Inches to MM Converter
Convert inches to millimeters, including tape-measure fractions such as 3/8 or 1 3/8.
Multiply inches by 25.4 to get millimeters, because an inch is exactly 25.4 mm. So 1 3/8 in is 1.375 in, which is 34.925 mm, or about 35 mm. For a fraction, divide the top number by the bottom one first: 3/8 in is 0.375 in, and 0.375 x 25.4 = 9.525 mm.
This converter uses the exact definition of the inch, so the millimeter value is exact apart from display rounding. The whole-millimeter figure is rounded; for machining or fit-critical work, use the decimal value and check it against the tolerance on your drawing.
In millimeters
34.925 mm
- Decimal inches
- 1.375 in
- Centimeters
- 3.4925 cm
- Meters
- 0.034925 m
- Nearest whole millimeter
- 35 mm
Millimeters = inches x 25.4, exactly.
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About the Inches to MM Converter
The inch has been exactly 25.4 millimeters since 1959, and NIST lists the factor as exactly 0.0254 meter per inch, so every inch to mm answer here is one multiplication by 25.4 with no rounding beyond the display. You can type the length the way a tape measure shows it: a whole number, a decimal such as 2.5, a fraction such as 3/8, or a whole number and a fraction such as 1 3/8. The converter turns the fraction into a decimal first, so 1 3/8 becomes 1.375 in and then 34.925 mm. Common marks work out cleanly: 1/16 in is 1.5875 mm, 1/4 in is 6.35 mm, 1/2 in is 12.7 mm and a foot of 12 in is 304.8 mm. The page also gives centimeters, meters and the nearest whole millimeter. Metric drill sizes can be finer than a whole millimeter, so compare the decimal value. To go the other way, use the mm to inches converter.
Frequently asked questions
How do I convert a fraction of an inch to mm?+
Divide the top number by the bottom one, then multiply by 25.4. For 5/16 in, 5 / 16 = 0.3125 and 0.3125 x 25.4 = 7.9375 mm. For a mixed number, add the whole number first: 1 3/8 in is 1.375 in, or 34.925 mm.
Which metric size is closest to a fraction of an inch?+
Convert first, then compare. 3/8 in is 9.525 mm, so a 9.5 mm part is 0.025 mm under and a 10 mm part is 0.475 mm over. For a close fit, compare the decimal.
What is 2.5 inches in mm?+
63.5 mm, because 2.5 x 25.4 = 63.5. That is 6.35 cm.
Should I round to the nearest millimeter?+
For most metric parts, yes: 34.925 mm is about 35 mm. For a close fit, keep the decimals, since the exact value can sit nearly half a millimeter away from the rounded one.
Conversion factor of 1 inch = 2.54 E-02 meter (25.4 mm), exact: National Institute of Standards and Technology, Special Publication 811, Guide for the Use of the International System of Units, Appendix B.8, Factors for Units Listed Alphabetically. Fractions are converted to decimal inches before multiplying by 25.4.
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