.0000 508.000 DN 500
24 in 24 in 24.0000 609.600 DN 600
30 in 30 in 30.0000 762.000 DN 750
36 in 36 in 36.0000 914.400 DN 900
48 in 48 in 48.0000 1219.200 DN 1200

Fraction to Decimal to Metric Conversion Reference

The original printable chart also includes common inch fraction, decimal inch, and millimeter conversions. Use this section when you need a quick inch-to-mm reference in addition to the nominal pipe size chart.

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

This reference lists the millimeter-to-inch conversions included in the original chart.

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

Need the original printable version? Download the Nominal Pipe to Inch to MM Conversion Chart PDF .

Why Nominal Pipe Size Doesn’t Always Match the Physical Dimensions

It’s a common misconception that a pipe’s nominal size equals its actual diameter. In reality:

  • For NPS sizes 12 inches and smaller, the outside diameter (OD) is fixed for each nominal size, regardless of schedule (wall thickness).
  • The inside diameter (ID) decreases as the pipe wall gets thicker — meaning higher schedules (e.g., Sch 80, Sch 160) have smaller internal diameters than lighter ones (e.g., Sch 40).
  • For sizes above NPS 14, the nominal value and actual OD begin to align more closely.

This system ensures that pipes of the same nominal size will fit the same fittings and flanges, even if they have different wall thicknesses.

Why It’s Important to Convert NPS to Fraction, Decimal, Metric & DN

Understanding how NPS relates to metric and DN values is crucial for engineers, contractors, and suppliers who work across different standards.

Key reasons for accurate conversion:

  • Global compatibility: International projects often require matching imperial pipe sizes (NPS) with metric specifications (DN).
  • Procurement clarity: Many suppliers list dimensions in either NPS or DN. Having both references prevents ordering errors.
  • Design and calculation accuracy: Hydraulic, flow, and pressure calculations often use metric or decimal units, making conversions essential.
  • Quality control: On-site verification often uses millimeter measurements — having metric equivalents helps confirm correct installation.

Common Mistakes & FAQs in Pipe Size Conversion

“Nominal 2 inch” vs. Actual OD/ID

An NPS 2 pipe does not have an OD of 2 inches. Its actual OD is 2.375 inches (60.3 mm). Always check the 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 — a convenient label, not a measured value.
  • NPS (Nominal Pipe Size): The imperial labeling system used primarily in North America.
  • Use DN and NPS interchangeably as reference labels, not as exact measurement indicators.

What Schedules Do & Why They Don’t Change OD

The term “schedule” refers to pipe wall thickness.

  • Increasing the schedule (e.g., from Sch 40 to Sch 80) increases wall thickness and reduces internal diameter.
  • However, the outside diameter remains the same for a given NPS.

This standardization ensures compatibility across different pipe grades and pressures.

What to Keep in Mind When Converting Pipe Sizes

Understanding pipe size conversions is essential for accurate design, installation, and procurement. Nominal Pipe Size (NPS) is a standardized naming system used in the imperial measurement system, but it does not directly represent the pipe’s actual physical dimensions. 

  • For NPS sizes 12 inches and smaller, the outside diameter remains fixed while the inside diameter varies according to the pipe’s wall thickness or schedule.
  • For mixed systems, always verify both the outside and inside diameters from standard tables before selecting fittings or flanges. Relying solely on nominal labels can lead to mismatched components and costly installation issues.

Use this conversion chart as a quick reference for identifying equivalent pipe sizes across unit systems, but consult detailed dimensional standards (ASME, ISO, or EN) when precise measurements are required.