Physics becomes useful when we can measure things consistently. A number without a unit is incomplete, so physicists always write quantities with units such as meters $(m)$, seconds $(s)$, and kilograms $(kg)$.

The International System of Units, or SI, is the standard system used in physics. The most common base units are meter $(m)$ for length, kilogram $(kg)$ for mass, second $(s)$ for time, ampere $(A)$ for electric current, kelvin $(K)$ for temperature, mole $(mol)$ for amount of substance, and candela $(cd)$ for luminous intensity.