Inches and Millimeters Converter (in to mm & mm to in

Inches (in) to Millimeters (mm), Exact Standards & Fractional Sizing

The inch-to-millimeter relationship is an exact mathematical constant legally established in 1959:

• **

• Exact Conversion Constants (NIST Special Publication 811)**:

1 Inch = 25.4000 Millimeters (Exact)

Length (mm) = Length (in) × 25.4

Length (in) = Length (mm)25.4

• **

• Common Fractional Inch to Metric Equivalents**:

• 1/16'' (0.0625 in): 1.5875 mm

• 1/8'' (0.1250 in): 3.1750 mm

• 1/4'' (0.2500 in): 6.3500 mm

• 3/8'' (0.3750 in): 9.5250 mm (close to 10mm)

• 1/2'' (0.5000 in): 12.7000 mm

• 3/4'' (0.7500 in): 19.0500 mm

• 1'' (1.0000 in): 25.4000 mm

• **

• Engineering Units**:

• 1 Thou (Mil): 0.001 in = 0.0254 mm = 25.4\ μm

• 1 Millimeter: 39.3701 thou

How to Use This Calculator

Enter your Length Magnitude (e.g. 1.0 or 25.4 in length), Conversion Direction — choose from Inches to Millimeters (in → mm: multiply by 25.4), Millimeters to Inches (mm → in: divide by 25.4), and Machining & Hardware Benchmark — choose from Custom Dimension, 1.0 Inch Exact Definition (1.0 in = 25.4000 mm), 1/4" Drive Socket / Bolt (0.25 in = 6.3500 mm). Results update instantly as you type — adjust any value to compare scenarios.

The Formula

Formula Reference: Inches and Millimeters Converter (in to mm & mm to in)

Input variables:

lengthMagnitudeInput — Length Magnitude

conversionDirectionSelect — Conversion Direction

Computed outputs:

convertedMillimeters — Length in Millimeters (mm)

convertedInchesDecimal — Length in Inches (in)

nearestFractionalInch — Nearest Fractional Inch (to 1/64")

lengthCentimeters — Length in Centimeters (cm)

engineeringThouMils — Machining Thou / Mils (0.001 in)

Mathematical relationships extracted from calculation logic:

rem = val - whole

num64 = Math.round(rem * 64)

num = num64 / g

den = 64 / g

mm = inches * 25.4

cm = mm / 10.0

thou = inches * 1000.0

Reference constants:

mm = 25.4

Standard: NIST Handbook 44 and BIPM SI unit definitions

Worked Example

A machinist reading an aerospace blueprint specifying a 0.250-inch hole diameter converts to metric: 0.250 × 25.4 = 6.350 mm, selecting a 6.35 mm reamer for the operation.

Understanding Your Result

Unit conversion precision: round to the same number of significant figures as your original measurement. Converting 6 feet to meters gives 1.8288 m — but if your measurement was approximate (±1 inch), expressing more than 2 decimal places is false precision. For international trade and technical specifications, SI units (meters, kilograms, liters) are the standard. US customary units (feet, pounds, gallons) differ from Imperial UK units: 1 US gallon = 3.785 L vs. 1 UK gallon = 4.546 L.