Millimeters to Inches Converter

CNC Machining & Precision Dimensional Metrology (1959 Agreement)

The millimeter (mm) and inch (in) are locked by international treaty:

• **

• Exact Defining Invariant**:

1\ in \ 25.4\ mm Exact = 2.54\ cm = 0.0254\ m

(in) = \25.4, \ (mm) = \× 25.4

• **

• Machining Units**:

1\ thou (mil) \ 0.001\ in = 0.0254\ mm = 25.4\ \μ

• **

• Standard Tooling & Fastener Benchmarks**:

• 0.50\ mm = 0.0197\ in = 19.7\ thou

• 3.175\ mm = 0.1250\ in = 1/8"

• 6.35\ mm = 0.2500\ in = 1/4"

• 10.00\ mm = 0.3937\ in \≈ 25/64"

• 12.70\ mm = 0.5000\ in = 1/2"

• 25.40\ mm = 1.0000\ in = 1"

How to Use This Calculator

Enter your Length Value to Convert (e.g. 25.4), Conversion Direction — choose from Millimeters (mm) to Inches (in = mm ÷ 25.4), Inches (in) to Millimeters (mm = in × 25.4), and CNC Machining & Tooling Benchmark — choose from Custom Length Value, 25.40 mm (Exact 1.0 Inch) → 1.0000 in (1,000 thou / 1" exact), 10.00 mm (Standard Socket Wrench) → 0.3937 in (approx 25/64"). Results update instantly as you type — adjust any value to compare scenarios.

The Formula

Conversion factors used in this calculator:

1 in = 25.4 mm exact

Worked Example

A CNC machinist setting up a lathe converts a 10.0 mm diameter stock into imperial machining units as 10.0 ÷ 25.4 = 0.3937 inches.

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.