The International System of Units defines a common system for expressing physical quantities. Its base units include the kelvin for thermodynamic temperature; derived units combine them to describe quantities such as force and energy.[ref]
The mole is the SI base unit of amount of substance. One mole contains exactly specified entities, as fixed by the Avogadro constant. It counts entities rather than a fixed mass or volume. [ref]
The newton is the SI unit of force:
(1)
This combines the units of mass and acceleration in Newton's second law. [ref].
The pascal is the SI unit of pressure:
(1)
For a uniform normal force of over area , the pressure is . The unit expresses a force per area, rather than the total force alone. [ref].
The joule is the SI unit of energy, work, and heat. One newton of force acting along one metre of displacement does one joule of work. The relation makes mechanical and electrical transfers comparable in the same energy unit. [ref]
The coulomb is the SI unit of electric charge. A constant electric current of transports in a time of ; a constant current of for transports . The prefix in explains the much smaller charge scale used in the worked Coulomb's law example. [ref]
The volt is the SI unit of electric potential and electric potential difference: , one joule per coulomb. A positive electric charge of moved through a potential increase of gains of potential energy; the electric field does of work. [ref].
A comparison using a logarithm of two powers is expressed as . Positive loss instead uses . For wave amplitudes, a factor twenty is appropriate only when the corresponding power ratio equals the square of the amplitude ratio.

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System of units by Ciro Santilli 40 Updated 2025-07-16
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The key thing in a good system of units is to define units in a way that depends only on physical properties of nature.
Ideally (or basically necessarily?) the starting point generally has to be discrete phenomena, e.g.
What we don't want is to have macroscopic measurement artifacts, (or even worse, the size of body parts! Inset dick joke) as you can always make a bar slightly more or less wide. And even metals evaporate over time! Though the mad people of the Avogadro project still attempted otherwise well into the 2010s!
Standards of measure that don't depend on artifacts are known as intrinsic standards.
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