The SI System: All 7 Base Units Explained

What Is the SI System?

The International System of Units (Système International d'Unités, or SI) is the modern form of the metric system and the world's most widely used system of measurement. Adopted in 1960 by the General Conference on Weights and Measures (CGPM), SI is built on exactly 7 base unitsfrom which all other measurement units are derived. Since the landmark 2019 revision, every base unit is defined by fixing the value of a fundamental constant of nature, making SI independent of any physical artifact.

The 7 SI Base Units

1. The Meter (Length)

The meter is defined by fixing the speed of light in vacuum at exactly 299,792,458 meters per second. This means one meter is the distance light travels in 1/299,792,458 of a second. Originally defined in 1793 as one ten-millionth of the distance from the equator to the North Pole, the meter has been redefined several times to increase precision. Use our length converter to convert meters to feet, inches, miles, and more.

2. The Kilogram (Mass)

Since 2019, the kilogram is defined by fixing the Planck constant at exactly 6.62607015 × 10⁻³⁴ joule-seconds. Previously, the kilogram was defined by a physical platinum-iridium cylinder stored in Paris — the last SI unit based on an artifact. The new definition ensures the kilogram will never change due to surface contamination or wear of a physical object.

3. The Second (Time)

The second is defined as the duration of 9,192,631,770 periods of the radiation corresponding to the transition between two hyperfine levels of the ground state of the cesium-133 atom. Atomic clocks based on this definition are accurate to about one second in 300 million years.

4. The Ampere (Electric Current)

The ampere is defined by fixing the elementary charge (the charge of a proton) at exactly 1.602176634 × 10⁻¹⁹ coulombs. One ampere is one coulomb of charge flowing per second.

5. The Kelvin (Temperature)

The kelvin is defined by fixing the Boltzmann constant at exactly 1.380649 × 10⁻²³ joules per kelvin. Zero kelvin (absolute zero) is the lowest possible temperature, where all thermal motion ceases. The kelvin scale starts at absolute zero, with each degree kelvin equal in size to one degree Celsius. Use our temperature converter to convert between Kelvin, Celsius, and Fahrenheit.

6. The Mole (Amount of Substance)

The mole is defined by fixing the Avogadro constant at exactly 6.02214076 × 10²³ entities per mole. One mole of any substance contains exactly this many atoms, molecules, or other specified particles. This unit is fundamental to chemistry for relating macroscopic quantities to atomic-scale counts.

7. The Candela (Luminous Intensity)

The candela measures the perceived brightness of a light source in a specific direction. It is defined by fixing the luminous efficacy of monochromatic radiation of frequency 540 × 10¹² Hz at exactly 683 lumens per watt. This connects human visual perception to measurable physical quantities.

Worked Examples

Example 1: Derived unit — the newton

Force is measured in newtons (N). One newton = 1 kg · m/s². It combines three base units: kilogram, meter, and second.

Example 2: Derived unit — the joule

Energy in joules (J) = kg · m²/s². One joule is the energy transferred when a force of one newton moves an object one meter.

Example 3: SI prefixes

SI uses prefixes for powers of 10: kilo (10³), mega (10⁶), giga (10⁹), milli (10⁻³), micro (10⁻⁶), nano (10⁻⁹). For example, 1 kilometer = 1,000 meters; 1 milligram = 0.001 grams.

Real-World Applications

Science and Engineering

SI is the universal language of science. Research papers, equipment calibrations, and international collaborations all use SI units. Using a single coherent system eliminates conversion errors that have caused costly failures (such as the 1999 Mars Climate Orbiter crash, caused by a metric/imperial mixup).

International Trade

Products sold internationally must have measurements in SI units. Most countries require metric labeling by law, and even the US includes metric measurements on many consumer products alongside customary units.

Everyday Life

Outside the US, SI units are used daily: temperatures in Celsius, distances in kilometers, mass in kilograms, and volumes in liters. Even within the US, fields like medicine, science, and the military use SI extensively.

Frequently Asked Questions

What are the 7 SI base units?

The 7 SI base units are: meter (length), kilogram (mass), second (time), ampere (electric current), kelvin (temperature), mole (amount of substance), and candela (luminous intensity).

When was the SI system established?

The modern SI system was established in 1960. It evolved from the metric system, which originated in France in the 1790s. The most recent major revision occurred in 2019, when all 7 base units were redefined in terms of fundamental physical constants.

How are SI units defined today?

Since the 2019 revision, all 7 SI base units are defined by fixing the numerical values of 7 fundamental constants of nature, making the definitions independent of any physical artifact.