The International System of Units, abbreviated SI from the French “Système International d’Unités,” is the globally accepted standard for measurement. It provides a common language of seven base units that scientists, engineers, doctors, and everyday people use to quantify everything from the length of a room to the energy in food. Since May 20, 2019, every SI unit is defined by fixed constants of nature rather than physical objects, making the system more precise and universal than ever before.
The Seven Base Units
The entire system rests on seven fundamental measurements. Every other unit in science and daily life is built from combinations of these:
- Second (s) for time
- Metre (m) for length
- Kilogram (kg) for mass
- Ampere (A) for electric current
- Kelvin (K) for temperature
- Mole (mol) for amount of substance
- Candela (cd) for luminous intensity
These seven cover every physical quantity you can measure, either directly or by combining them. Length and time together give you speed. Mass, length, and time together give you force. The system is designed so that no matter how complex a measurement gets, it traces back to these building blocks.
Derived Units for Everyday and Scientific Use
When you combine base units mathematically, you get derived units. Many of these are named after the scientists who pioneered the concepts they measure. The newton (N), named after Isaac Newton, measures force and equals one kilogram accelerated by one metre per second squared. The joule (J), named after James Prescott Joule, measures energy. A watt (W) measures power and is simply one joule per second. The pascal (Pa), named after Blaise Pascal, measures pressure and equals one newton spread over one square metre.
These relationships are straightforward once you see the pattern. If a 65-kilogram person climbs a 20-metre ladder, the work done is about 13,000 joules (or 13 kilojoules). A lightbulb rated at 60 watts uses 60 joules of energy every second. Food energy is measured in kilojoules: a typical teenage boy’s daily metabolic rate is about 11,000 kilojoules, while a girl’s is around 9,200 kilojoules.
In medicine, specialized derived units matter too. Radiation dose is measured in grays (Gy) for absorbed dose and sieverts (Sv) for the biological effect of that dose. Radioactivity itself is counted in becquerels (Bq). These standardized units allow hospitals and regulatory agencies worldwide to communicate radiation exposure levels without confusion.
Prefixes: Scaling From Atoms to Galaxies
One of the system’s most practical features is its set of prefixes, which scale any unit up or down by powers of ten. You already use several of these daily: a kilometre is 1,000 metres, a milligram is one-thousandth of a gram, and a gigabyte is one billion bytes. The prefix system currently spans 30 named multipliers.
The most commonly encountered prefixes run from pico (one trillionth) through tera (one trillion). On the small end, nanotechnology works at the nanometre scale, which is one billionth of a metre. On the large end, data storage now routinely reaches terabytes, and global data flows are measured in petabytes (quadrillions of bytes).
In 2022, the General Conference on Weights and Measures added four new prefixes to handle the extremes of modern science. Ronna (R) covers 10²⁷ and quetta (Q) covers 10³⁰ for enormous quantities, while ronto (r) at 10⁻²⁷ and quecto (q) at 10⁻³⁰ handle the vanishingly small. These additions were driven partly by the need to express things like the mass of the Earth (about 6 ronnagrams) and data volumes projected in coming decades, without resorting to unwieldy strings of zeros.
How the System Came to Be
The SI grew out of the metric system, which France developed in the late 1700s to replace the tangle of local measurement standards that made trade and science unreliable. The key international milestone was the Treaty of the Metre, signed on May 20, 1875, by 17 nations. That treaty established the International Bureau of Weights and Measures (known by its French abbreviation, BIPM), based near Paris, to maintain measurement standards and coordinate updates.
For most of its history, the system relied on physical reference objects. The kilogram, for instance, was defined by a platinum-iridium cylinder kept in a vault outside Paris. Every other kilogram in the world was ultimately compared to that one artifact. The problem was that physical objects can change: even the reference kilogram showed tiny variations in mass over time due to surface contamination.
The 2019 Redefinition
On May 20, 2019, the system underwent its most significant overhaul. All seven base units are now defined by fixing the numerical values of seven natural constants, including the speed of light, the Planck constant, and the Boltzmann constant. The kilogram is no longer tied to a metal cylinder. Instead, it is defined through the Planck constant, a value from quantum physics that relates energy to the frequency of a particle of light.
For most people, nothing changed in practical terms. A kilogram still feels like a kilogram, and a metre is still a metre. But for scientists and engineers working at extreme precision, the shift was transformative. Any laboratory with the right equipment can now reproduce the exact definition of any SI unit independently, without referencing a physical artifact stored in France. The system became, in principle, accessible to anyone, anywhere.
SI Units in Medicine and Health
Hospitals and laboratories around the world rely on SI units to report blood tests, prescribe medication dosages, and measure radiation exposure. The concentration of substances in your blood, such as glucose or cholesterol, is typically reported in millimoles per litre in countries that follow SI conventions. The U.S. Food and Drug Administration has noted that SI units are the worldwide standard for international clinical trials, meaning drug studies routinely measure and report lab results using them.
Radiation medicine provides a clear example of why standardization matters. When a patient receives a CT scan or radiation therapy, the dose they absorb is recorded in milligrays, and the estimated biological risk is expressed in millisieverts. Because these units are universal, a radiologist in Tokyo and an oncologist in London can compare results directly, reducing the chance of dangerous miscommunication.
Who Uses the SI Today
Nearly every country in the world has officially adopted the SI. The notable holdouts for everyday use are the United States, Myanmar, and Liberia, which still rely on older systems like the U.S. customary units (inches, pounds, Fahrenheit) for daily life. Even in those countries, however, science, medicine, the military, and much of industry use SI units internally. NASA, for example, works in metric, and U.S. pharmaceutical labels increasingly include metric measurements.
The system’s strength is its coherence. Because every unit connects logically to the seven base units, there are no arbitrary conversion factors within the system. One litre of water has a mass of one kilogram and occupies one cubic decimetre. That kind of clean, interlocking structure is why the SI has become the default framework for global science, trade, and communication.