Most Common Unit Systems
A measurement system, or system of measurement, is made up of interdependent measurement units.
Numerous systems have appeared over history and many continue to see widespread use.
Earlier communities often set unit definitions locally and at times arbitrarily, so the magnitude tied to a familiar unit name could shift from place to place; for example, a "foot" might match a ruler's own foot length.
Growing trade and scientific collaboration created demand for standardized, rational measurements, prompting development of the International System of Units (SI) as a universal, coherent option.
Even with SI heavily promoted, other systems remain entrenched, especially United States customary units (UCS) and the imperial framework. Because older systems persist locally, knowing how to relate and convert among them is still valuable.
The three unit systems most commonly encountered today are:
- International System of Units (SI)
- United States customary units (UCS)
- Imperial system
International System of Units
The International System of Units (SI) represents the modern metric scheme. It draws on seven base units and leverages metric prefixes to express decimal multiples and submultiples. SI pursues coherence and rational structure, with many definitions tied to invariant natural constants, historically including the speed of light and the triple point of water.
SI continues to evolve: definitions are updated when more stable constants are identified or measured more precisely. It is the most widely used system worldwide.
SI base units
- ampere (symbol: A) - base unit for electric current
- kelvin (symbol: K) - base unit for temperature
- second (symbol: s) - base unit for time
- meter (symbol: m) - base unit for length
- kilogram (symbol: kg) - base unit for mass
- candela (symbol: cd) - base unit for luminous intensity
- mole (symbol: mol) - base unit describing amount of substance
Metric prefixes (in everyday use)
The table lists each prefix alongside its symbol, decimal factor, and power of ten.
| Text | Symbol | Factor | Power |
|---|---|---|---|
| exa | E | 1000000000000000000 | 1018 |
| peta | P | 1000000000000000 | 1015 |
| tera | T | 1000000000000 | 1012 |
| giga | G | 1000000000 | 109 |
| mega | M | 1000000 | 106 |
| kilo | k | 1000 | 103 |
| hecto | h | 100 | 102 |
| deca | da | 10 | 101 |
| (none) | (none) | 1 | 100 |
| deci | d | 0.1 | 10-1 |
| centi | c | 0.01 | 10-2 |
| milli | m | 0.001 | 10-3 |
| micro | μ | 0.000001 | 10-6 |
| nano | n | 0.000000001 | 10-9 |
| pico | p | 0.000000000001 | 10-12 |
| femto | f | 0.000000000000001 | 10-15 |
| atto | a | 0.000000000000000001 | 10-18 |
SI-derived units (examples)
- radian (symbol: rad) - unit of angle
- newton (symbol: N) - unit of force or weight
- watt (symbol: W) - unit of power
- volt (symbol: V) - unit of voltage, electrical potential difference, electromotive force
- degree Celsius (symbol: °C) - unit of temperature
Non-SI units accepted for use with SI (examples)
- minute, hour, day (symbol: min, h, d respectively) - units of time
- degree (symbol: °C) - unit of temperature
- liter (symbol: L) - unit of volume
- bar (symbol: bar) - unit of pressure
- millimeter of mercury (symbol: mmHg) - unit of pressure
History of SI (key points)
- Metric concepts began forming around 1791 through committees such as the French Academy of Sciences.
- France officially launched the metric system in 1799.
- The metric system spread across Europe during the 19th century.
- By the 1970s, metrication based on SI was effectively complete in most countries; notable exceptions include the United Kingdom, the United States, Liberia, and Myanmar.
Country highlights (key dates)
United States:
- 1866 - metric system legalized (but not widely used).
- 1975 - metric system formally adopted for government, military, trade, and commerce.
- 1992 - a Fair Packaging and Labeling Act amendment required both metric and US customary units on food labels for federally regulated consumer commodities (amended in 2010 to allow voluntary metric-only labeling).
- 2012 - a petition to "Make the Metric system the standard in the United States" prompted a White House response noting UCS definitions in metric terms and that metric adoption is a matter of choice rather than federal mandate.
United Kingdom:
- 1862 - legal allowance to use metric units.
- 1965 - government set a 10-year plan for metrication.
- 1969 - Metrication Board created.
- 1980 - Metrication Board abolished.
- 1989 - the UK chose not to mandate metric use; later retail movements have sometimes shifted back to imperial units in certain contexts.
- Following the UK's exit from the EU, some retailers have moved toward more imperial usage.
United States customary units (UCS)
UCS arose from English units used in the British Isles and continued in American practice. The United States also employs SI extensively in scientific, medical, industrial, and governmental settings.
History (key events)
- 1824 - creation of the imperial system (split units used in the UK and US).
- 1893 - Mendenhall Order redefined most US customary units in terms of meters and kilograms.
- 1959 - international yard and pound agreement refined UCS definitions vs. metric units.
- 1975 - Metric Conversion Act designated the metric system as the preferred system for U.S. trade and commerce.
Units of length (everyday UCS with SI equivalents; based on 1 yard = 0.9144 meters)
- inch (symbol: in) - 0.0254 meters
- foot - 0.3048 meters
- yard - 0.9144 meters
- mile - 1609.344 meters
Units of area
Square feet, square inches, and square yards remain typical examples.
The acre is the lone widely used area unit in UCS that is not simply defined through length; acre = 4,046.873 m2.
Units of volume
General approximate units of volume
- cubic inch (in3) - 0.0000164 meters3
- cubic foot (ft3) - 0.0283 meters3
- cubic yard (yd3) - 0.765 meters3
Approximate liquid volumes (U.S.)
- teaspoon (symbol: tsp) - 4.929 milliliters
- tablespoon (symbol: tbsp) - 14.787 milliliters
- fluid ounce (symbol: fl oz) - 29.574 milliliters
- cup (symbol: cp) - 236.588 milliliters
- pint (symbol: pt) - 473.176 milliliters
- quart (symbol: qt) - 946.353 milliliters
- gallon (symbol: gal) - 3785.41 milliliters
Approximate dry volumes (U.S.)
- dry pint (symbol: pt) - 550.610 milliliters
- dry quart (symbol: qt) - 1,101.221 milliliters
- dry gallon (symbol: gal) - 4,404.884 milliliters
Units of weight and mass (UCS)
Within UCS, mass commonly follows the avoirdupois scheme; troy weight appears less frequently. The avoirdupois pound, set in 1959, equals exactly 453.59237 grams (the entries below list rounded comparisons).
Example conversions listed (approximate SI equivalents):
- ounce (symbol: oz) - 28.350 grams
- pound (symbol: lb) - 453.592 grams
- ton (symbol: ton) - 907.185 kilograms
- long ton (symbol: long ton) - 1,016.047 kilograms
Units of temperature
Daily-life temperatures in the United States typically rely on degrees Fahrenheit; scientific contexts use degrees Celsius and kelvin.
Imperial system (British Imperial)
The imperial system (British Imperial) was defined in 1824, replacing the older English (Winchester) standards. The UK has formally adopted SI in many domains, but imperial-derived units remain common in everyday life; Canada and some former British territories also still use imperial measures in practice.
Key dates (high-level)
- 1818 - metric system adoption discussion in Parliament.
- 1824 - Weights and Measures Act created the imperial system.
- 1960, 1965, 1969, 1978, 1989, 1995 - sequential policy and metrication events in the UK (partial metrication completed officially in 1995, with specific exceptions such as draught beer, road signs, and speedometers).
Imperial units (examples and metric equivalents)
Units of length
- inch (symbol: in) - 0.0254 meters
- foot (symbol: ft) - 0.3048 meters
- yard (symbol: yd) - 0.9144 meters
- chain (symbol: ch) - 20.1168 meters
- furlong (symbol: fur) - 201.168 meters
- mile (symbol: mi) - 1,609.344 meters
- league (symbol: lea) - 4,828.032 meters (3 miles)
Units of area
- perch - 25.293 square meters
- rood - 1011.714 square meters
- acre - 4046.856 square meters
Units of volume (imperial)
Imperial units share names with some UCS units but use different values; the imperial system does not separate dry versus liquid volumes.
- fluid ounce (symbol: fl oz) - 28.413 milliliters
- gill (symbol: gi) - 142.065 milliliters
- pint (symbol: pt) - 568.261 milliliters
- quart (symbol: qt) - 1,136.523 milliliters
- gallon (symbol: gal) - 4,546.09 milliliters
Units of weight and mass (imperial)
- ounce (symbol: oz) - 28.350 grams
- pound (symbol: lb) - 453.592 grams
- stone (symbol: st) - 6.350 kilograms
- ton (symbol: t) - 1,016.047 kilograms