Inches to Millimeters Converter
Convert inches (in) to millimeters (mm) using documented conversion factors. Includes interactive calculations, conversion formula, worked steps, and common reference tables.
Live Interactive Calculator
millimeter = inch × 25.4Conversion Formula
To convert from inches to millimeters, apply the mathematical formula:
Where in represents inches and mm represents millimeters.
Conversion Factor
The base relationship defined under primary measurement standards:
Reference: NIST SP 811 / ISO 80000-3
International Length Standards & Exact Definitions
All modern length units are legally tied to the metric base unit, the meter (m), defined by the BIPM as the distance traveled by light in a vacuum during an interval of 1/299,792,458 of a second.
Under the 1959 International Yard and Pound Agreement, English and US customary measurements are exact fractions of the meter:
Because these definitions are exact, conversions between yards, feet, inches, centimeters, and meters have zero theoretical error.
How to Convert Inches to Millimeters
Converting measurements between inches and millimeters follows a structured calculation process:
- Identify the quantity in inches (in) that you wish to convert.
- Multiply the input by the conversion factor ratio
1 in = 254 mm.. - The resulting value is the equivalent amount expressed in millimeters (mm).
Common Inches to Millimeters Reference Table
Frequently referenced values calculated using documented measurement conversion factors.
| Inch (in) | Millimeter (mm) | Arithmetic Expression |
|---|---|---|
| 0.125 in | 3.175 mm | 0.125 × 25.4 |
| 0.25 in | 6.35 mm | 0.25 × 25.4 |
| 0.5 in | 12.7 mm | 0.5 × 25.4 |
| 0.75 in | 19.05 mm | 0.75 × 25.4 |
| 1 in | 25.4 mm | 1 × 25.4 |
| 1.5 in | 38.1 mm | 1.5 × 25.4 |
| 2 in | 50.8 mm | 2 × 25.4 |
| 3 in | 76.2 mm | 3 × 25.4 |
| 4 in | 101.6 mm | 4 × 25.4 |
| 5 in | 127 mm | 5 × 25.4 |
| 6 in | 152.4 mm | 6 × 25.4 |
| 10 in | 254 mm | 10 × 25.4 |
| 12 in | 304.8 mm | 12 × 25.4 |
| 24 in | 609.6 mm | 24 × 25.4 |
Fractional Inch to Millimeter Reference Chart
Standard 1/16-inch shop increments with exact international millimeter equivalents (1 in = 25.4 mm exact).
| Fractional (in) | Decimal (in) | Millimeters (Exact mm) | Common Application & Tool Size |
|---|---|---|---|
| 1/16″ | 0.0625″ | 1.5875 mm | Sheet metal gauge, fine cotter pins, thin shims |
| 1/8″ | 0.1250″ | 3.1750 mm | Small drill bits, pop rivets, cable clamps |
| 3/16″ | 0.1875″ | 4.7625 mm | Brake lines, aircraft rivets, masonry screws |
| 1/4″ | 0.2500″ | 6.3500 mm | Standard 1/4-20 bolts, hex keys, camera tripod mounts |
| 5/16″ | 0.3125″ | 7.9375 mm | Automotive fasteners, fuel line hoses, heavy bolts |
| 3/8″ | 0.3750″ | 9.5250 mm | Drive sockets, plumbing supply lines, structural screws |
| 7/16″ | 0.4375″ | 11.1125 mm | Seat belt anchors, trailer hitches, chassis bolts |
| 1/2″ | 0.5000″ | 12.7000 mm | Standard copper water pipe, structural bolts, heavy drive sockets |
| 9/16″ | 0.5625″ | 14.2875 mm | Bicycle pedals, automotive suspension fasteners |
| 5/8″ | 0.6250″ | 15.8750 mm | Garden hoses, spark plug sockets, heavy machinery pins |
| 11/16″ | 0.6875″ | 17.4625 mm | Heavy-duty truck lug nuts, industrial piping fittings |
| 3/4″ | 0.7500″ | 19.0500 mm | Main water supply conduits, heavy timber framing, commercial conduit |
| 13/16″ | 0.8125″ | 20.6375 mm | Spark plug wrenches, industrial machinery bearings |
| 7/8″ | 0.8750″ | 22.2250 mm | Bicycle handlebars, construction crane cabling, heavy flange bolts |
| 15/16″ | 0.9375″ | 23.8125 mm | Structural steel connecting bolts, heavy axle spindle nuts |
| 1″ | 1.0000″ | 25.4000 mm | Exact international base reference: 25.4 mm exactly |
Finite Exact Decimal Property: Because 25.4 is equal to 127/5, every binary fraction of an inch (denominators of 2, 4, 8, 16, 32, 64) produces a completely exact terminating decimal value in millimeters without recurring digits or rounding errors.
Machining & Hardware Notice: While a 1/4″ bolt (6.35 mm) and an M6 metric bolt (6.00 mm) appear close in diameter, they are incompatible. Standard 1/4″-20 UNC thread pitch is 1.27 mm per crest, whereas coarse M6 pitch is 1.00 mm. Attempting to interchange them cross-threads and damages tapped holes.
Worked Calculation Examples
Practical step-by-step calculations showing how to convert inches to millimeters with exact mathematical equations.
Application context: 1/4-inch hardware bolt (6.35 mm)
Application context: 1/2-inch socket wrench or copper pipe (12.7 mm)
Application context: 1.0 inch defined internationally as exactly 25.4 mm
Application context: 10 inches machine dimension (254 mm exact)
About the Units
Authoritative technical specifications and historical definitions for both units.
Inch (in)
A customary unit of length equal to 1/12 of a foot or 1/36 of a yard. Defined as exactly 25.4 millimeters (0.0254 meters).
Millimeter (mm)
A metric unit of length equal to one-thousandth of a meter (0.001 m). Extensively used in precision mechanical engineering and architecture blueprints.
Need to convert Millimeters back to Inches?
Reverse Formula: inch = millimeter × 0.03937
Related Length Converters
Contextual conversions related to this pair within the measurement graph.