Complete Guide to SI Units: The Seven Base Units

March 2026 · 6 min read

In global scientific research, engineering, and international trade, there is one universal measurement language: the International System of Units (Systeme International d'Unites, abbreviated SI). It consists of seven base units from which all other physical quantities can be derived. In 2019, SI underwent a historic redefinition that made these base units more precise and stable than ever before.

The Seven SI Base Units

Quantity	Unit Name	Symbol	Defining Constant (post-2019)
Length	meter	m	Speed of light c = 299,792,458 m/s
Mass	kilogram	kg	Planck constant h = 6.62607015 x 10⁻³⁴ J·s
Time	second	s	Cesium-133 atom: 9,192,631,770 radiation periods
Electric current	ampere	A	Elementary charge e = 1.602176634 x 10⁻¹⁹ C
Temperature	kelvin	K	Boltzmann constant k = 1.380649 x 10⁻²³ J/K
Amount of substance	mole	mol	Avogadro constant NA = 6.02214076 x 10²³
Luminous intensity	candela	cd	Luminous efficacy of 540 x 10¹² Hz radiation

The Historic 2019 Redefinition

On May 20, 2019 (World Metrology Day), the definitions of SI base units underwent a fundamental change. The most significant was the redefinition of the kilogram.

Farewell to Physical Artifacts

Before 2019, the kilogram was the only base unit still defined by a physical object — a platinum-iridium cylinder called the International Prototype of the Kilogram (IPK), stored at the BIPM in Paris. The problem was that this artifact could change mass over time (however minutely), making the kilogram's definition inherently unstable.

Key Point: After the 2019 redefinition, all seven base units are based on invariant physical constants, no longer depending on any physical artifact. This means any laboratory with sufficiently precise equipment can independently reproduce the exact value of these units.

SI Prefixes: From Tiny to Enormous

SI uses a series of prefixes to express multiples or fractions of units, all based on powers of 10:

Prefix	Symbol	Factor	Example
tera	T	10¹²	1 TB = 1 trillion bytes
giga	G	10⁹	1 GHz = 1 billion hertz
mega	M	10⁶	1 MW = 1 million watts
kilo	k	10³	1 km = 1,000 meters
centi	c	10⁻²	1 cm = 0.01 meters
milli	m	10⁻³	1 mm = 0.001 meters
micro	μ	10⁻⁶	1 μm = one millionth of a meter
nano	n	10⁻⁹	1 nm = one billionth of a meter

Derived Units: From Basic to Complex

All other physical quantities can be derived from the seven base units. Common derived units include:

Newton (N): Unit of force = kg·m/s²
Pascal (Pa): Unit of pressure = N/m²
Joule (J): Unit of energy = N·m
Watt (W): Unit of power = J/s
Volt (V): Unit of voltage = W/A
Hertz (Hz): Unit of frequency = 1/s

SI Units in Everyday Life

While SI may seem purely "scientific," it is deeply woven into daily life:

Your body weight is measured in kilograms
Car speeds are displayed in kilometers per hour
Room temperature is measured in degrees Celsius (derived from kelvin)
Electricity bills are calculated in kilowatt-hours (kWh)
Phone screen brightness is measured in nits (cd/m²)

Try the Unit Converter Tool →

Conclusion

The SI is one of humanity's most successful standardization achievements. It enables scientists, engineers, and businesses worldwide to communicate using a single measurement language. The 2019 redefinition has made this system more precise and stable than ever before.

References

BIPM. "The International System of Units (SI)." SI Brochure, 9th edition, 2019. https://www.bipm.org/en/publications/si-brochure
NIST. "The International System of Units (SI)." NIST Special Publication 330, 2019. https://www.nist.gov/pml/owm/metric-si/si-units
ISO. "ISO 80000-1:2022 Quantities and units — Part 1: General." International Organization for Standardization, 2022. https://www.iso.org/standard/76921.html
Stock, Michael, et al. "The revision of the SI — the result of three decades of progress in metrology." Metrologia, Vol. 56, No. 2, 2019. https://doi.org/10.1088/1681-7575/ab0013