Ancient Units (3000 BCE – 500 CE)
Early length units were based on the human body or everyday objects:
- Cubit: The distance from the elbow to the tip of the middle finger, about 44–52 cm. Used throughout the ancient Near East. Egyptian and Mesopotamian cubits differed, which caused confusion in trade.
- Foot: The length of a human foot, about 25–35 cm. Various cultures used different foot standards.
- Pace: Two steps, about 1.5 m. Used by Roman armies for measuring distances.
- Stade: 600 Greek feet (about 185 m), the length of a footrace in ancient Greece. This is the origin of "stadium".
- Mile: From the Roman "mille passus" — 1,000 paces, about 1,480 m. This was the ancestor of the modern mile.
Ancient standardization was difficult because different regions used different lengths for the same name. Trade required conversion, and this was often done by local officials whose standards had official status.
Medieval Standardization (500 – 1500 CE)
Medieval Europe had a chaotic collection of length units, often varying from town to town. Monarchs periodically issued decrees fixing standards:
- Inch: Traditionally defined as the length of three barleycorns placed end to end. This gave a consistent but approximate standard.
- Foot: Often defined as the length of the king's foot, which changed with each monarch. Edward I of England (1272–1307) standardized the foot as 12 inches.
- Yard: The distance from King Henry I's nose to the tip of his outstretched thumb — a definition that was convenient but not replicable.
- Rod/pole/perch: 16.5 feet, used in land measurement.
- Chain: 66 feet (4 rods), invented by Edmund Gunter in 1620 for surveying.
- Furlong: 10 chains = 660 feet, the length of a furrow in a standard plowed field.
- Mile: Redefined as 8 furlongs = 5,280 feet. This gave the modern mile its awkward length because it was derived from the furrow and chain rather than from a decimal system.
The Metric Revolution (1791 – 1799)
The French Revolution brought both political and metrological change. Before the revolution, France had over 250,000 different units of measure, many with local variants. The revolutionary government commissioned a committee (including Lavoisier, Lagrange, and Laplace) to design a rational replacement.
The plan was to base the fundamental length unit on a natural constant. The meter was defined as exactly one ten-millionth of the distance from the North Pole to the equator along a meridian passing through Paris. Surveyors spent seven years measuring the arc between Dunkirk and Barcelona to establish this distance accurately.
Initial results were slightly off (due to a small calculation error in the flattening of the Earth), but by then the meter had been defined and legalized. Later refinements (1960, 1983) moved away from the Earth-based definition entirely.
Other metric units
The meter was part of a full system:
- Liter: Volume of a cube 0.1 m on each side.
- Gram: Mass of 1 cm³ of water at 4 °C.
- Second: Retained from earlier (the sexagesimal division of the day).
- Prefixes: Decimal, from milli (10⁻³) to myria (10⁴), later replaced by kilo, mega, etc.
International Standardization (1875 – 1960)
In 1875, the Treaty of the Meter established the Bureau International des Poids et Mesures (BIPM) in Sèvres, France, and distributed platinum-iridium prototype meters and kilograms to member nations. The meter was defined as the distance between two marks on a specific bar, which was kept at the BIPM.
By 1960, the meter was redefined using the wavelength of light from krypton-86, providing a more reproducible standard. In 1983, it was redefined once more using the speed of light in a vacuum, which remains the current definition.
The SI System (1960 – Present)
The International System of Units (SI) was formally established in 1960 as a modernized version of the metric system. Its seven base units (meter, kilogram, second, ampere, kelvin, mole, candela) cover every physical quantity used in science and engineering.
In 2019, the SI was revised so that all base units are defined by fixing constants of nature:
- Meter: Defined by the speed of light (299,792,458 m/s exactly).
- Kilogram: Defined by the Planck constant (6.62607015 × 10⁻³⁴ J·s exactly).
- Second: Defined by the caesium-133 hyperfine transition (9,192,631,770 Hz exactly).
These definitions are stable, reproducible anywhere in the universe, and do not depend on any physical artifact.
The Survivors: Customary and Imperial Units
Despite the metric system's dominance, some customary and imperial units remain in daily use:
- United States: Feet, inches, yards, miles, pounds, gallons, Fahrenheit.
- United Kingdom: Miles for road distances, pints for beer, stones for body weight (informally).
- Aviation: Feet for altitude, nautical miles for distance, knots for speed — worldwide, even in metric countries.
- Construction: Inches, feet, and pounds remain common in the US construction industry.
Since 1959, US customary units have been defined in terms of SI units (1 in = 2.54 cm exactly, 1 lb = 0.45359237 kg exactly), so they are technically part of the metric world.
Why the Metric System Won
The metric system succeeded because it was designed for consistency:
- Decimal: Multiples are powers of ten, so calculations are simple.
- Coherent: Derived units are products of base units without any numeric factors.
- Universal: The same units are used for all quantities, and prefixes are consistent.
- Reproducible: Modern definitions rely on fundamental constants, not on physical objects.
- International: Maintained by the BIPM, with formal recognition in nearly every country.
By contrast, customary systems accumulated over centuries with little coordination, and they contain many conversion factors that do not reflect any natural or logical relationship — 12 inches per foot, 3 feet per yard, 5,280 feet per mile.
A Timeline of Key Definitions
| Year | Event |
|---|---|
| ~3000 BCE | Egyptian cubit standardized on a royal master rod |
| ~500 BCE | Roman mile = 1,000 paces |
| 1324 | Edward II defines the inch as 3 barleycorns |
| 1593 | Elizabeth I defines the mile as 5,280 feet |
| 1799 | French meter prototype deposited in Paris |
| 1875 | Treaty of the Meter establishes BIPM |
| 1959 | International yard and pound agreement (US, UK, and Commonwealth) |
| 1960 | SI formally established; meter = krypton-86 wavelength |
| 1983 | Meter redefined via speed of light |
| 2019 | SI redefined via fundamental constants (kg, A, K, mol) |