107 Fun Facts About Metric System

Which of these quantities is measured by one of the seven SI base units?

Time. Its unit, the second, was once a set fraction of the Earth's day, but the planet's spin proved too unsteady, so since 1967 atomic clocks have defined it.

The metric prefix "kilo" multiplies a unit by what factor?

1,000. Computing muddied it by calling 1,024 bytes a kilobyte, so in 1998 a separate prefix, kibi, was coined to leave kilo meaning exactly a thousand.

Since a six-nation agreement in 1959, which American unit of length has been defined as exactly 0.9144 metres?

The yard. English-speaking nations had slightly different yards, so to agree on one value the American yard was shortened and the British one lengthened.

A popular myth says the United States and which two other countries are the last holdouts against the metric system?

Liberia and Myanmar. NIST calls it a myth spread by an old CIA Factbook list: every country has adopted the SI, and even US feet and inches are officially defined in terms of the metre.

The 1795 law that defined the metre and gram also introduced which unit of currency?

The franc. The same reform swept away royal money too: the old livre was exchanged for the new currency at 81 livres to 80 francs.

On the eve of the Revolution, roughly how many different units of measure were in use across France?

About 800. Each could change from town to town and even from trade to trade, so by one estimate they hid a quarter of a million different local measures.

The 1791 definition of the metre made it one ten-millionth of the distance from the equator to what?

The North Pole. Surveyors could measure only a stretch of that line and scale it up, and the metre they produced was later found to be about 0.2 mm short.

In medicine and engine sizes, one 'cc' is exactly the same volume as one what?

Millilitre. A cc is a cubic centimetre, which is why a 5 cc syringe holds 5 ml and a 750 cc motorbike engine displaces three-quarters of a litre.

Which British inventor called in 1783 for a global decimal measurement system based on water's density?

James Watt. He was tired of converting the pounds and ounces in German chemists' reports, which changed from one part of the empire to the next.

Which chemist worked with René Just Haüy in 1793 to define the grave, the unit that became the kilogram?

Antoine Lavoisier. He and Haüy made the measurements that fixed the unit, the year before Lavoisier was sent to the guillotine.

Which French ruler rolled back the metric system, letting shopkeepers use the old pound and foot again?

Napoleon. His 1812 'mesures usuelles' kept the familiar names but tied them to metric sizes, so a livre became exactly 500 grams, and France went fully metric again only in 1840.

An 1875 measurement treaty's signing date, 20 May, is marked as what?

World Metrology Day. The kilogram's new definition took effect on that day in 2019, and UNESCO made it an official international day in 2023.

The international prototypes of the metre and kilogram were made of platinum alloyed with which metal?

Iridium. Copies in the same 90:10 alloy were sent out to the nations that signed the Metre treaty; the United States received kilograms number 4 and 20.

Which country's weather forecasts switched from Fahrenheit to Celsius on April Fools' Day 1975?

Canada. It was the opening move of a metric campaign that put kilometres on the highways two years later, though Canadians still tend to give their own weight in pounds.

Le Grand K, the metal cylinder that defined the kilogram until 2019, is about the size of what?

Golf ball. Its platinum alloy is almost twice as dense as lead, so a full kilogram of it sits easily in the palm of a hand.

The 2019 SI revision defines the kilogram by fixing the value of which constant?

The Planck constant. With its value fixed, a Kibble balance can weigh an object against a precisely measured electromagnetic force, so no master cylinder in a vault is needed.

The metre's definition fixes which quantity at 299,792,458 metres per second?

The speed of light. Since 1983 a metre has been however far light travels in a vacuum in 1/299,792,458 of a second, so sharper measurements refine the metre rather than light's speed.

Before 1983, the metre was defined by wavelengths of light from which element?

Krypton-86. A metre was 1,650,763.73 wavelengths of its orange-red glow, so any well-equipped lab could reproduce the unit without travelling to compare a metal bar.

The SI launched in 1960 with six base units; which one was added as the seventh in 1971?

Mole. For decades it was defined through 12 grams of carbon-12, so the chemists' unit quietly depended on the kilogram cylinder locked in a French vault.

The second has been defined since 1967 by the hyperfine transition frequency of which atom?

Caesium-133. Louis Essen built the first accurate caesium clock in Britain in 1955, and twelve years later its 9,192,631,770 ticks became the world's second.

Chemistry's constant 6.02214076 × 10^23 is a number named after which Italian scientist?

Amedeo Avogadro. He never knew the figure: it was named in his honour in 1909, half a century after his death, by the physicist Jean Perrin.

In 1954 the kelvin was defined by fixing 273.16 K as the temperature of what?

The triple point of water. It is the one temperature at which ice, liquid water and vapour all coexist; since 2019 the kelvin has instead been defined by fixing the Boltzmann constant.

The Celsius scale was known in English by what name until it was renamed in 1948?

Centigrade. The new name honoured Anders Celsius, whose original 1742 thermometer ran the other way, with 0 at boiling and 100 at freezing.

Which of these electrical units honours a French scientist?

The ampere. Defined at an 1881 congress in Paris, it honours André-Marie Ampère, who showed in 1820 that two current-carrying wires attract or repel each other.

The candela is the SI unit of what?

Luminous intensity. The unit's name simply means candle, and an ordinary wax candle still gives off roughly one candela.

A litre is exactly equal to which volume?

One cubic decimetre. That is a cube just 10 cm on each side; the name comes from the old French litron, itself descended from a Greek word for a pound weight.

From 1901 to 1964, the litre was defined as the volume of one kilogram of what?

Pure water. Careful measurement showed this litre was about 28 parts per million too big, so the water definition was scrapped in 1964.

Which unit of land area equals a square 100 metres on each side?

The hectare. Its name means a hundred ares, the 10-metre squares of France's first metric law, and it works out at about two and a half acres.

The tonne, or metric ton, corresponds exactly to which SI unit?

The megagram. A tonne is a million grams, yet that official name is rarely used; 'tonne' survives from the tun, a huge wine cask that weighed about as much when full.

Why does the SI rule out a unit like the 'millikilogram'?

Only one prefix per name. The kilogram is the one base unit that already carries a prefix, so its multiples and fractions are built on the gram instead.

Which four metric prefixes were adopted in 2022?

Ronna, quetta, ronto and quecto. R and Q were the only letters not already used in measurement, and the new names mean the Earth can be said to weigh six ronnagrams.

Which now-obsolete prefix of the original metric system multiplied a unit by 10,000?

Myria. English keeps the same Greek root in 'myriad'; the prefix itself was dropped as obsolete when the SI was drawn up in 1960.

Kilo, hecto and deca came from Greek; which language gave the metric system milli, centi and deci?

Latin. The designers of 1795 drew multiples from Greek and fractions from Latin, so the language of a prefix told you at once whether a unit had grown or shrunk.

The metric carat, fixed at exactly 200 milligrams, is the unit used for weighing what?

Gemstones. The carat began as the weight of a carob seed and varied from place to place until 1907, when it was fixed at a fifth of a gram.

Which single SI unit of energy replaced older units such as the calorie?

The joule. It honours James Joule, whose experiments in the 1840s showed that mechanical work turns into heat at a fixed rate, which is why one unit can measure both.

Metric units have been legal for US trade since the year after which war ended?

The Civil War. The Metric Act of 1866 made the system lawful in contracts and commerce but compelled no one, so Americans have had the right to go metric for over 150 years.

Which drinks have been sold in America only in metric bottle sizes since around 1980?

Wine and spirits. The 750 ml bottle replaced the 'fifth', once a fifth of a US gallon, making the liquor aisle one of the few all-metric corners of an American shop.

Which president signed the 1970s Metric Conversion Act, which made going metric national policy but purely voluntary?

Gerald Ford. Signing it in December 1975, he stressed that conversion would be 'completely voluntary'; the board created to coordinate the change was wound up in 1982.

Confusion between which two units helped cause the 1983 'Gimli Glider' fuel-exhaustion incident?

Pounds and kilograms. The airliner left with half the fuel it needed and ran dry in mid-air, but its captain, an experienced glider pilot, landed it on an old airstrip at Gimli.

As of 2024, British traders could still sell gold by which non-metric unit?

Troy ounce. Going metric became British government policy in 1965, yet the law still exempts precious metals, which trade in a unit used since the Middle Ages.

As of 2024, British law mandated imperial units alone for which of these?

Speed and distance on road signs. Petrol went over to litres and shop scales to kilograms, but limits are still posted in miles per hour and distances in miles.

Which unit, still used by ships and aircraft, has been fixed since 1929 at exactly 1,852 metres?

The nautical mile. It began as one minute of latitude, which lets navigators read distances straight off a chart, and no 'nautical kilometre' ever replaced it.

Before settling on the Earth's meridian, France proposed in 1790 to define the metre by what?

A one-second pendulum. French scientists dropped it for the meridian within a year, because gravity makes such a pendulum's length vary from place to place.

Where is the headquarters of the International Bureau of Weights and Measures (BIPM)?

Saint-Cloud, near Paris. France handed the new bureau the Pavillon de Breteuil, a former royal pavilion in the park of Saint-Cloud, where the prototype metre and kilogram were locked away.

Privateers captured Joseph Dombey in 1794 before he could deliver France's new metric standards to which American statesman?

Thomas Jefferson. Storms drove Dombey's ship into the Caribbean, where British privateers seized it; he died a prisoner on Montserrat, and the standards never reached Jefferson's hands.

Which US president signed an early-1990s executive order telling federal agencies to use the metric system?

George H. W. Bush. His Executive Order 12770 covered federal purchasing, grants and other business, but it placed no duty on the public, so everyday American life stayed customary.

Which is the only SI prefix whose symbol is a Greek letter?

Micro. Its symbol μ is so easily mistaken for an m that US medication-safety guidance tells prescribers to write 'mcg' rather than μg for micrograms.

Which NASA probe was lost at Mars in 1999 because one team's software used US customary units while another's expected metric?

Mars Climate Orbiter. The mismatch sent it far too close to the planet, and the spacecraft is thought to have burned up in the Martian atmosphere.

Henri Tresca, the engineer behind the X-shaped prototype metre bar, has his name engraved on which Paris landmark?

The Eiffel Tower. Gustave Eiffel had the names of 72 scientists and engineers lettered around the first level, and Tresca is third on the side facing the Trocadéro.

Which war broke out weeks before the International Metre Commission first met in Paris, forcing it to put off its decisions?

The Franco-Prussian War. The delegates still met in August 1870, without the Germans, but left the final decisions until the full commission could gather again in 1872.

In which French town did the world's delegates vote in 2018 to redefine the kilogram?

Versailles. The vote at the General Conference on Weights and Measures also redefined three other base units, so that from May 2019 no SI unit depended on a physical object.

Which US coin weighs exactly five grams?

The nickel. A modern penny weighs exactly half as much, 2.5 grams, so two pennies balance one nickel on a kitchen scale.

Why did the SI approve a capital L for the litre in 1979?

Small l looks like a 1. Capital symbols are normally kept for units named after people, so the litre was given a special exception, and both l and L remain official.

The Metre Convention first covered the units of only which two quantities?

Length and mass. The treaty's first job was to make and guard new prototypes of the metre and kilogram; only with a 1921 amendment did it take in other fields of physics.

The kelvin honours Lord Kelvin, who spent 53 years as a professor at the university of which Scottish city?

Glasgow. He was elected professor there at 22, and the title he later chose comes from the River Kelvin, which flows past the university.

The SI unit of pressure honours which French thinker, who showed that air pressure falls with altitude?

Blaise Pascal. In 1648 he had a barometer carried up a mountain in central France, and its mercury sank as it climbed, proof that the air above was thinning.

Which medieval English document demanded 'one measure of wine throughout our whole realm, and one measure of ale'?

Magna Carta. Clause 35 of the 1215 charter also called for one measure of corn and one width of cloth, nearly six centuries before the metric system delivered uniform units.

Interstate 19, the only US interstate with distances signed in kilometres, lies entirely within which state?

Arizona put up the signs in 1980, during America's push to go metric. Interstate 19 runs just 102 km from the Mexican border at Nogales north to Tucson.

Which soft drink maker introduced the first two-litre bottle in 1970, a rare metric size on American shelves?

PepsiCo launched it in 1970, and the two-litre bottle remains one of the few things Americans buy in a round number of metric units.

In Pulp Fiction, Vincent Vega blames the metric system for France calling which burger a 'Royale with Cheese'?

The Quarter Pounder really does lose its name in countries that never weigh in pounds, though French menus call it a Royal Cheese rather than the film's 'Royale with Cheese'.

On the original 1742 scale of astronomer Anders Celsius, zero degrees marked which property of water?

Boiling point was zero and freezing point 100 on Celsius's own scale, so a hotter day gave a lower reading. Other scientists flipped it within three years to the order used today.

The Celsius and Fahrenheit scales show exactly the same number at which bitterly cold temperature?

−40° is the only point where the two scales cross, because each Fahrenheit degree is just five-ninths the size of a Celsius degree.

Which president abolished the United States Metric Board, set up in the 1970s to coordinate a voluntary switch?

Ronald Reagan abolished the 17-member board in 1982, only seven years after Congress created it to encourage Americans to go metric.

Which character on The Simpsons rants that the metric system is 'the tool of the devil'?

Grampa Simpson follows the outburst with a boast in thoroughly old-fashioned units: 'My car gets forty rods to the hogshead and that's the way I likes it.'

Which European country switched its road speed limits from miles per hour to kilometres per hour in January 2005?

Ireland had put kilometres on its direction signs back in the 1970s, so for three decades drivers read metric distances beside speed limits in miles per hour.

The 'Calorie' printed on American food labels is really which larger metric unit of heat energy?

A kilocalorie, the scientist's 'large calorie', entered nutrition in 1887, when American chemist Wilbur Atwater used it to tally the energy in food.

The metre was first sized as a fraction of the globe, which is why Earth's circumference is almost exactly what?

40,000 km falls out of the 1790s plan to make one fourth of the way around the world exactly 10,000 km. The modern figure for that circle is 40,008 km, so the surveyors came remarkably close.

In American and British military slang, a distance of one kilometre is known as a what?

A klick is the soldier's kilometre: the US military works in metric so that its maps and orders match those of its NATO allies.

Which pioneer of alternating current gave his name in 1960 to the SI unit of magnetic flux density?

Nikola Tesla received the honour 17 years after his death. A fridge magnet manages about five thousandths of his unit, while a hospital MRI scanner runs at 1.5 to 3.

A0, the giant sheet that yields sixteen A4 pages when halved four times, is defined as having what area?

One square metre is the starting point of the whole A series of paper, so a sheet of standard 80-gram office A4, a sixteenth of A0, weighs exactly 5 grams.

Absolute zero, the 0 K starting point of the kelvin scale, is roughly what temperature in degrees Celsius?

−273° Celsius, or −273.15 to be exact, is the coldest temperature that theory allows. The tempting −460 is the very same point read on a Fahrenheit thermometer.

Anders Celsius, the astronomer whose name is on the metric world's temperature scale, was a professor in which country?

Sweden was home to Celsius, an astronomy professor's son who founded the Uppsala observatory in 1741 and proposed his thermometer scale the following year.

Which US agency, called the National Bureau of Standards until 1988, keeps America's official measurement standards?

NIST, the National Institute of Standards and Technology, was founded in 1901 as America's national physical laboratory; before that, weights and measures were a Treasury Department job.

Science students read a liquid's volume at the bottom of which curve, formed where the surface meets the glass?

The meniscus forms because surface tension pulls the liquid up the sides of the glass, so the true level is the bottom of the dip, read with the eye level with it.

In physics, weight is a force rather than a mass, so it is properly measured in which SI unit?

The newton is a small unit: an average 200-gram apple presses down with about two of them, a fitting scale for a unit named after Isaac Newton.

Which health figure is found by dividing a person's weight in kilograms by the square of their height in metres?

Body mass index goes back to Belgian statistician Adolphe Quetelet, who devised it between 1830 and 1850 to describe an 'average man' and never meant it as a medical test.

On a car tyre marked 205/55 R16, the width is 205 millimetres, but the 16 gives the rim diameter in what?

Inches for the rim sit beside millimetres for the width in the very same tyre code, while the middle number, 55, is the sidewall's height as a percentage of that width.

Which SI unit of frequency replaced the older phrase 'cycles per second' in 1960?

The hertz honours Heinrich Hertz, the first to prove conclusively that electromagnetic waves exist; newspapers only began swapping 'cycles' for his name in the late 1960s.

Which electrical unit honours the Italian credited with inventing the electric battery around 1800?

The volt recalls Alessandro Volta of Como, who built his 'voltaic pile' in 1799 and is also credited with discovering methane.

The becquerel, an SI unit named for a physicist who shared a 1903 Nobel Prize with the Curies, measures what?

Radioactivity is counted in becquerels as one atomic decay per second. Henri Becquerel stumbled on it in 1896, in the excitement that followed the discovery of X-rays.

The unit of sound level that carries the metric prefix for one tenth honours which inventor?

Bell, as in Alexander Graham Bell, gave his name to the decibel: telephone engineers needed a unit for signal loss on long lines in the 1920s. The full-sized bel is seldom used.

On a worksheet marked to SI style rules, which is the correct way to write a mass of fifty kilograms?

50 kg is right because SI symbols are not abbreviations: they take no plural s and no full stop, and a space always separates the number from the unit.

The abbreviation SI, used worldwide for modern metric measurement, is short for which name in English?

International System of Units is the English form of the French 'Système international d'unités', which is why the letters come out as SI rather than IS.

Which small item of stationery, like Japan's one-yen coin, has a mass of roughly one gram?

A paper clip is the classic classroom stand-in for a gram. A US penny, by comparison, weighs two and a half times as much.

The Olympic track race nicknamed the 'metric mile' is run over which distance?

1,500 m is three and three-fourths laps of a standard track, a little short of a true mile, and has been on the Olympic programme since the first modern Games of 1896.

A spring scale measures weight; which instrument finds mass by comparing an object with standard masses in a second pan?

A beam balance reads true wherever gravity is a little stronger or weaker, because both pans are pulled equally; a spring scale has to be recalibrated.

Which tall, narrow piece of lab glassware carries a printed scale for measuring the volume of a liquid?

A graduated cylinder is built slim on purpose: the tall column makes it more accurate and precise than a flask or beaker, though a volumetric flask is better still.

Which unit of electrical resistance takes the Greek letter omega as its symbol?

The ohm got its omega in 1867, when William Preece suggested the letter simply because the two words sound alike; the unit itself honours German physicist Georg Ohm.

Alongside metric units, Revolutionary France adopted a calendar whose ten-day 'décade' replaced which familiar period?

The week gave way to three ten-day décades in every 30-day month, with the tenth day replacing Sunday as the day of rest, partly to weaken the Church's hold.

The stère, an original metric unit equal to one cubic metre, was created for measuring stacks of what?

Firewood was the stère's whole purpose: France coined it in 1795 as a metric answer to the cord, and although it is no longer a legal unit there, wood dealers still use it.

The French word 'mètre', coined in the 1790s for the new unit of length, comes from a Greek word meaning what?

Measure is the sense of the Greek 'metron', the same root that gives English the '-meter' of thermometer and the meter of a poem.

Which unit of angle, created alongside the metric system, splits a right angle into 100 parts instead of 90?

The gradian, or 'metric degree', was a standard mode on scientific calculators from the 1970s to the 1990s; a hundredth of one, traced along the Earth's surface, is about a kilometre.

The French 'livre' and the German 'Pfund' both live on as an informal metric pound equal to what?

500 grams became the official pound of the German Customs Union in 1854, while France had already tidied its old livre to the same round figure in 1812.

To help redefine the kilogram, scientists counted the atoms in polished single-crystal spheres of which element?

Silicon spheres, grown as single crystals of the isotope silicon-28, had their volume and atomic spacing measured so precisely that the atoms inside could be counted.

Which metric unit of pressure, close to the air pressure at sea level, has a name from the Greek for 'weight'?

The bar and its thousandth, the millibar, were introduced by Norwegian meteorologist Vilhelm Bjerknes, a founder of modern weather forecasting.

Before it was redefined in 1956, the second was officially 1/86,400 of which natural period?

A mean solar day holds 24 × 60 × 60 seconds, but the Earth's spin varies and is slowly braking, so the unit was handed first to the Earth's orbit and later to the atom.

The first platinum metre and kilogram of 1799 took their French names from which institution, where they were deposited?

The National Archives housed the Mètre des Archives and Kilogramme des Archives, which stayed the official standards for 90 years, until new international prototypes replaced them in 1889.

Which instrument, renamed in 2016 for its late British inventor, weighs an object against an electromagnetic force?

The Kibble balance took its new name two months after physicist Bryan Kibble died. It weighs a mass by the electricity needed to hold it up, and labs now use it to realise the kilogram.

Which former Rhode Island governor made switching America to metric units part of his 2016 presidential campaign?

Lincoln Chafee launched his bid for the Democratic nomination in June 2015 and suspended it that October, after one debate, low polling and thin fundraising.

Which German mathematician measured Earth's magnetism in 1832 in units built on the millimetre, milligram and second?

Carl Friedrich Gauss showed that magnetism could be pinned down with ordinary mechanical units, the first 'absolute' measurement of its kind; an older unit of magnetic field was later named after him.

Which American physicist's 1907 Nobel Prize owed much to measuring the prototype metre in wavelengths of cadmium light?

Albert Michelson matched the metre bar against light waves to within a tenth of a wavelength in 1892–93, and went on to become the first American to win a science Nobel.

Which French scientist was jailed as a suspected spy in the Balearic Islands in 1808 while extending the metre's survey?

François Arago had been lighting signal fires on a mountaintop for his measurements, which islanders at war with France took for messages to the invading army.

Which SI unit, made official in 1999, measures how quickly an enzyme speeds up a chemical reaction?

The katal measures catalytic activity. It was first proposed as an SI unit in 1978 and had to wait 21 years for approval.

Which future president wrote an 1821 report on weights and measures that praised France's new metric system?

John Quincy Adams wrote it while secretary of state, a post that also had him overseeing the 1820 census; the report was a particular passion and surveyed the whole history of the subject.

Which French abbot proposed a decimal system of lengths in 1670, based on the size of the Earth?

Gabriel Mouton of Lyon took a minute of arc along a meridian and divided it by tens; one of his units, the virga, came out about two metres long, close to the Parisian toise.

Which Coast Survey chief gave his name to the 1893 order that made metric standards America's fundamental standards?

Thomas Mendenhall's order means the pound and the foot have been legally defined by metric standards ever since, so in a sense America has been metric since 1893.

Under SI rules, the first letter in the symbol of any unit named after a person is always written how?

As a capital: C for coulomb, F for farad, even the C in °C. The unit names themselves, though, are written small.