Convert mm to inches
PrintMillimeter to Inch Conversion Table
| Millimeter (mm) | Inch (in) |
|---|---|
| 0.01 mm | 0.0003937008 in |
| 0.1 mm | 0.0039370079 in |
| 1 mm | 0.0393700787 in |
| 2 mm | 0.0787401575 in |
| 3 mm | 0.1181102362 in |
| 5 mm | 0.1968503937 in |
| 10 mm | 0.3937007874 in |
| 20 mm | 0.7874015748 in |
| 50 mm | 1.968503937 in |
| 100 mm | 3.937007874 in |
| 1000 mm | 39.37007874 in |
Inches and millimeters are the two foundational length units used in mechanical engineering, precision machining, 3D printing, plumbing pipe fitting, bolt and socket wrench sizing, and architectural blueprinting. While the inch (in) is the traditional customary unit used across North American manufacturing and Imperial tooling, the millimeter (mm) is the universal metric subunit (1/1,000th of a meter) used in global ISO engineering, CAD design, and scientific hardware.
Our free online Inches to Mm Converter provides instant, high-precision length conversions in both directions:
- Inches to Millimeters: Multiply inches by
25.4(exact international standard). - Millimeters to Inches: Divide millimeters by
25.4(or multiply by0.039370079).
Exact Conversion Formulas: Inches & Millimeters
Since the International Yard and Pound Agreement of 1959, one international inch is defined as exactly 25.4 millimeters (2.54 centimeters). This establishes exact mathematical relationships:
| Conversion Direction | Exact Mathematical Formula | Quick Mental Approximation Rule |
|---|---|---|
| Inches to Millimeters | Millimeters = Inches × 25.4 |
Multiply inches by 25 (e.g., 2 in × 25 = 50 mm). |
| Millimeters to Inches | Inches = Millimeters ÷ 25.4 |
Divide mm by 25 (e.g., 100 mm ÷ 25 = 4 inches). |
Step-by-Step Calculation Example
To convert 15 inches to millimeters:
15 in × 25.4 = 381 millimeters
Thus, 15 inches is equal to exactly 381 mm.
Fractional Inches to Millimeters Chart (Tooling & Sockets)
In mechanics and woodworking, imperial fractional inch sizes map directly to metric socket wrench equivalents:
| Fractional Inches | Decimal Inches | Exact Millimeters (mm) | Nearest Metric Socket Wrench |
|---|---|---|---|
| 1/16 in | 0.0625 in | 1.5875 mm | 1.5 mm / 1.6 mm pin |
| 1/8 in | 0.1250 in | 3.1750 mm | 3 mm / 3.2 mm drill bit |
| 1/4 in | 0.2500 in | 6.3500 mm | 6.5 mm socket / 1/4″ drive shank |
| 3/8 in | 0.3750 in | 9.5250 mm | 10 mm socket (≈ 3/8″ drive) |
| 1/2 in | 0.5000 in | 12.7000 mm | 13 mm socket / 1/2″ pipe fitting |
| 5/8 in | 0.6250 in | 15.8750 mm | 16 mm spark plug socket |
| 3/4 in | 0.7500 in | 19.0500 mm | 19 mm socket (≈ 3/4″ lug nut) |
| 7/8 in | 0.8750 in | 22.2250 mm | 22 mm socket / O2 sensor socket |
| 1.0 in | 1.0000 in | 25.4000 mm | 25.4 mm (1 Inch Exact Benchmark) |
Decimal Inch to Millimeter Conversion Table
Below is a quick reference lookup table converting decimal measurements from 0.01 inches up to 1,000 inches:
| Decimal Inches (in) | Millimeters (mm) | Engineering & Hardware Benchmark |
|---|---|---|
| 0.01 in | 0.2540 mm | 10 mil shim stock thickness |
| 0.10 in | 2.5400 mm | Standard 2.54 mm breadboard header pin pitch |
| 1.00 in | 25.4000 mm | 1.0 Inch Exact International Benchmark |
| 2.00 in | 50.8000 mm | 50.8 mm (2-inch PVC pipe diameter) |
| 3.00 in | 76.2000 mm | 76.2 mm automotive exhaust pipe |
| 5.00 in | 127.0000 mm | 127 mm PC cooling fan size |
| 10.00 in | 254.0000 mm | 254 mm (10-inch table saw blade diameter) |
| 12.00 in (1 Foot) | 304.8000 mm | 304.8 mm (Standard 12-inch school ruler) |
| 20.00 in | 508.0000 mm | 508 mm automotive wheel rim diameter (20″) |
| 50.00 in | 1,270.0000 mm | 1.27 Meters |
| 100.00 in | 2,540.0000 mm | 2.54 Meters |
| 1,000.00 in | 25,400.0000 mm | 25.4 Meters |
History & Standards: Inch vs. Millimeter
The Definition of the Inch (in)
The English word inch derives from the Latin uncia (meaning 1/12th part of a Roman foot). Historically defined by King Edward II in 1324 as 3 dry round barleycorns laid end-to-end, the inch was standardized in 1959 by the International Yard and Pound Agreement as exactly 25.4 millimeters (2.54 cm). It remains the dominant unit for US fasteners, pipe threads, and mechanical tooling.
The Definition of the Millimeter (mm)
The millimeter is a decimal submultiple of the meter in the International System of Units (SI). Defined as 1/1,000th of a meter, 1 mm represents the distance traveled by light in a vacuum in 1 / 299,792,458,000th of a second. Millimeters are used worldwide in mechanical CAD engineering, 3D printing, electronics, and medicine.
To convert millimeters back to inches, use our Mm to Inches Converter. For centimeter length conversions, check our Inches to Cm Converter. For feet and yard conversions, visit our Inches to Feet Converter or Conversion Calculator.
Frequently Asked Questions (FAQ)
How many millimeters are in 1 inch?
There are exactly 25.4 millimeters in 1 inch.
How many inches are in 1 millimeter?
There are approximately 0.0393701 inches in 1 millimeter (1 ÷ 25.4).
What is a 1/2-inch bolt or pipe in millimeters?
A 1/2-inch fitting equals 0.5 × 25.4 = 12.7 millimeters. Mechanics typically use a 13 mm socket to turn 1/2-inch hardware.
What is a 3/8-inch socket in millimeters?
3/8 inches equals 0.375 × 25.4 = 9.525 mm. A 10 mm socket is the closest metric equivalent for 3/8-inch hardware.