The Ultimate SI Units Reference Table

Unit Name	Symbol	Quantity	Breakdown	Type
Meter	m	Length	m	Base
Kilogram	kg	Mass	kg	Base
Second	s	Time	s	Base
Ampere	A	Electric Current	A	Base
Kelvin	K	Thermodynamic Temperature	K	Base
Mole	mol	Amount of Substance	mol	Base
Candela	cd	Luminous Intensity	cd	Base
Hertz	Hz	Frequency	s⁻¹	Derived
Newton	N	Force	kg⋅m⋅s⁻²	Derived
Pascal	Pa	Pressure	kg⋅m⁻¹⋅s⁻²	Derived
Joule	J	Energy, Work	kg⋅m²⋅s⁻²	Derived
Watt	W	Power	kg⋅m²⋅s⁻³	Derived
Coulomb	C	Electric Charge	A⋅s	Derived
Volt	V	Voltage	kg⋅m²⋅s⁻³⋅A⁻¹	Derived
Farad	F	Capacitance	kg⁻¹⋅m⁻²⋅s⁴⋅A²	Derived
Ohm	Ω	Resistance	kg⋅m²⋅s⁻³⋅A⁻²	Derived
Siemens	S	Conductance	kg⁻¹⋅m⁻²⋅s³⋅A²	Derived
Weber	Wb	Magnetic Flux	kg⋅m²⋅s⁻²⋅A⁻¹	Derived
Tesla	T	Magnetic Flux Density	kg⋅s⁻²⋅A⁻¹	Derived
Henry	H	Inductance	kg⋅m²⋅s⁻²⋅A⁻²	Derived
Lumen	lm	Luminous Flux	cd⋅sr	Derived
Lux	lx	Illuminance	cd⋅sr⋅m⁻²	Derived
Becquerel	Bq	Radioactivity	s⁻¹	Derived
Gray	Gy	Absorbed Dose	m²⋅s⁻²	Derived
Sievert	Sv	Equivalent Dose	m²⋅s⁻²	Derived
Katal	kat	Catalytic Activity	mol⋅s⁻¹	Derived

The Universal Language of Science

Imagine if every country used a different way to measure "one meter." Construction would collapse, trade would halt, and science would be impossible. The International System of Units (SI) solves this by providing a single, globally agreed-upon standard for measurement. From the microscopic components of a computer chip to the vast distances between galaxies, the SI system is the foundation of modern civilization.

A History of Evolution

Before 1960, the world was split between systems like CGS (Centimeter-Gram-Second) and MKS (Meter-Kilogram-Second). The SI system was established to unify these. Originally defined by physical artifacts (like a metal bar for the meter), it has now evolved to be defined by fundamental constants of the universe, ensuring perfection.

The 7 Pillars

The entire system rests on just 7 Base Units. Every other unit you can think of—Volts, Pascals, Watts, Newtons—is just a clever combination of these seven. This makes dimensions consistent and calculations predictable across all fields of physics.

The 3 Classes of SI Units

1. Base Units

These are the "atoms" of the measuring world. They are dimensionally independent.
Example: You cannot express "Time" using "Mass." They are fundamentally different.

2. Derived Units

These are the "molecules," built by combining base units.
Example: Speed is simply Length divided by Time ($m/s$). Force ($Newton$) is Mass times Acceleration ($kg \cdot m/s^2$). The SI system assigns special names (like Newton, Joule, Watt) to common derived units to make life easier.

3. Prefixes (The Scale of the Universe)

The SI system is decimal (base-10). Instead of inventing new names for small or large quantities (like "inch" vs "mile"), we simply add a prefix.
A kilometer is 1000 meters. A millimeter is 1/1000th of a meter. This elegance allows scientists to switch from measuring atoms (nanometers) to cities (kilometers) just by moving a decimal point.

🚀 Did You Know?

In 2019, the definition of the Kilogram changed forever. It used to be the mass of a physical metal cylinder locked in a vault in Paris (Le Grand K). Now, it is defined by the Planck Constant ($h$) using a Kibble Balance. If the metal cylinder is destroyed tomorrow, the definition of the kilogram remains safe in the laws of physics.

Common Conversion Mistakes

Confusing Mass and Weight: In daily life, we say "I weigh 70 kg." In physics, Mass is kg, but Weight is a Force measured in Newtons ($N = mg$). Your mass is the same on the Moon, but your weight changes.
Case Sensitivity: Symbols matter!
- m = milli (10⁻³) vs M = Mega (10⁶)
- s = second vs S = Siemens (conductance)
- k = kilo (lowercase!) vs K = Kelvin (uppercase!)

Frequently Asked Questions

Why are there exactly 7 base units in the SI system?

The 7 base units were chosen because they are dimensionally independent. Any other physical quantity (like force, energy, or velocity) can be derived mathematically from a combination of these seven (Meter, Kilogram, Second, Ampere, Kelvin, Mole, Candela).

What changed in the 2019 redefinition of SI units?

On May 20, 2019, the SI system was fundamentally overhauled. Artifacts (like the platinum-iridium cylinder for the kilogram) were retired. The system is now defined entirely by fundamental constants of nature (Planck constant, speed of light, etc.), ensuring units remain stable forever.

What is the difference between Base Units and Derived Units?

Base units are the fundamental building blocks (e.g., Meter for length). Derived units are formed by combining base units (e.g., Newton for force = kg⋅m/s²). Base units are defined by constants; derived units are defined by equations.

Why is the Kilogram (kg) the only base unit with a prefix?

This is a historical quirk. When the metric system was created during the French Revolution, the "grave" (now kilogram) was the standard. Later, the "gram" was considered too small for daily trade, so the 1000-gram mass became the standard, but the name "kilogram" stuck.

Is the Liter (L) an SI unit?

Technically, no. The SI unit of volume is the cubic meter (m³). However, the Liter is "accepted for use with the SI" because it is convenient for measuring liquids. 1 Liter = 0.001 m³ = 1 cubic decimeter (dm³).

What are the SI prefixes for very large and very small numbers?

For huge numbers, we use prefixes like Giga (10⁹), Tera (10¹²), Peta (10¹⁵), Exa (10¹⁸). For tiny numbers, we use Nano (10⁻⁹), Pico (10⁻¹²), Femto (10⁻¹⁵), Atto (10⁻¹⁸). The list recently expanded to include Quetta (10³⁰) and Quecto (10⁻³⁰) in 2022!

Why is the Second defined by cesium atoms?

Atomic clocks based on Cesium-133 are incredibly stable. One second is officially defined as the time it takes for 9,192,631,770 cycles of radiation from a specific transition in the cesium atom.

Is Celsius an SI unit?

No, the SI base unit for Temperature is the Kelvin (K). However, the degree Celsius (°C) is an accepted derived unit. The size of one degree Celsius is exactly equal to one Kelvin, but the scales have different zero points (0 K = Absolute Zero, 0°C = Freezing point of water).

What is a "Coherent" system of units?

A coherent system (like SI) means that derived units are formed simply by multiplying or dividing base units without any numerical factors like "12" or "5280". For example, 1 Newton = 1 kg × 1 m/s². In the imperial system, 1 Horsepower = 550 ft⋅lb/s, which is messy.

How do I convert between prefixes like Micro and Milli?

Look at the powers of 10. Milli is 10⁻³ and Micro is 10⁻⁶. The difference is 10³. Since micro is smaller, there are 1000 microseconds in 1 millisecond.