SI: The International System of Units and the Science of Measurement

The International System of Units, known globally by the abbreviation SI (from the French Système international d'unités), serves as the modern foundation of the metric system. As the world's most widely used system of measurement, it holds official status in nearly every country, providing a standardized language for science, technology, industry, and global commerce. The system is coordinated by the International Bureau of Weights and Measures (BIPM).

SI base units (outer ring) and constants (inner ring)

Key Facts

Closeup of the National Prototype Metre, serial number 27, allocated to the United States

The SI is the modern form of the metric system and is used globally in science and industry.
It is based on seven base units corresponding to fundamental physical quantities.
The 2019 redefinition tied all base units to fixed physical constants of nature.
The system is a decimal system, meaning conversions occur via powers of ten.
The BIPM is the international organization responsible for coordinating the SI.

The Seven SI Base Units

At the heart of the SI are seven base units. These units are the building blocks from which all other measurements are constructed. Unlike previous iterations of measurement systems, the current SI defines these units through exact numerical values of fundamental constants.

SI Base Units and Quantities
Quantity	Unit Name	Symbol
Time	second	s
Length	metre	m
Mass	kilogram	kg
Electric current	ampere	A
Thermodynamic temperature	kelvin	K
Amount of substance	mole	mol
Luminous intensity	candela	cd

Arrangement of the principal measurements in physics based on the mathematical manipulation of length, time, and mass

The 2019 Redefinition and Physical Constants

A landmark shift occurred in 2019 when the SI was redefined. Previously, certain units relied on physical artifacts, such as the International Prototype of the Kilogram. Today, the base units are derived exclusively from constants of nature. For instance, the kilogram is now defined using the Planck constant (h), ensuring that measurements remain stable and reproducible anywhere in the universe.

Dependencies of the SI base units on seven physical constants, which are assigned exact numerical values in the 2019 redefinition. Unlike in the previous definitions, the base units are all derived exclusively from constants of nature. Here, means that is used to define .

These constants include the speed of light (c), the elementary charge (e), and the Avogadro constant (NA). By anchoring measurement to these unchanging values, the scientific community has achieved unprecedented precision.

Silicon sphere for the Avogadro project used for measuring the Avogadro constant to a relative standard uncertainty of 2×10−8 or less, held by Achim Leistner[15]

Derived Units and Mathematical Relationships

While base units represent fundamental quantities, most measurements in physics are derived units. These are created by mathematically combining the seven base units. For example, force is measured in newtons (N), which is a combination of kilograms, metres, and seconds (kg⋅m/s²).

Example of lexical conventions. In the expression of acceleration due to gravity, a space separates the value and the units, both the 'm' and the 's' are lowercase because neither the metre nor the second are named after people, and exponentiation is represented with a superscript '2'.

Common Derived Quantities

Frequency: measured in hertz (Hz)
Pressure: measured in pascals (Pa)
Energy: measured in joules (J)
Power: measured in watts (W)
Electric potential: measured in volts (V)

The Decimal System and SI Prefixes

One of the primary advantages of the SI is its decimal nature. This means that moving between different scales of measurement is always done by multiplying or dividing by powers of ten. To facilitate this, the SI utilizes 24 metric prefixes.

Countries using the metric (SI), imperial, and US customary systems as of 2019

These prefixes allow scientists to express incredibly large or incredibly small quantities without using cumbersome strings of zeros. For example, a mega- prefix represents 10⁶ (one million), while a nano- prefix represents 10⁻⁹ (one billionth). In 2022, new prefixes such as quetta (Q) and ronto (r) were adopted to accommodate the growing needs of data science and advanced physics.

Stone marking the Austro-Hungarian/Italian border at Pontebba displaying myriametres, a unit of 10 km used in Central Europe in the 19th century (but since deprecated)[28]

Non-SI Units and Common Usage

While the SI is the standard, several other units are widely used in specific contexts. The litre (L), for example, is a common unit for volume. Although not an SI unit itself, it is accepted for use and is equivalent to 10⁻³ cubic metres (m³).

While not an SI unit, the litre is widely used. It is equivalent to (10 cm)3 = 10−3 m3.

Other non-SI units include units of time like the minute, hour, and day, as well as the astronomical unit (au) used in astronomy. While these are not part of the formal SI base system, they are often used alongside it through established conversion factors.

Frequently Asked Questions

What is the difference between SI and the metric system?

The SI is the modern, internationally standardized form of the metric system. While the terms are often used interchangeably, the SI is a specific, highly regulated system coordinated by the BIPM.

Why were the SI units redefined in 2019?

The redefinition moved the system away from physical objects (like a metal cylinder for the kilogram) and toward universal physical constants. This ensures that measurements are more precise and do not change over time.

How many base units are there in the SI?

There are exactly seven base units: the second, metre, kilogram, ampere, kelvin, mole, and candela.

Is the litre an SI unit?

The litre is not an SI base unit, but it is a widely used unit for volume that is accepted for use with the SI. It is equivalent to one cubic decimetre.

What are SI prefixes used for?

SI prefixes are used to indicate decimal powers of ten, allowing for the easy expression of very large or very small quantities, such as kilometers for distance or micrometres for microscopic lengths.