A base unit is a standard unit of measurement adopted for a base quantity. In the International System of Units (SI), there are seven base units, which are the standard units of measurement defined by the International System of Units for the seven base quantities of what is now known as the International System of Quantities. The seven SI base units are:

• Second (time)
• Metre (length or distance)
• Kilogram (mass)
• Ampere (electric current)
• Kelvin (thermodynamic temperature)
• Mole (amount of substance)
• Candela (luminous intensity)

All other SI units can be derived from these base units by multiplying together different powers of the base units. The SI base units are a fundamental part of modern metrology and the foundation of modern science and technology. Prefixes can be added to the base units to scale measurements up or down when needed.