The metric system

1795 Paris, France

In short

Before the Revolution, every region of France had its own measures, and a pound or a foot changed from one town to another. Scientists proposed a system that was the same for everyone, based on multiples of ten. The metre was defined as one ten-millionth of the distance from the North Pole to the equator, and two astronomers spent years measuring the meridian between Dunkirk and Barcelona. A law of 1795 introduced the metre, the litre and the gram.

Why it matters Today almost the whole world measures with the same system, and science and trade understand each other more easily.

Source: Bureau International des Poids et Mesures (BIPM), history of the International System of Units

See it on the line Era quiz Read the full card Printable worksheet

Illustration: The metric system (Paris, France) — Illustration created with AI for Storiascopio

Where: Paris, France

Study card about 5 minutes of reading

The story

A chaos of measures

Imagine going to the market and not knowing how long an “arm” of cloth is, how much a “pound” of meat weighs or how much a “brenta” of wine holds. Until a little over two hundred years ago, in France and all over Europe, every region and even every town had its own measures, with the same name but different values. According to some estimates, in 18th-century France there were hundreds of thousands of different units of measurement. In Italy too there was the Florentine braccio, the Neapolitan canna, the Venetian foot and the Milanese pertica, all different.

This chaos caused constant problems: merchants were cheated, engineers and scientists could not compare their results, and peasants complained of being swindled by their lords, who used larger measures when collecting taxes and smaller ones when paying. Those who governed understood that measures were needed that were the same for everybody.

“For all times, for all peoples”

With the French Revolution the chance came. In 1790 the National Assembly asked the Academy of Sciences to invent a new system, rational, simple and the same for everyone. The commission included some of the greatest scientists of the age: the chemist Antoine Lavoisier, the mathematicians Pierre-Simon Laplace and Joseph-Louis Lagrange, and the Marquis de Condorcet. They decided that measures must be based on something natural and unchanging, that did not depend on a king or a country: the Earth itself. The slogan was: “For all times, for all peoples”.

It was decided that the unit of length, the metre, would be one ten-millionth of the distance from the North Pole to the equator, measured along the meridian that passes through Paris. The other values would be derived from the metre in a decimal way, that is, in groups of ten: for volume, the litre would be a cubic decimetre; for weight, the kilogram would be the weight of a litre of pure water. For multiples and fractions, fixed prefixes would be used: kilo, hecto, deca, deci, centi, milli.

Measuring the Earth

To find out how long the metre really was, in 1792 two astronomers, Jean-Baptiste Delambre and Pierre Méchain, set out to measure a long stretch of the meridian, from Dunkirk, on the North Sea, to Barcelona, in Spain, with a method called triangulation: measuring the angles between towers, bell towers and hilltops, and working out the distances with trigonometry. It was an adventurous undertaking that lasted seven years, amid wars, arrests, illness and suspicion: in several countries the scientists were mistaken for spies, and Méchain tormented himself for years over a small error in measurement that he had discovered and kept secret.

In the meantime, on 7 April 1795 (18 Germinal, year III of the revolutionary calendar) the law officially adopted the decimal metric system. In 1799 the final platinum standards of the metre and the kilogram were made and placed in the Archives of France. Meanwhile two “standard metres” were carved in marble in some Paris streets so that citizens could compare them with their own rulers: one is still preserved, in rue de Vaugirard.

How it was received

Habits, however, do not change in a day. People went on using the old measures, and in 1812 Napoleon had to allow the traditional ones to be used again, in a metric version (old names, but values tied to the metre). Only in 1837 did the metric system become compulsory in France. The Kingdom of Sardinia adopted it in 1845, and united Italy extended it to the whole country in 1861. In 1875 seventeen countries signed the Metre Convention, and an international bureau of weights and measures was set up at Sèvres, near Paris, which looks after the standards.

Why it happened

The metric system came from the need for simplicity and fairness in trade, and from the Enlightenment faith in reason: if the laws of nature are valid for everyone, why not measures too? The revolution, which wanted to wipe out differences and privileges, offered the political chance to do it.

It was possible thanks to the science of the time, able to measure the Earth and to build precise instruments, and to a community of scientists who worked in a coordinated way, despite the war.

What changed

The metric system is now used in almost the whole world, and is the common language of scientists, engineers and merchants. In 1960 it was updated into the “International System of Units” (SI). The definitions have changed: since 1983 the metre has been defined as the distance travelled by light in a vacuum in a very short time (1/299,792,458 of a second), and since 2019 even the kilogram no longer depends on a platinum cylinder but on a constant of physics.

Countries that have not fully adopted it, like the United States, which still use miles, pounds and gallons, know this well: the metric system is convenient, because it is decimal. A muddle of units can even be dangerous: in 1999 the American space probe Mars Climate Orbiter was lost because one team of engineers had used metric units and another American units.

On 20 May, the anniversary of the Metre Convention, World Metrology Day, the day of the science of measurement, is celebrated every year.

Key people and places

People

Antoine Lavoisier: French chemist (1743–1794): member of the commission that defined the new system of measures.
Pierre-Simon Laplace: French mathematician and astronomer (1749–1827): he took part in defining the metric system.
Jean-Baptiste Delambre: French astronomer (1749–1822): he measured the meridian from Dunkirk to Barcelona.
Pierre Méchain: French astronomer (1744–1804): he measured the southern part of the meridian.
Charles-Maurice de Talleyrand: French bishop and politician (1754–1838): in 1790 he proposed to the Assembly that a uniform system of measures be created.

Places

Sèvres, near Paris: The home of the International Bureau of Weights and Measures, which looks after the standards.
Rue de Vaugirard, Paris: Where a marble “standard metre” from 1796 is still preserved.

Good to know

In 18th-century France there were hundreds of thousands of different units of measurement, according to some estimates.
The metre was meant to be one ten-millionth of the distance from the North Pole to the equator.
The kilogram was defined as the weight of a litre of water.
In 1999 an American probe to Mars was lost because metric and American units had been mixed up.
Since 2019 the kilogram has been defined from a constant of physics, and no longer from a platinum cylinder.