Inches to Millimeters Converter

Precision Metrology & The 25.4 Millimeter Invariant

Under international metrology standards:

• **

• Exact Defining Invariant**:

1 in 25.4 mm = 25,400 μm = 2.54 cm

• **

• Formula Equations**:

Millimeters = Inches × 25.4, Inches = Millimeters25.4

• **

• Standard Hardware Benchmarks**:

• 1/16 in (0.0625") = 1.5875 mm

• 1/8 in (0.1250") = 3.1750 mm

• 1/4 in (0.2500") = 6.3500 mm

• 1/2 in (0.5000") = 12.7000 mm

• 3/4 in (0.7500") = 19.0500 mm

• 1.000 in = 25.4000 mm

How to Use This Calculator

Enter your Inches Whole Number (e.g. 0.5), Fractional Inch Portion (optional) — choose from 0 (Exact Whole Inches / Decimal Input), 1/16 Inch (+0.0625" = 1.5875 mm), 1/8 Inch (+0.1250" = 3.1750 mm), and Machining & Hardware Benchmark — choose from Custom Dimension, 1/2" (0.500 Inch) → 12.7000 mm (12,700 µm), 1.0" Standard Inch → 25.4000 mm (Exact Standard). 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 machining a 1/2" shaft sets the toolpath diameter to exactly 0.50 × 25.4 = 12.7000 mm.

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.