How to Convert Inches to Millimeters
Converting inches to millimeters is a critical operation in precision engineering, manufacturing, and any technical field requiring accurate cross-system measurements. The inch (in) is an imperial unit fundamental to US engineering and manufacturing, while the millimeter (mm) is the metric standard for precision measurements used worldwide. One inch equals exactly 25.4 millimeters, a clean and exact conversion factor that makes this one of the most reliable unit conversions in existence. This conversion is indispensable in CNC machining, where programs may be written in one system but executed on machines calibrated in the other. Aerospace and automotive industries rely on this conversion for parts interchangeability across international supply chains. PCB designers, semiconductor manufacturers, and electronics engineers routinely convert between these units for component placement and trace width specifications. Woodworkers using imported metric hardware on imperial-measured projects also need accurate conversions. Our tool uses the exact 25.4 mm per inch factor to deliver precise results instantly, ensuring your precision work maintains its accuracy across measurement systems.
Conversion Formula
To convert inches to millimeters, multiply the value in inches by 25.4. This is an exact conversion factor, not an approximation. The 1959 International Yard and Pound Agreement defined the inch as exactly 25.4 millimeters. This exact definition means there is zero rounding error in the base conversion, making it one of the most precise unit conversions available.
Millimeters = Inches × 25.4
0.5 inches = 12.7 millimeters
Step-by-Step Example
To convert 0.5 inches (1/2 inch) to millimeters:
1. Start with the value: 0.5 inches
2. Multiply by the conversion factor: 0.5 × 25.4
3. Calculate: 0.5 × 25.4 = 12.7
4. Result: 0.5 inches = 12.7 millimeters
Half an inch is exactly 12.7 mm, a very commonly encountered dimension in manufacturing and hardware.
Understanding Inches and Millimeters
What is a Inch?
The inch has been used for centuries, with its name derived from the Latin "uncia" meaning one-twelfth, referring to its relationship as one-twelfth of a foot. Historically, it was defined as the width of a man's thumb, or three barleycorns placed end to end. The modern international inch was standardized in 1959 as exactly 25.4 millimeters. In precision engineering, the inch is subdivided into fractions (1/2, 1/4, 1/8, 1/16, 1/32, 1/64) or decimal thousandths (0.001 inch, called a "thou" or "mil"). Despite metrication, the inch remains fundamental in US manufacturing and engineering.
What is a Millimeter?
The millimeter was established as part of the metric system during the French Revolution. Derived from the Latin "mille" (thousand), the millimeter is one-thousandth of a meter. It became the standard unit for precision measurements in engineering, manufacturing, and science throughout the metric world. The millimeter is used in technical drawings, machining tolerances, and specifications worldwide. The ISO (International Organization for Standardization) system is built on metric measurements, with millimeters being the preferred unit for mechanical engineering drawings.
Practical Applications
Inches to millimeters conversion is essential in precision machining and CNC programming, where dimensional accuracy to thousandths of an inch (converted to hundredths of a millimeter) is required. Electronics design uses this conversion for PCB layout, connector specifications, and component footprints. Automotive and aerospace manufacturing depends on precise conversion for international parts compatibility. Plumbing and pipe fitting uses both systems, requiring accurate conversion for proper sealing and connection. 3D printing specifications may use either system depending on the software and printer manufacturer.
Tips and Common Mistakes
Since 25.4 is an exact factor, this conversion introduces no rounding error at the base level. However, a common mistake is using 25 instead of 25.4, which creates a 1.57% error. For common fractions: 1/8 inch = 3.175 mm, 1/4 inch = 6.35 mm, 3/8 inch = 9.525 mm, 1/2 inch = 12.7 mm, 3/4 inch = 19.05 mm. Memorizing these common equivalents saves time. Another tip: when converting thousandths of an inch (thou/mils) to millimeters, multiply by 0.0254. So 1 thou (0.001 inch) = 0.0254 mm. Be careful not to confuse the unit "mil" (0.001 inch) with millimeters.
Frequently Asked Questions
One inch equals exactly 25.4 millimeters. This is not an approximation but a precise, internationally defined value established in 1959. Every other inch-to-metric conversion is derived from this fundamental relationship. The exactness of this definition makes inch-to-millimeter conversion one of the most accurate unit conversions possible.