Reference
How each unit got here.
Every family in the converter has a history: a body part, a royal decree, a platinum bar, or a committee. Below is the short version of how each quantity developed, and one fact worth keeping.
Length
Length began as anatomy and landscape. The cubit was a forearm, the foot a royal shoe, the fathom a sailor’s outstretched arms, the mile a thousand Roman double-paces (mille passus).
The metre was born in the 1790s as one ten-millionth of the distance from the equator to the North Pole through Paris, then recast as a platinum bar, a krypton wavelength, and, since 2019, a fixed speed of light. In 1959 the US, UK, Canada, Australia, New Zealand, and South Africa locked the international yard at exactly 0.9144 m, so the inch is 25.4 mm with no remainder.
Fun factA fathom is six feet of sounding line. The word also means “to understand” — to get to the bottom of something. That is why this converter is called Fathom.
Area
An acre was the strip a yoke of oxen could plough in a day: four rods wide by one furlong long, or 4,840 square yards. The hectare arrived with the metric system as a square 100 metres on a side.
The international acre follows the 1959 yard, so it is exactly 4,046.8564224 m². The US survey acre, retired for most federal work in 2023, differed by a few parts per million. A barn (10⁻²⁸ m²) was coined by Manhattan Project physicists who needed a unit for nuclear cross-sections “as big as a barn”.
Fun factA rood is a quarter-acre. English wills still occasionally leave “three roods of meadow”, which is why the unit refuses to die in the converter.
Volume
Wine, ale, and grain each had their own gallon in medieval England. The 1707 Queen Anne wine gallon — 231 cubic inches — became the US liquid gallon. Britain unified volume in 1824 around the imperial gallon, later fixed at 4.54609 litres.
The litre was originally a cubic decimetre and is now exactly 0.001 m³. US fluid ounces, pints, and quarts all divide the 231-cubic-inch gallon; imperial fluid ounces divide a larger gallon into 160 parts, so a US pint is 16 US fl oz and an imperial pint is 20 imperial fl oz.
Fun factAn oil barrel is 42 US gallons because that was the wooden tierce nineteenth-century Pennsylvania drillers could roll. OPEC still quotes production in those barrels, not cubic metres.
Mass
The grain — the mass of a barleycorn — is one of the oldest surviving units and still underpins the avoirdupois pound (7,000 grains) and the troy pound (5,760 grains) used for precious metals.
The kilogram was a cylinder of platinum-iridium in a vault at Sèvres until 2019, when SI redefined it via the Planck constant. The international pound has been exactly 0.45359237 kg since 1959. Short tons (2,000 lb), long tons (2,240 lb), and tonnes (1,000 kg) remain three different “tons”.
Fun factA stone is 14 pounds and is still how many people in the UK describe body weight. The US never adopted it for people, which is why American bathroom scales look oddly decimal to British eyes.
Temperature
Daniel Fahrenheit (1724) used a brine zero, human-body heat, and a 180-degree span between ice and steam. Anders Celsius (1742) originally put 100 at ice and 0 at steam; colleagues flipped the scale after his death. William Thomson, Lord Kelvin, later counted from absolute zero.
The kelvin is now defined by Boltzmann’s constant. Celsius shares the kelvin’s size and offsets by 273.15. Rankine is Fahrenheit counted from absolute zero and still appears in some US engineering tables. Réaumur and Newton scales survive mainly as historical curiosities.
Fun fact−40 °C is exactly −40 °F. It is the only temperature where the two everyday scales agree, which is why meteorology quizzes love it.
Speed
A knot is one nautical mile per hour, read from a chip log: a wooden board, a line knotted at intervals, and a 28-second sandglass. Statute miles per hour followed roads and railways; kilometres per hour followed metric road signs.
The international nautical mile is exactly 1,852 metres, so a knot is exact in SI. Mach number is not a unit of speed but a ratio to the local speed of sound; converters that treat “Mach 1” as a fixed km/h are quoting a sea-level convention, not a law of nature.
Fun factThe phrase “knots in the line” is literal. Early logs really did have knots, and a sailor counted how many ran out while the glass emptied.
Time
Hours, minutes, and seconds are Babylonian sexagesimal leftovers: 60 because it divides cleanly by 2, 3, 4, 5, and 6. Days followed the Sun; weeks, oddly, the seven classical “planets”.
The second was a fraction of the mean solar day, then of the tropical year, and since 1967 a count of caesium-133 hyperfine transitions. Leap seconds paper over the Earth’s slowing spin. The SI prefixes (ms, µs, ns) and the engineering minute/hour/day sit alongside that atomic second.
Fun factThere are not 24 × 60 × 60 SI seconds in every calendar day. When a leap second is inserted, that UTC day is 86,401 seconds long.
Pressure
Evangelista Torricelli’s 1643 mercury column gave millimetres of mercury (torr). Blaise Pascal’s work on fluids later named the SI unit. Engineers kept pounds per square inch because boilers and tyres were already stamped that way.
The pascal is one newton per square metre. A bar is 100,000 Pa, close to one atmosphere (101,325 Pa) and still the language of meteorologists and divers. Standard atmospheres, torr, and psi persist because instruments and codes of practice change slower than textbooks.
Fun factA millibar and a hectopascal are the same number. Weather maps quietly switched labels without changing a single isobar.
Energy
James Joule measured the mechanical equivalent of heat in the 1840s, tying foot-pounds to the warming of water. Calories counted food and chemistry; British thermal units counted steam coal.
The joule is a newton-metre, or a watt-second. The thermochemical calorie and the International Table calorie differ slightly; nutrition “calories” on packets are kilocalories. Electronvolts dominate particle physics because a joule is an absurdly large bag for one electron.
Fun factA dietary Calorie (capital C) is 1,000 small calories — about 4,184 J. A banana is roughly 400 kJ, which sounds more alarming until you remember the factor of a thousand.
Power
James Watt needed a way to sell steam engines against horses. He defined one horsepower as 550 foot-pounds per second after watching brewery horses, and billed customers in those units for decades.
The watt is a joule per second. Mechanical horsepower (≈ 745.7 W), metric Pferdestärke (75 kgf·m/s), and boiler horsepower are not the same, which is why engine plates still cause arguments. Kilowatts won for electricity because they divide a grid cleanly.
Fun factWatt’s “horse” was a strong brewery horse working a mill, not a typical riding horse. A real horse can sustain closer to one-third of a mechanical horsepower over a working day.
Force
Weight was force long before mass was distinguished from it. The kilogram-force and the pound-force let engineers write “kg” on a spring scale and mean a pull, not a lump of matter.
The newton (1 kg·m/s²) made force coherent in SI. The dyne (CGS) is 10⁻⁵ N. Pound-force remains on every US torque wrench and tyre gauge. A kilogram-force is the weight of one kilogram at standard gravity, 9.80665 m/s².
Fun factThe slug exists so that F = ma still works in feet and pounds: one slug accelerated at 1 ft/s² produces one pound-force. It is the US customary cousin of the kilogram, and almost nobody says it out loud.
Torque
Torque — a twist — was measured as a force at the end of a spanner long before it had a dedicated SI name. Mechanics still say “foot-pounds” for the twist that tightens a bolt.
The newton-metre is the SI unit. Pound-force-feet and pound-force-inches dominate US workshop charts. Energy is also a newton-metre (the joule), so context has to say whether you are twisting or lifting; they are different quantities that share a dimension.
Fun factA 1 N·m torque is roughly the twist of a small apple hanging from a metre-long string. That is the actual image used in some metrology lectures.
Acceleration
Galileo timed balls on ramps and found a constant acceleration near the Earth’s surface. The gal (or galileo) later honoured him in CGS as 1 cm/s², still used in gravimetry.
SI uses metres per second squared. Standard gravity g₀ is defined as 9.80665 m/s² so that “one g” is exact in converters even though local g varies with latitude and altitude. Inches per second squared appear in vibration specs.
Fun factA gal is tiny on purpose. Geoscientists hunting oil or mapping magma chambers care about millionths of g, so milligals and microgals are everyday words in that trade.
Density
Specific gravity — density relative to water — let merchants check whether wine had been watered down without owning a kilogram. Archimedes had the idea two millennia earlier.
Kilograms per cubic metre is the SI unit; grams per cubic centimetre is the same number as specific gravity for water at 4 °C. Pounds per cubic foot still run through US lumber, HVAC, and soil reports.
Fun factIce is about 917 kg/m³, which is why it floats. The 9% expansion as water freezes is also why pipes burst and why granite can split on a cold night.
Flow rate
Irrigation and mill-races were measured in whatever container a district already had: miner’s inches in the American West, cusecs (cubic feet per second) on British rivers, litres per second once metric waterworks arrived.
Cubic metres per second is the SI unit. Litres per minute dominate taps and aquarium pumps; gallons per minute split, inevitably, into US and imperial. Petroleum still likes barrels per day.
Fun factA miner’s inch was the water that would flow through a one-inch hole under a specified head — and California, Colorado, and Arizona never quite agreed on the head, so the “inch” wandered by tens of percent.
Angle
Babylonian astronomers divided the circle into 360 parts, probably because 360 is close to a year’s days and divides neatly. Radians — arc length over radius — waited for calculus.
The radian is the SI unit and is dimensionless. Degrees remain the public language of navigation and school geometry. Gradians (400 to a circle) were a French metric experiment that surveyors in some countries still use. Turns (1 = 360°) show up in modern APIs.
Fun factA mil in artillery is not a millimetre of angle: NATO defines 6400 mils in a circle so that 1 mil is almost exactly 1 metre at 1 km. That is a battlefield gift, not a coincidence.
Frequency
Frequency was “cycles per second” until 1930, when the IEC named the hertz after Heinrich Hertz, who generated and detected radio waves in the 1880s.
RPM is still how engines and turntables are specified (revolutions per minute). Musical pitch is hertz; mains electricity is 50 or 60 Hz depending on which ocean you grew up beside. Angular frequency in radians per second is 2π times hertz.
Fun factA-440, the note orchestras tune to, was not always 440 Hz. Nineteenth-century pitch wandered from about 420 to 450 Hz, which is why period-instrument ensembles still argue about “Baroque pitch”.
Data
Claude Shannon named the bit in 1948. The byte settled at eight bits once IBM System/360 made that width a commercial fact. Prefixes then collided: engineers used 2¹⁰ = 1,024 as a “kilo”, while the SI kilo is 1,000.
IEC binary prefixes (kibi, mebi, gibi) were introduced in 1998 so that a kibibyte is 1,024 bytes and a kilobyte can mean 1,000. Drive makers use decimal SI; memory and some operating systems historically used binary while still printing “GB”. Fathom labels both.
Fun factA “floppy disk” 1.44 MB was a hybrid lie: 1,440 KiB, advertised with a decimal-looking 1.44 and a binary megabyte. The industry has been confusing customers on purpose since 1987.
Data rate
Baud, named for Émile Baudot, counts symbols per second on a telegraph line. Once a symbol could carry more than one bit, bits per second and baud parted company.
Networking standardised on bits per second (and kilobits, megabits, gigabits). Bytes per second remain the language of file copies. The factor of eight between MB/s and Mb/s is the most common trap on broadband boxes.
Fun factA “56k modem” never really hit 56,000 bit/s. US law limited copper phone power, so the practical ceiling was 53.3 kbit/s — a number burned into a generation’s patience.
Numeral systems
Roman numerals carved well in stone and poorly in arithmetic. Hindu–Arabic digits, with a true zero, reached Europe through the Islamic world and made place value ordinary. Binary is as old as I Ching hexagrams and as new as Boolean algebra.
Positional bases from 2 to 36 are a single idea: each place is a power of the base. Hexadecimal became the human face of machine bytes because two hex digits are one byte. Roman numerals survive on clocks, monarchs, and Super Bowl branding.
Fun factThere is no zero in classical Roman numerals, which is why year “0” does not exist in the AD/BC system and historians still argue about whether the millennium started in 2000 or 2001.
Fuel economy
Miles per gallon is an efficiency: distance per volume. Litres per 100 kilometres, common in Europe and much of the world, is a consumption: volume per distance. They are reciprocals, not scaled copies.
US mpg uses the US gallon; UK mpg uses the imperial gallon, so the same car posts two different mpg numbers. Japan’s km/L sits with mpg as an efficiency. Converting 10 L/100 km is not “multiply by ten” — it is an inverse.
Fun factCAFE stickers in the US once used a harmonic mean of city and highway mpg. That is why a car rated 20 mpg city and 40 mpg highway is not a 30 mpg combined car — it is 26.7.
Cooking
Recipes were “a teacup of flour” until Fannie Farmer, in 1896, campaigned for level measured cups in Boston. The US cup became 8 US fluid ounces. Commonwealth cups drifted toward 250 ml.
A US cup is 236.588 ml; a metric cup is 250 ml; a traditional UK cup was often 10 imperial fluid ounces (284 ml). Tablespoons and teaspoons follow their local fluid ounce, which is why a UK tablespoon is not a US tablespoon.
Fun factA pinch is sometimes standardised as 1/16 of a teaspoon, a dash as 1/8. Cooks who actually cook still use two fingers and a shrug, which is historically correct.
Electric current
André-Marie Ampère gave current its name and its right-hand rule. Early standards used silver deposits and the mass of metal plated in an hour.
The ampere is now fixed by the elementary charge: one ampere is a flow of 1 / (1.602176634×10⁻¹⁹) elementary charges per second. CGS electromagnetic units (abampere) are 10 A; electrostatic units are a different beast entirely.
Fun factHousehold circuits in much of the world are 10–16 A at the breaker. A single USB-C laptop charger can pull more current at 5 V than a 1920s house was wired to deliver at all.
Voltage
Alessandro Volta’s pile (1800) was the first battery. Voltage — electric potential difference — inherited his name. Telegraphers later spoke of “pressure” in volts the way plumbers speak of head.
The volt is a joule per coulomb, or a watt per ampere. Mains settled at about 100–127 V in the Americas and 220–240 V almost everywhere else, a split that still decides whether you pack a transformer. The CGS statvolt is nearly 300 V.
Fun factA lemon battery makes about 0.9 V. It takes a series of them to light an LED, which is why school science-fair lemons always look slightly ashamed.
Resistance
Georg Ohm related current, voltage, and resistance in 1827 and was ignored for years. The ohm later became the SI unit; the siemens (mho, ohm backwards) is its reciprocal.
A 2019 SI rewrite tied the ohm to the von Klitzing constant from the quantum Hall effect, so resistance standards are now counted in fundamental constants rather than a mercury column of specified bore.
Fun factResistor colour bands include a gold stripe for 5% tolerance because gold was the stripe they could print that did not look like orange under factory lights. The joke writes itself.
Electric charge
Charge was “quantity of electricity”. Charles-Augustin de Coulomb torsion-balanced pith balls in the 1780s; the coulomb later became the SI unit, equal to an ampere-second.
The elementary charge is now an exact SI number. Ampère-hours remain how batteries are sold: a 2 Ah cell can theoretically deliver 2 A for an hour. Faraday’s constant counts the charge of a mole of electrons and still runs electrochemistry.
Fun factA mole of electrons is about 96,485 C — Faraday’s constant. That is also why a car battery that stores a few hundred ampere-hours is moving moles of charge, not metaphors.
Magnetic field
Carl Friedrich Gauss mapped the Earth’s field in the 1830s; the gauss became the CGS unit of magnetic flux density. Nikola Tesla’s name later went to the much larger SI unit.
One tesla is 10,000 gauss. Earth’s field is about 30–60 µT (0.3–0.6 G). MRI scanners run at 1.5–3 T, which is why a paperclip in that room is a bad idea. Oersted and ampere per metre measure magnetising field, a different quantity that converters must not mix with tesla.
Fun factA fridge magnet is roughly 5 mT on its face — about 100 times Earth’s field, and about a thousandth of a hospital magnet. Scale is the whole story.
Illuminance
Candlepower was the light of a specified spermaceti candle. The candela replaced it as an SI base unit; lux is lumens per square metre on a surface, foot-candles the imperial cousin.
The candela is now defined by the luminous efficacy of 540 THz monochromatic light. Lux governs workplaces and photography; nits (cd/m²) govern screens. A foot-candle is the illuminance of one lumen on a square foot, about 10.76 lx.
Fun factA full moon on a clear night is about 0.25 lux. A brightly lit office is 300–500 lux. Your eyes span that range without a settings menu, which is why camera histograms look so dramatic.
Radioactivity
The curie was originally the activity of one gram of radium-226, a tribute by the nuclear community to Marie and Pierre. The becquerel, one decay per second, is the SI replacement.
One curie is exactly 3.7×10¹⁰ Bq. Rutherford (one megabecquerel) appears in older papers. Activity counts decays, not the harm those decays do — that is dose, a different family.
Fun factA banana yields a few becquerels of potassium-40. The “banana equivalent dose” is a joke unit that is also, unfortunately, the most successful public-radiation metaphor ever coined.
Absorbed dose
Early X-ray workers measured exposure by how long it took to redden skin. The rad (radiation absorbed dose) put a number on energy deposited per mass: 100 erg/g.
The gray, named for Louis Harold Gray, is 1 J/kg and equals 100 rad. Absorbed dose does not by itself say how dangerous a radiation type is; that weighting is equivalent dose, in sieverts.
Fun factA dental X-ray is typically a few microsieverts of equivalent dose, not grays. Mixing gray and sievert is how radiation stories go wrong in newspapers.
Equivalent dose
The rem (roentgen equivalent man) tried to say “as harmful as this much X-ray”. Quality factors for alpha, beta, and neutron radiation made a single number possible, if a little fictional.
The sievert is 1 J/kg weighted by radiation type and, in some contexts, tissue. One sievert is 100 rem. Public limits are millisieverts per year; medical doses are quoted per procedure.
Fun factA transatlantic flight is about 30–80 µSv from cosmic rays. Airline crews are among the most irradiated workers who are not in a nuclear plant, which is why their hours are treated as a dose budget.
Typography
Metal type was measured in points that varied by country: the Didot point in continental Europe, the American point that Adobe later froze. A pica is 12 points; an em is the body height of the type.
Desktop publishing defined the point as exactly 1/72 of an international inch. CSS later defined a px as 1/96 in at the reference pixel. Printers still talk picas; screens talk pixels; both sit on that 1959 inch.
Fun factThe word “leading” (pronounced ledding) is the extra space between lines, named for the strips of lead compositors slipped between type. Every modern design tool still uses a metal-shop metaphor.
Viscosity
Jean Léonard Marie Poiseuille studied blood flow in capillaries. The poise (CGS) and the stokes (kinematic viscosity) remember that nineteenth-century physiology.
SI uses pascal-seconds for dynamic viscosity and square metres per second for kinematic. Centipoise remains how motor oil and honey are quoted because water is about 1 cP at 20 °C — a convenient benchmark. Saybolt and Engler seconds survive in older petroleum specs.
Fun factPitch, the black tar in the University of Queensland pitch-drop experiment, has a viscosity around 10¹¹ times that of water. The eighth drop fell in 2014. Nobody has ever seen a drop hit the beaker.