Before the 1790s, France alone had hundreds of different units, and a “foot” or a “pound” could change from one town to the next. Merchants lost money on it and tax collectors exploited it. The French Revolution set out to replace all of this with a system “for all people, for all time”, based on nature rather than on any king’s body.
A unit taken from the Earth
In 1791 the French Academy of Sciences proposed a new unit of length, the meter: one ten-millionth of the distance from the North Pole to the equator, measured along the meridian through Paris. To find it, two astronomers, Jean-Baptiste Delambre and Pierre Méchain, spent most of the 1790s surveying the arc between Dunkirk and Barcelona by triangulation, through war, arrests and bad weather.
In 1799 the result was fixed in a platinum bar, the “meter of the Archives”. We now know the survey contained small errors, so the meter is about 0.2 mm shorter than the definition intended, but that hardly mattered: what counted was that there was one agreed standard.
Mass from water, and everything in tens
The other units were built from the meter. A liter was the volume of a cube one tenth of a meter on each side, and the kilogram was the mass of a liter of water. Larger and smaller units used the same decimal prefixes everywhere: kilo for a thousand, centi for a hundredth, milli for a thousandth. Converting kilometers to meters means moving a decimal point; converting miles to feet means remembering 5,280.
From France to the world
The new system spread slowly, even in France, but by the mid-1800s it had a clear advantage for science and trade. On 20 May 1875, seventeen nations, including the United States, signed the Metre Convention in Paris. It created the International Bureau of Weights and Measures (BIPM) near Paris, which in 1889 distributed new platinum-iridium prototype meters and kilograms to member countries. The date is still celebrated every year as World Metrology Day.
In 1960 the system was renamed the International System of Units, or SI, with seven base units: the meter, kilogram, second, ampere, kelvin, mole and candela.
From metal bars to constants of nature
A metal bar can be damaged, and copies drift. So the meter was redefined: in 1960 as a number of wavelengths of light from krypton-86, and since 1983 as the distance light travels in a vacuum in exactly 1/299,792,458 of a second. The speed of light is now exact by definition, and any well-equipped laboratory can realize the meter without visiting Paris.
The kilogram held out longest. Until 2019 it was defined by a single metal cylinder kept in a vault at the BIPM. Since 20 May 2019 it has been defined by fixing the value of the Planck constant, completing a move first imagined in the 1790s: units based on nature, not on objects.
Why it matters for your conversions
Today even US customary units are defined through the SI. The inch has been exactly 2.54 cm since 1959, which is why the length converter can give exact answers, and the pound is exactly 0.45359237 kg, the basis of the weight converter. The liter of the 1790s survives too, in every bottle measured by the volume converter.
Try it
- Length & distance converter — mm to miles, drawn on a dual ruler
- Weight & mass converter — grams to tons, on a balance
- Volume converter — cups to gallons, US and imperial