About

In precision engineering and international manufacturing, the conversion between imperial inches and metric millimeters is a frequent necessity. Mismatched units in blueprints or CAD files can lead to catastrophic machining errors, wasted materials, and assembly failures. This tool addresses the specific needs of CNC operators, mechanical engineers, and automotive technicians who require absolute exactness. Unlike standard approximations, this calculator adheres to the international yard and pound agreement of 1959, establishing the definition that one inch is exactly 25.4 millimeters. It supports complex fractional inputs often found in US hardware specifications (e.g., 3/16, 5/64) and handles floating-point arithmetic with extended precision to prevent rounding artifacts common in basic JavaScript operations.

Formulas

The conversion relies on the internationally agreed constant defining the relationship between the Imperial and Metric systems.

L_mm = L_in × 25.4

For fractional inputs, the tool first evaluates the rational number before applying the scaling factor:

If L_in = ab, then L_mm = ab × 25.4

Reference Data

Fraction (in)	Decimal (in)	Metric (mm)	Application Note
1/64″	0.015625	0.3969	PCB Trace Widths
1/32″	0.03125	0.7938	Sheet Metal Gauge
1/16″	0.0625	1.5875	Standard Washers
1/8″	0.1250	3.1750	Rivet Diameters
1/4″	0.2500	6.3500	Audio Jacks
3/8″	0.3750	9.5250	Fluid Hoses
1/2″	0.5000	12.7000	Bolt Shanks
5/8″	0.6250	15.8750	Spark Plugs
3/4″	0.7500	19.0500	Plumbing Pipe (ID)
1″	1.0000	25.4000	Calibration Standard
2″	2.0000	50.8000	Structural Tubing
3.937″	3.9370	100.0000	10cm Reference

Frequently Asked Questions

Since the International Yard and Pound Agreement of 1959, signed by the US, UK, Canada, Australia, New Zealand, and South Africa, the inch is legally defined as exactly 25.4 millimeters. There is no rounding involved in this definition.

The parsing logic identifies the integer part (1) and the fractional part (1/2), sums them to a decimal (1.5), and then proceeds with the multiplication. Ensure a space exists between the whole number and the fraction.

The tool calculates up to 6 decimal places, which exceeds standard machining tolerances (typically +/- 0.005mm for CNC work). This ensures that rounding errors do not accumulate in subsequent engineering steps.

Yes, while the primary flow is Inches to Millimeters, entering a value in the Millimeter field and calculating effectively reverses the operation using division by 25.4.