SI units

The International System of Units (SI) defines measurement units from exact constants, not from physical artefacts. This makes units reproducible anywhere with good experiments.

Seven base units

Quantity	Unit	Symbol	Defining constant idea
time	Second	$\mathrm{s}$	caesium-133 transition frequency
length	Metre	$\mathrm{m}$	speed of light $c$ plus the second
mass	Kilogram	$\mathrm{kg}$	Planck constant $h$
electric current	Ampere	$\mathrm{A}$	elementary charge $e$
temperature	kelvin	$\mathrm{K}$	Boltzmann constant $k$
amount of substance	mole	$\mathrm{mol}$	Avogadro constant $N_A$
luminous intensity	candela	$\mathrm{cd}$	luminous efficacy $K_{cd}$

SI defining constants

Symbol	Defining constant	Exact value
$\Delta {\nu }_{Cs}$	caesium-133 hyperfine transition frequency	$9192631770\mathrm{Hz}$
$c$	speed of light in vacuum	$299792458\mathrm{m}{\mathrm{s}}^{-1}$
$h$	Planck constant	$6.62607015\times 10^{-34}\mathrm{J}\mathrm{s}$
$e$	elementary Charge	$1.602176634\times 10^{-19}\mathrm{C}$
$k$	Boltzmann constant	$1.380649\times 10^{-23}\mathrm{J}{\mathrm{K}}^{-1}$
$N_A$	Avogadro constant	$6.02214076\times 10^{23}\mathrm{mo}{\mathrm{l}}^{-1}$
$K_{cd}$	luminous efficacy of 540 THz radiation	$683\mathrm{lm}{\mathrm{W}}^{-1}$

Practical rule: reduce derived units to base units when an equation feels suspicious. Example: Newton becomes $\mathrm{kg}\mathrm{m}{\mathrm{s}}^{-2}$, so Joule becomes $\mathrm{kg}{\mathrm{m}}^2{\mathrm{s}}^{-2}$.