Why is 1 inch exactly 25.4 mm?

By international agreement in 1959, the inch was redefined to be exactly 25.4 millimeters. This creates a fixed conversion that never changes, essential for global manufacturing.

How do I convert 1/4 inch to millimeters?

0.25 inches × 25.4 = 6.35 mm. Quarter-inch drill bits and fasteners are common in mechanical work.

I have a 1/16-inch precision tool. What's that in mm?

1/16 inch = 0.0625 inches × 25.4 = 1.5875 mm. Sixteenths of an inch are very common in machining.

Is this conversion exact?

Yes, by definition. 1 inch = 25.4 mm exactly, no rounding. This is the official conversion used in engineering worldwide.

Inch to Millimeter Converter

Enter a value to instantly convert between length units.

From

Precision4 dp

1 Inch = 25.4 Millimeter

Key Formulas

Centimeter → Inch

in = cm × 0.393701

Kilometer → Mile

mi = km × 0.621371

Foot → Meter

m = ft × 0.3048

Inch → Centimeter

cm = in × 2.54

Formula

mm = in × 25.4

One inch is defined as exactly 25.4 millimeters by international agreement since 1959. This conversion bridges imperial and metric precision measurements in manufacturing, engineering, and craftsmanship. When a bolt is specified in inches but your shop manual lists dimensions in millimeters, or when ordering fasteners online from different countries, this conversion ensures parts fit correctly. The 25.4 factor is one of the few exact values in unit conversion.

Source: International Yard and Pound Agreement (1959)

Frequently Asked Questions

Real-World Examples

1 inch = 25.4 mm (the exact definition since 1959)

1 in = 25.4 mm

A half-inch bolt = 12.7 mm, common in engineering

0.5 in = 12.7 mm

A 2-inch pipe fitting = 50.8 mm external diameter

2 in = 50.8 mm

A 3.5-inch hard drive disk = 88.9 mm diameter

3.5 in = 88.9 mm

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Inch to Millimeter Converter

Key Formulas

Formula

Frequently Asked Questions

Real-World Examples

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