Select a unit category to see detailed information about available units and their relationships.
The earliest known measurement systems emerged in ancient Mesopotamia and Egypt. The cubit, based on the length of a forearm, was one of the first standardized units. The Egyptians developed the royal cubit around 3000 BCE, subdivided into palms and digits. Meanwhile, the Sumerians created the sexagesimal system (base 60), which still influences how we measure time and angles today. [Encyclopædia Britannica – History of Measurement]
The Romans developed a practical system of measurements that spread across their vast empire. The foot (pes) was the fundamental unit of length, divided into 12 inches (unciae). The mile (mille passus) was defined as 1000 paces. Roman weights used the libra (pound) and uncia (ounce), which gave us the abbreviation "lb" for pound. This system influenced medieval European measurements and persists in the imperial system. [World History Encyclopedia – Weights and Measures in the Roman World]
During the French Revolution, the need for a universal, decimal-based measurement system led to the creation of the metric system. The meter was defined as one ten-millionth of the distance from the North Pole to the Equator. The gram was based on the mass of one cubic centimeter of water at 4 °C. This system was designed to be simple, logical, and accessible to all people. [BIPM – History of the SI]
The modern SI system was established in 1960, building on the metric system with seven base units: meter, kilogram, second, ampere, kelvin, mole, and candela. In 2019, the SI underwent a historic revision, fixing the values of the Planck constant, elementary charge, Boltzmann constant, and Avogadro constant to redefine the units in terms of fundamental physical constants. [NIST – SI Redefinition] [NPL – SI Units]
The SI system has seven base units from which all other units are derived: the meter for length, the kilogram for mass, the second for time, the ampere for electric current, the kelvin for temperature, the mole for amount of substance, and the candela for luminous intensity. Derived units are combinations of base units, such as the newton (kg·m/s²) for force and the joule (kg·m²/s²) for energy. [NIST – SI Base Units]
Converting between units involves multiplying by a conversion factor — the ratio between two units of the same dimension. For example, 1 meter equals 3.28084 feet, so to convert meters to feet you multiply by 3.28084. This works for any units measuring the same physical quantity, from length and mass to pressure and energy. For a comprehensive set of exact conversion factors, see [NIST SP 811 – Guide for the Use of the SI].
Temperature is unique because scales have different zero points. The Kelvin scale starts at absolute zero (the lowest possible temperature). Celsius uses the freezing and boiling points of water at 0 °C and 100 °C. Fahrenheit places freezing at 32 °F and boiling at 212 °F. Converting between these scales requires both multiplication and addition due to the offset — unlike most other unit conversions that need only multiplication. [UK Met Office – What is Temperature?]
Metric prefixes allow very large or very small quantities to be expressed without changing the base unit. Each prefix represents a power of ten: kilo means 1,000 (10³), milli means one-thousandth (10⁻³), and micro means one-millionth (10⁻⁶). Moving between prefixes is simply a matter of shifting the decimal point. The full official table of SI prefixes is maintained by the BIPM. [BIPM – SI Prefixes]
The SI system is the modern form of the metric system and the world's most widely used measurement system. It is maintained by the International Bureau of Weights and Measures (BIPM) in Sèvres, France. The SI is used in science, international trade, and by the vast majority of countries worldwide. Its key advantages include coherence (derived units are products of base units with no extra factors), decimal multiples, and definitions based on invariant constants of nature. [BIPM – Measurement Units]
The imperial system evolved from British units standardized in the 19th century Weights and Measures Act. The US customary system shares many units with the imperial system but differs in key areas — the US liquid gallon (3.785 L) is smaller than the imperial gallon (4.546 L). These systems use feet, pounds, gallons, and Fahrenheit. While gradually replaced by SI in many applications, they remain common in the United States and United Kingdom for everyday use. [UK Metric Association – History of Imperial Measures]
Various specialized systems exist for specific fields. The astronomical system uses units like light-years and astronomical units (AU) for cosmic distances — 1 AU being the mean Earth–Sun distance of roughly 149.6 million km. The nautical system uses nautical miles and knots for maritime and aviation navigation. Digital storage conventionally uses powers of 1024 (kibibytes, mebibytes) for binary data, as standardized by IEC 80000-13. [IAU – Measuring the Universe]
The trend in modern metrology is toward defining units by fundamental physical constants rather than physical artifacts. This provides stability and precision that no material object can match. The 2019 SI revision marked a landmark milestone in this direction, redefining the kilogram via the Planck constant, eliminating the last artifact-based definition. Future developments in quantum metrology and optical atomic clocks continue to push the frontiers of measurement precision. [PTB – What is the New SI?]