From the French Revolution to the present day, the evolution of the SI is one of humanity's greatest scientific achievements.
The International System of Units (SI) was born from the need to standardize measurements after the French Revolution. Before the 18th century, each region of Europe — and the world — had its own units of length, mass, and volume, often based on the human body (the king's foot, the fathom, the span) or on local objects (a barleycorn, a standard barrel). This diversity made trade between regions confusing and prone to fraud, and collecting taxes fairly was nearly impossible. The French Revolution, with its rationalist spirit, saw the standardization of measurements as a necessary political and scientific reform.
In 1790, the French National Assembly commissioned the Academy of Sciences to create a universal system of measurements based on nature, not the authority of a monarch. The result was the decimal metric system: the meter was initially defined as one ten-millionth of a quarter of the Earth's meridian, a measurement of the Earth itself, and the kilogram as the mass of one cubic decimeter of pure water at 4 °C. The choice of a decimal base — multiples and submultiples of 10 — was a radical break from traditional base-12 and base-16 systems, and made calculations much simpler.
Adoption of the metric system was neither immediate nor peaceful. France imposed it by law in 1795, but effective use took decades to consolidate, and Napoleon even temporarily allowed the return of old units. Throughout the 19th century, however, the metric system spread across Europe and the world, driven by international trade, science, and diplomatic agreements such as the Metre Convention of 1875, which created the Bureau International des Poids et Mesures (BIPM), still the world authority in metrology today.
It was only in 1960, at the 11th General Conference on Weights and Measures (CGPM), that the system gained its modern form and the official name International System of Units (SI), with the formal definition of the seven base units and a coherent set of derived units. From then on, the SI underwent periodic revisions to keep pace with scientific progress and make definitions increasingly precise and stable, independent of physical artifacts that could degrade or be lost.
The most important revision took place on May 20, 2019, when the four remaining base units that still depended on a physical prototype — the kilogram, ampere, kelvin, and mole — were redefined by fundamental physical constants of nature, such as the Planck constant, the elementary charge of the electron, the Boltzmann constant, and the Avogadro number. With this, the SI became entirely based on universal constants, eliminating dependence on the historic International Prototype of the Kilogram, a platinum cylinder kept since 1889 in a vault in Sèvres, France.
The seven SI base units are: meter (length), kilogram (mass), second (time), ampere (electric current), kelvin (temperature), mole (amount of substance), and candela (luminous intensity). From these seven, all other units — newton, joule, watt, pascal, hertz, volt, ohm, and dozens of others — are derived in a mathematically coherent way, without arbitrary conversion factors between them.
Today, the SI is used by virtually every country in the world, with a few notable exceptions — mainly the United States, which keeps the imperial system in everyday life, although it uses the SI in science, medicine, and international trade. Its decimal logic and scientific basis eliminate most conversion errors between different systems and provide a common language for global science, industry, and trade. TudoConversor uses the official SI factors and definitions in all its conversions.
The 26th General Conference on Weights and Measures, held in 2018, marked a historic overhaul of the SI: from May 20, 2019, all seven base units came to be defined by fundamental physical constants, rather than physical artifacts or specific material properties. The kilogram, previously defined by a platinum-iridium cylinder kept in Sèvres, France, came to be defined by the Planck constant. The ampere began using the elementary charge of the electron, and the kelvin, the Boltzmann constant. This change made the SI fully independent of physical objects that could degrade or be lost.
The 2019 redefinition was the result of decades of precision metrology research, involving laboratories from several countries that developed the Kibble balance — an instrument capable of relating mass and the Planck constant with uncertainty below 20 parts per billion. Before this redefinition, the international prototype of the kilogram had shown microgram variations in periodic comparisons with its official copies, an instability unacceptable for a standard that underpins measurements worldwide. The new definition ensures that the kilogram is reproducible in any equipped laboratory, without depending on a single object.