The inch (in) is a unit of length in the imperial and United States customary systems. It is defined as exactly 25.4 mm by international agreement since 1959. Inches are commonly used for display screen sizes, construction lumber dimensions, and plumbing fixture measurements in the US and UK.
The millimeter (mm) is a unit of length in the metric system, equal to one thousandth of a meter. It is the standard unit for precision engineering, machining, and scientific instrumentation worldwide. Millimeters provide finer granularity and are preferred in mechanical design, 3D printing, and medical device specifications.
Conversion Factor
The exact conversion relationship is \(1\,\text{in} = 25.4\,\text{mm}\). To convert inches to millimeters, multiply the inch value by 25.4. Conversely, to convert millimeters to inches, divide by 25.4.
Step 3: The result is 254 millimeters. Thus, 10 inches equals exactly 254 mm.
The factor 25.4 was established by international agreement in 1959 when the United States, United Kingdom, and other nations redefined the inch in terms of the metric system. The inch was set to exactly 25.4 mm, making conversion mathematically exact and eliminating previous variations between countries.
To convert millimeters to inches, divide the millimeter value by 25.4. For example, 254 mm ÷ 25.4 = 10 in. Alternatively, multiply by the reciprocal \(1/25.4 \approx 0.03937\). This is useful when working with metric drawings or components that need to be expressed in imperial units.
"On fait la science avec des faits, comme on fait une maison avec des pierres ; mais une accumulation de faits n'est pas plus une science qu'un tas de pierres n'est une maison." "Science is built up of facts, as a house is built of stones; but an accumulation of facts is no more a science than a heap of stones is a house." — Henri Poincaré (French Mathematician, Theoretical Physicist & Mining Engineer)