SI Unit Reference Guide: The 7 Base Units Explained
The International System of Units (SI) is the world's most widely used measurement system. It is built on just 7 independent base units — everything else in science and engineering is derived from these. Understanding the SI system is the foundation of quantitative reasoning in any scientific field.
The 2019 SI Redefinition
In 2019, all 7 SI base units were redefined by fixing the numerical values of fundamental physical constants: the Planck constant h, the speed of light c, the elementary charge e, the Boltzmann constant k B, the Avogadro constant N A, the luminous efficacy K cd, and the cesium hyperfine frequency Δν Cs. This makes all SI units reproducible in any laboratory anywhere in the universe — no physical artifact required.
The 7 SI Base Units
| Quantity | Unit Name | Symbol | What It Measures | Real-World Reference |
|---|---|---|---|---|
| Length | Meter | m | Distance and length | Height of a doorknob from the floor (~1 m) |
| Mass | Kilogram | kg | Amount of matter | A 1-liter bottle of water ≈ 1 kg |
| Time | Second | s | Duration | A human heartbeat ≈ 1 second |
| Electric current | Ampere | A | Flow of electric charge | A phone charger draws ~1–2 A |
| Temperature | Kelvin | K | Thermodynamic temperature | Room temperature ≈ 293 K (20°C) |
| Amount of substance | Mole | mol | Count of particles | 1 mol = 6.022 × 10²³ particles (N_A) |
| Luminous intensity | Candela | cd | Light intensity | A common candle emits ~1 cd |
Common Derived SI Units
There are 22 named derived SI units. These are the most important for students in physics, chemistry, and engineering courses.
| Unit | Symbol | Quantity | In Base Units | Plain-English Definition |
|---|---|---|---|---|
| Newton | N | Force | kg·m/s² | 1 kg accelerated at 1 m/s² |
| Pascal | Pa | Pressure | kg/(m·s²) | 1 N per m² |
| Joule | J | Energy | kg·m²/s² | 1 N applied over 1 m |
| Watt | W | Power | kg·m²/s³ | 1 J per second |
| Hertz | Hz | Frequency | s⁻¹ | 1 cycle per second |
| Volt | V | Electric potential | kg·m²/(A·s³) | 1 W per A |
| Ohm | Ω | Resistance | kg·m²/(A²·s³) | 1 V per A |
Frequently Asked Questions
What are the 7 SI base units?
The seven SI base units are: meter (m) for length, kilogram (kg) for mass, second (s) for time, ampere (A) for electric current, kelvin (K) for thermodynamic temperature, mole (mol) for amount of substance, and candela (cd) for luminous intensity. All other units in the metric system are derived from these seven.
What is the difference between a base unit and a derived unit?
Base units are the seven fundamental, independently defined units of the SI system. Derived units are combinations of base units using multiplication and division. For example, the newton (N) is derived as kg·m/s², the pascal (Pa) is N/m² = kg/(m·s²), and the joule (J) is N·m = kg·m²/s². There are 22 named derived SI units in total.
Why was the kilogram redefined in 2019?
Before 2019, the kilogram was defined by a physical artifact — the International Prototype of the Kilogram (IPK), a platinum-iridium cylinder stored in France. This was problematic because physical objects can change mass over time. In 2019, the SI redefined the kilogram by fixing the numerical value of the Planck constant (h = 6.62607015 × 10⁻³⁴ J·s), making the unit reproducible anywhere from fundamental physics without a physical object.