The History of the Metric System: From Revolutionary France to the Entire World
The metric system is now used by 195 countries and underpins all of science and international trade. But it started as a radical political project during the French Revolution. Here is how it conquered the world.
Walk into any laboratory, hospital, or international shipping depot in the world and you will find one measurement system in use: the metric system. Today it is so universal that it feels inevitable. But 250 years ago, Europe was a chaos of incompatible local units — and the metric system was a deliberate invention, born from revolution and designed to replace all of them.
Before the Metric System: Measurement Chaos
Before 1790, France alone used over 700 different units of measurement — and the same unit name often meant different things in different cities. A 'pound' in Paris was not the same as a 'pound' in Lyon. A 'foot' could vary by region. Merchants, farmers, and tax collectors all worked with systems designed to benefit local lords rather than enable fair trade.
The problem was not unique to France. Across Europe, measurement was local, inconsistent, and frequently manipulated. When you bought a yard of cloth, the yard depended entirely on who was selling it. Scientific communication across borders was a constant headache, with researchers needing to specify not just their measurements but exactly which local standard they were using.
The French Revolution and the Birth of the Metre
In 1789, the French Revolution swept away the old order — and with it, reformers saw an opportunity to create a rational, universal system of measurement. The National Assembly commissioned a study, and in 1791 the French Academy of Sciences proposed a system based on natural constants rather than arbitrary human standards.
The metre was defined as one ten-millionth of the distance from the North Pole to the equator, measured along the meridian passing through Paris. This was not just a unit — it was a statement: measurement should belong to nature and to all people, not to kings and local customs. The motto of the new system was 'for all people, for all time.'
Two astronomers, Jean-Baptiste Delambre and Pierre Méchain, spent six years measuring the arc from Dunkirk to Barcelona to establish the exact length of the metre. Their survey, completed in 1798, contained a small error — but the metre had already been defined and platinum standards had been cast. The error was acknowledged and the metre kept anyway.
The International System: SI Units
The modern metric system — known officially as the International System of Units, or SI from the French Système International d'Unités — was formalised in 1960 at the 11th General Conference on Weights and Measures. It defined seven base units that underpin all measurement in science and industry:
- Metre (m) — length
- Kilogram (kg) — mass
- Second (s) — time
- Ampere (A) — electric current
- Kelvin (K) — thermodynamic temperature
- Mole (mol) — amount of substance
- Candela (cd) — luminous intensity
All other units — newtons, joules, watts, pascals, volts — are derived from combinations of these seven. The elegance of SI is that you can express any physical quantity using just these building blocks.
How Many Countries Use the Metric System?
As of 2026, the metric system is the official measurement system of every country in the world except three: the United States, Liberia, and Myanmar (Burma). Even in those three countries, metric is used extensively in science, medicine, military, and international trade.
The United Kingdom officially uses metric for most purposes — road signs being a notable exception, which still show miles and miles per hour. Australia completed its metric conversion in the 1970s, Canada in the 1970s–80s, and India in 1957. Most countries in Africa and Asia adopted metric upon independence, often as a deliberate break from colonial measurement systems.
The Metric System in Science
Science is entirely metric — no exceptions. When NASA lost the Mars Climate Orbiter in 1999, the cause was a unit mismatch: one team had used imperial units while another used metric. The $327 million spacecraft burned up in the Martian atmosphere. The incident is one of the most expensive measurement errors in history and a stark reminder of why universal standards matter.
The 2019 Redefinition: Metric Meets Quantum Physics
On 20 May 2019, all SI base units were redefined in terms of fundamental physical constants rather than physical artefacts. The kilogram, which had been defined by a platinum-iridium cylinder sitting in a vault near Paris since 1889, was redefined using Planck's constant. The ampere was redefined using the elementary charge of an electron. The metric system became truly universal — not just for all people, but for all possible civilisations in the universe.
Convert Units Instantly
Whether you are converting between metric and imperial or working purely within the SI system, our free converters handle it instantly. No sign-up required.