SI units (Système International d'Unités, or International System of Units) are the modern, universally agreed-upon form of the metric system. They are the global standard for measurement in science, technology, industry, and everyday life in most countries.

Think of them as a complete, coherent, and consistent language for measuring the physical world.

The 7 Base SI Units

The entire system is built upon seven base units, each representing a fundamental physical quantity.

Quantity  Unit Name    Unit Symbol

Length     metre (meter)m

Mass  kilogram  kg

Time   second    s

Electric current   ampere    A

Thermodynamic temperature   kelvin  K

Amount of substance    mole   mol

Luminous intensity  candela   cd

Derived SI Units

All other units are created by mathematically combining the base units. Many have special names for convenience.

Examples:

    Area: square metre (m²)

    Volume: cubic metre (m³)

    Speed: metre per second (m/s)

    Force: newton (N) = kg·m/s²

    Pressure: pascal (Pa) = N/m² = kg/(m·s²)

    Energy: joule (J) = N·m = kg·m²/s²

    Power: watt (W) = J/s = kg·m²/s³

    Electric charge: coulomb (C) = A·s

    Voltage: volt (V) = J/C = kg·m²/(A·s³)

Key Features & Advantages

    Decimal-Based: Uses powers of 10, making conversions simple (just move the decimal point).

    Coherent: Derived units are defined without conversion factors (e.g., 1 Joule = 1 Newton × 1 metre).

    Prefixes: A set of standard prefixes denotes multiples and fractions, so you can use the same unit for vastly different scales.

Common SI Prefixes

Prefix Symbol    FactorExample

giga-   G  10⁹1 GHz = 1,000,000,000 hertz

mega- M  10⁶1 MW = 1,000,000 watts

kilo-    k   10³1 km = 1,000 metres

centi-  c   10⁻²    1 cm = 0.01 metres

milli-   m  10⁻³    1 mA = 0.001 amperes

micro- μ   10⁻⁶    1 µm = 0.000001 metres

nano-  n   10⁻⁹    1 ns = 0.000000001 seconds

How They Are Defined (Modern Approach)

Since 2019, all seven base units are defined by fixing the numerical values of fundamental constants of nature, not by physical artifacts. This makes them incredibly stable and reproducible anywhere in the universe.

For example:

    The metre is defined by fixing the speed of light in a vacuum (*c*) to be exactly 299,792,458 m/s.

    The second is defined by fixing the frequency of a specific transition in a cesium-133 atom.

    The kilogram is defined by fixing the Planck constant (*h*) to be exactly 6.62607015 × 10⁻³⁴ kg·m²/s.

Common Misconceptions & Clarifications

    SI vs. Metric: SI is the official, modern metric system. Older metric systems (like cgs) are not identical to SI.

    The kilogram (kg) is the base unit, not the gram (g). This is a historical quirk because mass was originally defined by a physical object (the International Prototype Kilogram).

    SI is not identical to everyday metric units. For example, the SI unit of volume is the cubic metre (m³), but the litre (L) is an accepted metric unit for everyday use (1 L = 0.001 m³ = 1 dm³).

    Non-SI units accepted for use with SI: Some units are so practical they are officially accepted alongside SI units, like the minute/hour/day, degree/litre/tonne, and even the electronvolt (eV) in physics.

In summary, SI units are the complete, logically constructed, universal foundation for measurement, rooted in the constants of nature. They allow scientists, engineers, and people worldwide to communicate measurements clearly and without error.

compilation by DeepSeek, deepseek.com