UnitConv
UnitConv

Unit Origins & Stories

Every unit we use daily has a name with a story behind it. The metre was born from the size of the Earth; horsepower from a contest with horses. Trace the origins, history and present-day definitions of the units around you.

Metre (m)

Origin

From the Greek métron (μέτρον), meaning "a measure". The very act of measuring became the unit’s name.

The Story

In 1791 the French Academy of Sciences defined one metre as one ten-millionth of the distance from the equator to the North Pole — a quarter of the meridian. A platinum-iridium International Prototype Metre was later made, serving as the world standard for length from 1889.

Today’s Definition

Since 1983 the metre has been defined as the distance light travels in a vacuum in 1/299,792,458 of a second, fixing the speed of light c at exactly 299,792,458 m/s.

Metre (m) →

Sources: Wikipedia (en): Metre

Kilogram (kg)

Origin

The Greek-derived prefix kilo- ("thousand") plus the French gramme (from Late Latin gramma) — literally "a thousand grams".

The Story

For over a century the kilogram was defined by the International Prototype of the Kilogram (nicknamed "Le Grand K"), a cylinder of 90% platinum and 10% iridium kept in a vault at the BIPM near Paris. It was the only physical mass standard — but over the decades the prototype and its copies drifted apart by tens of micrograms.

Today’s Definition

On 20 May 2019 the kilogram was redefined by fixing the Planck constant h at exactly 6.62607015×10⁻³⁴ J·s, and the 130-year-old physical prototype was finally retired.

Kilogram (kg) →

Sources: Wikipedia (en): Kilogram ·Wikipedia (en): International Prototype of the Kilogram

Second (s)

Origin

From the Latin secunda (pars) minuta, "the second small division". Dividing an hour into 60 gives the "minute" (the first small division); dividing that by 60 again gives the "second".

The Story

The second was long defined as 1/86,400 of a mean solar day (24 hours × 60 × 60). But the Earth’s rotation wobbles slightly, so a more stable reference was needed.

Today’s Definition

In 1967 the second was defined as the duration of 9,192,631,770 periods of the radiation from the transition between two hyperfine levels of the ground state of the caesium-133 atom — anchoring time to the atomic clock.

Second (s) →

Sources: Wikipedia (en): Second

Degree Celsius (°C)

Origin

Named after the Swedish astronomer Anders Celsius (1701–1744). The name "degree Celsius" and the symbol °C were adopted by the General Conference on Weights and Measures in 1948.

The Story

Celsius proposed his scale in 1742, but the original ran the opposite way to today’s: 0 was the boiling point of water and 100 the freezing point (higher numbers meant colder). The scale was reversed after his death (Carl Linnaeus and others are credited), giving 0 = freezing and 100 = boiling.

Today’s Definition

Today it is the everyday scale on which water freezes at 0°C and boils at 100°C (at standard pressure). One degree Celsius is defined to be exactly the same size as one kelvin.

Degree Celsius (°C) →

Sources: Wikipedia (en): Celsius ·Wikipedia (en): Anders Celsius

Degree Fahrenheit (°F)

Origin

Named after the German-born physicist and instrument-maker Daniel Gabriel Fahrenheit (1686–1736).

The Story

Fahrenheit established his scale around 1724. He set 0°F near the lowest temperature produced by a freezing mixture of water, ice and ammonium chloride (salt), and used roughly human body temperature as one reference point (the details are debated).

Today’s Definition

In the modern definition water freezes at 32°F and boils at 212°F — a span of exactly 180 degrees. It is still in everyday use in the United States and a few other places.

Degree Fahrenheit (°F) →

Sources: Wikipedia (en): Fahrenheit ·Wikipedia (en): Daniel Gabriel Fahrenheit

Kelvin (K)

Origin

Named after the Irish-born physicist William Thomson, 1st Baron Kelvin (1824–1907), who chose his title after the River Kelvin that flows past the University of Glasgow.

The Story

The SI unit of thermodynamic temperature, the kelvin is an absolute scale starting from absolute zero (the theoretical lowest temperature, where molecular motion stops, about −273.15°C). One kelvin is the same size as one degree Celsius, so adding 273.15 to a Celsius value gives kelvins.

Today’s Definition

Since the 2019 redefinition the kelvin is defined by fixing the Boltzmann constant k at exactly 1.380649×10⁻²³ J/K.

Kelvin (K) →

Sources: Wikipedia (en): Kelvin ·Wikipedia (en): William Thomson, 1st Baron Kelvin

Horsepower (hp)

Origin

Literally "the power of a horse" — a unit of power devised by the Scottish engineer James Watt (1736–1819).

The Story

Watt defined horsepower to market his steam engines by comparing them with the draught horses that were the main power source of the day, basing it on the rate at which a horse could lift a load. The SI unit of power, the watt, is in turn named after Watt himself.

Today’s Definition

One mechanical horsepower equals about 33,000 foot-pounds-force per minute (ft·lbf/min), which is roughly 745.7 watts.

Horsepower (hp) →

Sources: Wikipedia (en): Horsepower

Mile (mi)

Origin

From the Latin mille passus (milia passuum), "a thousand paces". Mille means "thousand".

The Story

A Roman mile was 1,000 "paces" — a pace being a full stride, one step of each foot — totalling about 1,480 m. Lengths varied across regions until an English statute of 1593 fixed the mile at 8 furlongs, or 5,280 feet.

Today’s Definition

The modern international mile is defined as exactly 1,609.344 m (1.609344 km).

Mile (mi) →

Sources: Wikipedia (en): Mile

Inch (in)

Origin

From the Latin uncia, "a twelfth" — the same root as the ounce. An inch is one twelfth of a foot.

The Story

It is traditionally said to derive from the width of a thumb. Medieval England also defined an inch as the length of three barleycorns laid end to end (1 inch = 3 barleycorns).

Today’s Definition

The modern international inch is defined as exactly 25.4 mm (2.54 cm).

Inch (in) →

Sources: Wikipedia (en): Inch

Knot (kn)

Origin

Literally from "knot". One knot is one nautical mile per hour (about 1.852 km/h), used for the speed of ships and aircraft.

The Story

In the age of sail, sailors threw a wooden chip on a rope (the log line) tied with knots at regular intervals into the sea. Counting how many knots ran out over a fixed time, measured by a sandglass, gave the ship’s speed — hence the name.

Today’s Definition

The nautical mile it is based on corresponds to one minute of latitude along a meridian and is now defined as exactly 1,852 m, so one knot is 1,852 m/h.

Knot (kn) →

Sources: Wikipedia (en): Knot (unit) ·Wikipedia (en): Nautical mile

Calorie (cal)

Origin

From the Latin calor, "heat". It is a unit of heat or energy.

The Story

One calorie is roughly defined as the heat needed to raise one gram of water by 1°C (1 K), though the value depends on the reference water temperature (giving variants like the 15°C calorie). Note that the dietary "Calorie" (Cal) is really a kilocalorie (kcal, 1,000 cal).

Today’s Definition

One calorie equals about 4.184 joules. The SI unit of energy is the joule, which is what science generally uses.

Calorie (cal) →

Sources: Wikipedia (en): Calorie

Frequently Asked Questions

Why were units redefined using physical constants?

Physical artefacts like a prototype can be lost, damaged or drift over time. Physical constants such as the speed of light and the Planck constant are the same everywhere in the universe, so basing units on them lets anyone reproduce the same unit anytime, anywhere, with far greater precision. In 2019 the kilogram, kelvin and others moved to this approach.

How was the metre first defined?

In 1791 the French Academy of Sciences defined one metre as one ten-millionth of the distance from the equator to the North Pole (a quarter of the meridian). After an era of physical prototypes, since 1983 it has been defined as the distance light travels in a vacuum in 1/299,792,458 of a second.

Is the dietary "Calorie" the same as the physics "calorie"?

They are actually different units. The food-label "Calorie" (Cal) is 1,000 times the physics calorie (cal) — that is, a kilocalorie (kcal). One physics calorie is the heat needed to raise one gram of water by about 1°C, roughly 4.184 joules.

Related Pages

Unit Systems GuideInternational System of Units (SI) Try a conversion