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MM to Inches

Millimeters to inches for tools, parts and specs.

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MM to inches

Convert millimeters to inches: divide by 25.4

1 millimeter = 0.03937007874 inches

Formula: in = mm ÷ 25.4

Millimeters to inches table

Millimeters (mm)Inches (in)
10.03937007874
20.07874015748
30.1181102362
40.157480315
50.1968503937
60.2362204724
70.2755905512
80.3149606299
90.3543307087
100.3937007874
150.5905511811
200.7874015748
250.9842519685
501.968503937
1003.937007874

Calculated on your device with exact factors from NIST and the SI.

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How to convert MM to inches

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MM to Inches: what to know

Millimeters are the standard unit for engineering drawings, fasteners, jewelry, camera lenses and 3D printing, while US hardware is often sized in inches. Converting millimeters to inches is a division by 25.4, because one inch is defined as exactly 25.4 mm (1959 international yard and pound agreement; NIST Handbook 44).

For example, an 8 mm bolt is 8 ÷ 25.4 = 0.315 inches, a 1.75 mm 3D printer filament is 0.0689 inches, and a 300 mm ruler is 11.811 inches long. Since the factor is exact, any difference you see between a metric part and an inch part is a real difference in size, not a rounding artifact.

Inch tools usually come in fractions such as 1/8, 1/4 or 5/16. To match a metric size to the nearest fraction, convert to decimal inches and multiply by the denominator: 10 mm is 0.3937 inches, and 0.3937 × 16 = 6.3, so the closest sixteenth is 6/16 (3/8 inch, 9.525 mm). The two are close but not interchangeable for a snug fit.

Popular uses

  • Matching metric bolts, drill bits and sockets to inch sets
  • Converting part tolerances on engineering drawings
  • Reading rainfall, which some sources report in mm
  • Checking jewelry, watch and lens dimensions

Tips for the best result

  • Keep three decimals of an inch (a thousandth) for machining work.
  • Metric and inch wrenches that look the same can round off a bolt head; use the right set.
  • Paper thickness and film are often in thou: 1 thou is 0.0254 mm.

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