System of units

Characteristics and components

Most systems distinguish a small set of base units and a larger set of derived units. Base units are chosen so other quantities can be expressed algebraically from them (for example, speed = length/time). Key characteristics include:

• Coherence: A coherent system produces derived units that are algebraically consistent with base units without extra conversion factors.
• Prefixes: Multiplicative prefixes (milli-, kilo-, etc.) extend the range of numerical values conveniently.
• Dimensional structure: Each quantity has dimensions (such as M for mass, L for length, T for time) used in analysis and checking equations.

Common systems

Examples illustrate differences in choice and application:

• International System (SI): The globally preferred scientific system with seven base units (e.g., metre, kilogram, second). It is coherent and uses prefixes and was modernised to tie unit definitions to fundamental physical constants.
• CGS and MKS: Historical systems based on centimetre–gram–second or metre–kilogram–second; useful in some branches of physics.
• Imperial and US customary: Collections of historically developed units used for commerce and daily life in some countries; they require constant conversion to SI for scientific work.

History and development

Systems of units evolved from local measures such as the cubit or foot toward national and then international standards as trade, science and industry expanded. The 19th and 20th centuries saw concerted efforts to harmonise units internationally, culminating in formal organisations and treaties that maintain and refine systems like the SI.

Uses and significance

Standardised systems underpin science, engineering, manufacturing, health and trade. They enable reproducible experiments, compatible engineering designs, clear legal definitions for commerce and interoperability of technology worldwide. Clear unit systems also reduce costly errors in fields such as aerospace and medicine.

Notable facts and distinctions

Dimensional analysis uses the structure of a unit system to verify formulas and derive relationships. Converting between systems requires known conversion factors; loss or ambiguity in conversion has led to historical accidents, so modern practice emphasises international standards. National and international bodies maintain legal metrology, calibrate reference standards and publish conventions so measurements remain consistent across borders.