Millimetre (mm): metric unit of length

Technical definition and conversions

Formally, 1 millimetre = 10⁻³ metres. Because the inch is defined as exactly 25.4 millimetres, one millimetre equals 1/25.4 of an inch (about 0.03937 in). This exact relationship makes conversion between metric and imperial systems straightforward in engineering and trade. The abbreviation mm is universally recognized in technical drawings and specifications.

History and standardisation

The millimetre arose with the development of the metric system in the late 18th and early 19th centuries as a convenient subdivision of the metre. Over time it became entrenched in international standards and practice; the modern SI formalised the metre and its decimal submultiples, including the millimetre. National and international standards bodies use the millimetre in dimensional tolerancing, product specifications and scientific reporting.

Common uses and examples

Millimetres appear across many fields because they strike a balance between usability and precision:

• Manufacturing and engineering tolerances (e.g., part dimensions and fits).
• Construction and architectural detail work for small components.
• Medical imaging and device dimensions where sub-centimetre accuracy matters.
• Everyday items: thickness of sheet metal, smartphone dimensions, bolt thread pitches.

Distinctions and notable facts

Spelling varies by region: "millimetre" (British) and "millimeter" (American). The millimetre should not be confused with the micrometre (micron), which is one thousandth of a millimetre (10⁻⁶ metres) and written as μm. For authoritative definitions of SI units and guidance on usage see general references to the metric system and units of length, such as the overview of units of length available through standard references here and here.