Number-loving people all over the world want to communicate and compare their numbers with each other. This requires everyone expressing measurements using similar units. The International System of Units is the most widely used system of measurement and was developed so that measurements could be compared between different disciplines.

There are seven base SI units based on invariant constants in nature, such as the speed of light in a vacuum. The seven SI units are: second (s) for time, meter (m) for length, kilogram (kg) for mass, ampere (A) for electric current, kelvin (K) for temperature, mole (mol) for amount of substance, and candela (cd) for luminous intensity.

These base units can be combined to create a variety of derived units for measurements of diverse quantities such as force measured in newtons (N = kg·m/s2) and energy measured in joules (J = kg·m2/s2).