The 7 Fundamental SI Units
The official 2019 definitions of the seven independent base quantities that form the foundation of modern physics.
| Quantity | Symbol | SI Unit | Dimension | Official Definition (2019) |
|---|---|---|---|---|
Length | l, x, r | meter m | [L] | Distance traveled by light in vacuum in 1/299,792,458 second. Defined by:Speed of Light (c) |
Mass | m | kilogram kg | [M] | Defined by fixing the numerical value of the Planck constant (h) to 6.62607015×10⁻³⁴ J⋅s. Defined by:Planck Constant (h) |
Time | t | second s | [T] | Duration of 9,192,631,770 periods of radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom. Defined by:Caesium Frequency (ΔνCs) |
Electric Current | I, i | ampere A | [A] | Defined by fixing the elementary charge (e) to be 1.602176634×10⁻¹⁹ C. Defined by:Elementary Charge (e) |
Thermodynamic Temperature | T | kelvin K | [K] | Defined by fixing the Boltzmann constant (k) to 1.380649×10⁻²³ J⋅K⁻¹. Defined by:Boltzmann Constant (k) |
Amount of Substance | n | mole mol | [N] | Contains exactly 6.02214076×10²³ elementary entities. This number is the Avogadro constant. Defined by:Avogadro Constant (NA) |
Luminous Intensity | Iv | candela cd | [J] | Luminous intensity in a given direction of a source that emits monochromatic radiation of frequency 540×10¹² Hz. Defined by:Luminous Efficacy (Kcd) |
Showing 7 base units
The 7 Base Pillars of Reality
Just as the millions of words in English are built from just 26 letters, the infinite complexity of our physical universe—from the spin of an electron to the explosion of a supernova—is described using just 7 Fundamental Units. These are the axioms of the physical world. They cannot be broken down further.
Independence
The key characteristic of a fundamental unit is Independence. You cannot create "Time" by mixing "Mass" and "Length." They are distinct dimensions of reality. This is why we need exactly seven of them to cover all bases (Mechanics, Thermodynamics, Electromagnetism, Optics).
2019 Revolution
On May 20, 2019, humanity achieved a dream: defining all measurements using Fundamental Constants of Nature. We stopped measuring "Kilogram" based on a metal rock in France, and started defining it using the Planck Constant. Now, our units are universal—aliens could replicate them!
Why These Specific Seven?
The choice of these seven is a mix of science and history:
- Length, Mass, Time (M, L, T): These three are needed for Mechanics (Motion, Force, Energy).
- Electric Current (A): Needed to bridge into Electromagnetism.
- Temperature (K): Needed for Thermodynamics (Heat energy).
- Amount of Substance (mol): Needed for Chemistry (Atomic counting).
- Luminous Intensity (cd): The odd one out. Needed for Photometry (Human perception of light).
🧠 Student Tip: Dimensional Homogeneity
In any physics equation, the dimensions on the left side must equal the dimensions on the right.
Example: $F = ma$.
Dimensions: $[MLT^-2] = [M] \times [LT^-2]$. It matches!
Symbols vs Units
Students often confuse the quantity symbol with the unit symbol.
| Concept | Quantity Symbol (Italic) | Unit Symbol (Standard) |
|---|---|---|
| Length | l, x, r | m |
| Mass | m | kg |
| Time | t | s |
Frequently Asked Questions
What exactly is a "Fundamental Unit"?
A fundamental unit (or base unit) is a measurement that cannot be derived from any other unit. It is an independent quantity. For example, you cannot express "Time" using formulas for Mass or Length. It stands alone.
Why are there only 7 fundamental units?
Physicists have determined that these 7 quantities (Length, Mass, Time, Current, Temperature, Amount of Substance, Luminous Intensity) are sufficient to describe all known physical phenomena in the classical world.
Why is Electric Current (Ampere) fundamental and not Charge?
Historically, it was easier to measure current (using magnetic force between wires) than static charge. So, the Ampere was chosen as the base unit. Charge (Coulomb) is technically a derived unit ($C = A \cdot s$).
Why is the Kilogram defined by Planck's Constant now?
The old definition relied on a metal cylinder in Paris, which was slowly losing atoms (mass). The 2019 redefinition uses the Planck Constant ($h$), a fixed number in nature, ensuring the kilogram never changes, even in a million years.
Is the Mole really a unit or just a number?
The Mole represents a specific count of elementary entities ($6.022 \times 10^{23}$), similar to how a "Dozen" represents 12. In Chemistry, it bridges the gap between the microscopic world of atoms and the macroscopic world of grams.
What happened to the "Supplementary Units"?
Radian (angle) and Steradian (solid angle) were once a separate class called "Supplementary." In 1995, the CPM decided to reclassify them as dimensionless derived units.
Can we add more fundamental units in the future?
It is unlikely for classical physics. However, if we discover new physics (like dark matter interaction dimensions) or modify the system structure, definitions could evolve, but the core 7 have stood the test of time since 1960.
Why is Candela (Luminosity) a base unit? It seems specific.
Candela is unique because it measures light as perceived by the human eye. It connects physics to human physiology (photometry). Since "human perception" cannot be derived purely from energy watts, it needed its own base unit.
What is the symbol for Dimension of Temperature?
The dimension symbol is usually capital Theta ($[\Theta]$) or sometimes $[K]$. Be careful not to confuse $[T]$ (Time) with Temperature.
How do nuclear clocks relate to the definition of Second?
Currently, the second is defined by Caesium atomic clocks (microwave frequency). Scientists are working on "Optical Lattice Clocks" and "Nuclear Clocks" which beat much faster and are more precise. A redefinition of the second may happen in the 2030s!