Convert Inches to Millimeters
1 inch (in) equals 25.4 millimeters (mm).
1 in = 25.4 mm
Type in either box. Scientific notation works: 1.5e-3.
Converting a list? Use the bulk inch to millimeter converter to paste a column from Excel.
How to Convert Inch to Millimeter
- 1 in = 25.4 mm
- 1 mm = 0.03937007874 in
- Example: convert 15 in to mm: 15 in = 15 × 25.4 mm = 381 mm
Reference conditions. in: The international yard and pound agreement (1959) fixed 1 yard = 0.9144 m exactly, so the inch, foot and mile are exact.
Formula
mm = in × 25.4
Derivation from SI units:
- 1 in = 2.54 cm = 0.0254 m (exact)
- 1 mm = 10⁻³ m
- 1 in = 0.0254 m ÷ 0.001 m per mm = 25.4 mm
Both units are defined exactly, so the factor is exact. Inch definition: inch; Millimeter definition: millimeter.
Worked examples
- A 0.25 in bolt is 6.35 mm in diameter.
0.25 in × 25.4 = 6.35 mm - A 0.125 in sheet is 3.175 mm thick.
0.125 in × 25.4 = 3.175 mm
Inch to Millimeter conversion table
| Inches (in) | Millimeters (mm) |
|---|---|
| 0.125 in | 3.175 mm |
| 0.25 in | 6.35 mm |
| 0.5 in | 12.7 mm |
| 0.75 in | 19.05 mm |
| 1 in | 25.4 mm |
| 1.5 in | 38.1 mm |
| 2 in | 50.8 mm |
| 3 in | 76.2 mm |
| 4 in | 101.6 mm |
| 5 in | 127 mm |
| 6 in | 152.4 mm |
| 8 in | 203.2 mm |
| 10 in | 254 mm |
| 12 in | 304.8 mm |
| 15 in | 381 mm |
| 20 in | 508 mm |
| 24 in | 609.6 mm |
| 27 in | 685.8 mm |
| 32 in | 812.8 mm |
| 36 in | 914.4 mm |
| 48 in | 1219.2 mm |
| 55 in | 1397 mm |
| 65 in | 1651 mm |
| 72 in | 1828.8 mm |
Where it's used
US drawings, bolts and sheet thicknesses in inches are converted to millimetres for metric machining and CAD. Multiply by exactly 25.4.
Inch to other length units
| Conversion | 1 in equals |
|---|---|
| inch to meter | 0.0254 m |
| inch to kilometer | 0.0000254 km |
| inch to centimeter | 2.54 cm |
| inch to millimeter | 25.4 mm |
| inch to micrometer | 25400 µm |
| inch to nanometer | 25400000 nm |
| inch to angstrom | 254000000 Å |
| inch to mile | 0.00001578282828 mi |
| inch to yard | 0.02777777778 yd |
| inch to foot | 0.08333333333 ft |
| inch to mil | 1000 mil |
| inch to nautical mile | 0.00001371490281 nmi |
Frequently asked questions
How many mm is 1/8 inch?
1/8 in = 0.125 in = 3.175 mm.
How many millimeters are in 1 inch?
1 in = 25.4 mm.
How do I convert inches to millimeters?
Multiply the value in in by 25.4. For example, 15 in × 25.4 = 381 mm.
Is the in to mm conversion exact?
Yes. Both units are defined exactly, and 1 in is exactly 25.4 mm.
Sources
- Guide for the Use of the International System of Units (SI), NIST Special Publication 811, 2008 Edition, National Institute of Standards and Technology , Appendix B.8, "inch (in)".
- The International System of Units (SI), 9th edition (SI Brochure), Bureau International des Poids et Mesures , Table 7, SI prefixes.
Last reviewed: . Factors are stored exactly and computed with 50-digit decimal arithmetic. See how we verify conversion factors.
Cite this page
APA
Scientific Converters. (2026, October 15). Convert Inches to Millimeters. Retrieved October 17, 2026, from https://scientificconverters.com/length/inches-to-millimeters/
IEEE
Scientific Converters, "Convert Inches to Millimeters," scientificconverters.com. Accessed: Oct. 17, 2026. [Online]. Available: https://scientificconverters.com/length/inches-to-millimeters/