There have been numerous systems of units of measurement for physical quantities such as mass, length and energy. Some of these systems of units are inconsistent and worse have no repeatable definition.

Early systems, and even modern imperial systems, are based on the lengths of part of the human body. The best example is the foot. Other units have the same name but different values in different countries. A good example is the gallon. An imperial (UK) gallon is the volume of ten pounds of water at 17°. A US gallon is defined to be 231 cubic inches, which as now an inch is defined to be exactly 2.54 centimetres, is exactly 3.785411784 litres!

In 1875 the *Metre Convention* standardised the definition of the metre for length. This led to the International System of Units (SI). There are now seven base units. They were originally defined in terms of physical quantities. Some of these
were later refined to be in terms of specially created physical entities. Most are now defined exactly. Each unit has a symbol. It usually starts with a lower case letter unless it is named after a person.

In May 2019 all SI units got redefined. A number of fundamental physical constants, such a Planck's constant, were given exact values. All SI units are now defined exactly in terms of fundamental physical constants and other SI units. Their values can never change and are not dependent on any physical artifact.

Many units are too large or too small in some environments. The convention is to prefix the unit with a modifier which is a power of 10 and is usually a power of 1,000=10^{3}. Some of the multiplier names have curious derivation.

- Yotta (Y) 10
^{24}is derived from the Greek word for eight as it is 1,000^{8}. - Zetta (Z) 10
^{21}was originally Hepta and is derived from the Greek word for seven as it is 1,000^{7}. - Exa (E) 10
^{18}is derived from the Greek word for six as it is 1,000^{6}. - Peta (P) 10
^{15}is derived from the Greek word for five as it is 1,000^{5}. - Tera (T) 10
^{12}is derived from the Greek word for monster. - Giga (G) 10
^{9}is derived from the Greek word for giant. - Mega (M) 10
^{6}is derived from the Greek word for great. - kilo (k) 10
^{3}is derived for the Greek word for thousand. - hecto (h) 10
^{2}is derived for the Greek word for hundred. - deca (da) 10 is derived for the Greek word for ten, it is not often used.
- deci (d) 10
^{-1}is derived from the Latin for tenth. - centi (c) 10
^{-2}is derived from the Latin for hundred. - milli (m) 10
^{-3}is derived from the Latin for thousand. - micro (μ) 10
^{-6}derived from the Greek word for small and is the only multiplier which doesn't use a letter from the Latin alphabet. - nano (n) 10
^{-9}is derived from the Greek for dwarf. - pico (p) 10
^{-12}is derived from the Spanish for peak or beak. - femto (f) 10
^{-15}is derived from the Danish for fifteen. - atto (a) 10
^{-18}is derived from the Danish for eighteen. - zepto (z) 10
^{-21}is derived from the Latin for seven as it is 1,000^{-7}. - yocto (y) 10
^{-24}is derived from the Greek word for eight as it is 1,000^{-8}.

There are a number of fundamental constants of nature. The values of these constants have been determined by experiment. The 2019 redefinition of the SI units has meant that some constants have been given an exact value to allow all SI units to be defined exactly in terms of other SI units and fundamental constants.

- Speed of Light (c) is exactly 299,792,458ms
^{-1}. - Elementary charge (e) is exactly 1.602176634x10
^{-19}C. - Planck constant (h) is the quantum of electromagnetic action and is exactly 6.62607015x10
^{_34}Js. It is named after Max Planck. - Boltzmann constant (k) relates energy to telperature in a gas is exactly 1.380649x10
^{-23}JK^{-1}. It is named after Ludwig Boltzmann. - Gravitational Constant (G) is about 6.674 28x10
^{-11}m^{3}kg^{-1}s^{-2}and is only known to about 5 significant figures. - Julian Year is exactly 365.25 days.
- Astronomical Unit (AU) was originally the average of Earth's aphelion and perihelion distances. It is now defined to be exactly 149,597,870.7km.
- Light Year (l.y.) is the distance light travels in one Julian Year. It is defined to be exactly 9,460,730,472,580.8km.
- Parsec (pc), one parallax second. It is defined to be exactly 648,000/πAU.

The SI base units are a choice of seven well-defined units which by convention are regarded as dimensionally independent. Every other unit can be derived from one or more base units.

There are many units of time. Some are short, some are very long. Most are based of periods of the Earth and Moon. The week has varied between cultures and has been six, seven and even ten days in duration. The unit of time which has been adopted as an international standard is the second.

The second was originally defined in terms of an Earth day. A day consists of 24 hours. An hour consists of 60 minutes. A minute consists of 60 seconds. Hence a day is 8,6400 seconds.

Due to the fact that Earth days vary in length, an exact definition of a second was required.

A Caesium-133 atom has 55 electrons. Of these electrons; 54 of them are tightly bound in their stable orbitals. The outermost electron in the `6s`

shell is not affected by the others and has two possible spin states. There is a small energy
difference between the spin states which emits microwave radiation on transition. The energy difference is small and is called a hyperfine level. The second was tied to the frequency of this radiation in 1967.

The second, symbol s, is the SI unit of time. It is defined by taking the fixed numerical value of the caesium frequency Δν Cs, the unperturbed ground-state hyperfine transition frequency of the caesium-133 atom, to be 9,192,631,770 when
expressed in the unit Hertz (Hz), which is equal to s^{-1}.

There have been many units of length. Many were based on the dimensions of parts of the human body or of other common objects. The SI unit of length is the metre.

The metre was originally defined to be 1/10,000,000th of the distance between the Equator and the North Pole along the great circle through Paris. It was later redefined in terms of the wavelength of a transition of a Krypton-86 atom.

In 1983 the metre was redefined in terms of the second and the speed of light in a vacuum.

The metre, symbol m, is the SI unit of length. It is defined by taking the fixed numerical value of the speed
of light in vacuum c to be 299,792,458 exactly when expressed in the unit ms^{-1}, where the second is defined in terms
of the caesium frequency Δν Cs.

The kilogram is curious because it has the kilo multiplier. It was also the remaining SI base unit to be based on an artifact.

The kilogram was originally defined to be the mass of 1 litre of pure water at its freezing point. In 1889 a 90% Platinum
10% Iridium artifact was created which was called the International Prototype Kilogram (IPK). It was also known as
*Le Grand K*. It was stored near Paris. There were six official copies stored around the world for calibration.
The problem was that the prototpe mass changed over time which meant that the kilogram also changed!

In May 2019, the definition of the kilogram changed to be defined in terms of the Planck constant h. The Planck constant has been fixed, rather like the speed of light was fixed to define the metre. The Kibble balance is a device used to measure the Planck constant. Now that the Planck constant is fixed, a Kibble balance can accurately measure mass.

The kilogram, symbol kg, is the SI unit of mass. It is defined by taking the fixed numerical value
of the Planck constant h to be 6.626 070 15x10^{-34} when expressed in the unit J s, which is equal to
kgm^{2}s^{-1}, where the meter and the second are defined in terms of c and Δν Cs.

The ampere or amp is named after André-Marie Ampère.

The ampere, symbol A, is the SI unit of electric current. It is defined by taking the fixed numerical
value of the elementary charge e to be 1.602176634x10^{-19} when expressed in the unit C, which is
equal to A s, where the second is defined in terms of Δν Cs.

The kelvin is named after William Thomson, Lord Kelvin.

The kelvin, symbol K, is the SI unit of thermodynamic temperature. It is defined by taking the fixed
numerical value of the Boltzmann constant k to be 1.380649x10^{-23} when expressed in the unit J·K^{-1},
which is equal to kg·m^{2}·s^{-2}·K^{-1}, where the kilogram, metre and second are defined in terms
of h, c and Δν Cs

The mole, symbol mol, is the SI unit of amount of substance. One mole contains exactly 6.02214076x10^{23}
elementary entities. This number is the fixed numerical value of the Avogadro constant, N_{A}, when expressed in the
unit mol^{-1} and is called the Avogadro number. The amount of substance, symbol n, of a system is a measure of
the number of specified elementary entities. An elementary entity may be an atom, a molecule, an ion, an electron,
any other particle or specified group of particles.

The candela, symbol cd, is the SI unit of luminous intensity in a given direction. It is defined by taking the
fixed numerical value of the luminous efficacy of monochromatic radiation of frequency 540x10^{12} Hz, K_{cd},
to be 683 when expressed in the unit lm·W^{-1}, which is equal to cd·sr·W^{-1}, or
cd·sr·kg^{-1}·m^{-2}·s^{3}, where the kilogram, metre and second are defined in terms of
h, c and Δν Cs

Derived units are defined in terms of combinations of one or more base units or derived units.

The steradian, symbol sr, is the SI unit of solid angle. It is the solid angle subtended at the centre of a unit sphere by a unit of area on its surface.

The coulomb, symbil C, is the SI unit of electric charge 1C = 1A·s. It is named after Charles-Augustin de Coulomb.

The hertz is the SI unit of frequency. The symbol is Hz. It is named after Heinrich Rudolf Hertz. It is defined in terms of
the number of cycles per second of sound waves, light waves or other vibrations 1Hz = 1s^{-1}

The litre, symbol l, is a unit of volume 1l = 1,000cm^{3}.

The newton, symbol N, is the unit of force 1N = 1kg·m^{2}s^{-3}. It is named after Sir Isaac Newton.

The jouls, symbol J, is the unit of energy 1J = 1kg·m^{2}·s^{-2}. It is named after James Prescott Joule.