Metric units are units based on the metre, gram or second and decimal (power of ten) multiples or sub-multiples of these. According to Schadow and McDonald,^[1] metric units, in general, are those units "defined 'in the spirit' of the metric system, that emerged in late 18th century France and was rapidly adopted by scientists and engineers. Metric units are in general based on reproducible natural phenomena and are usually not part of a system of comparable units with different magnitudes, especially not if the ratios of these units are not powers of 10. Instead, metric units use multiplier prefixes that magnifies or diminishes the value of the unit by powers of ten." The most widely used examples are the units of the International System of Units (SI). By extension they include units of electromagnetism from the CGS and SI units systems, and other units for which use of SI prefixes has become the norm. Other unit systems using metric units include:

• International System of Electrical and Magnetic Units
• Metre–tonne–second (MTS) system of units
• MKS system of units (metre, kilogram, second)

The first group of metric units are those that are at present defined as units within the International System of Units (SI). In its most restrictive interpretation, this is what may be meant when the term metric unit is used.

The unit one (1) is the unit of a quantity of dimension one. It is the neutral element of any system of units.^[2]

In addition to the unit one, the SI defines 7 base units and associated symbols:

• The second (s) is the unit of time.
• The metre (m) is the unit of length.
• The kilogram (kg) is the unit of mass.
• The ampere (A) is the unit of electric current.
• The kelvin (K) is the unit of thermodynamic temperature.
• The mole (mol) is the unit of amount of substance.
• The candela (cd) is the unit of luminous intensity.

The SI also defines 22 derived units and associated symbols:

• The hertz (Hz) is equal to one reciprocal second (1 s⁻¹).
• The radian (rad) is equal to one (1).
• The steradian (sr) is equal to one (1).
• The newton (N) is equal to one kilogram-metre per second squared (1 kg⋅m⋅s⁻²).
• The pascal (Pa) is equal to one newton per square metre (1 N⋅m⁻²).
• The joule (J) is equal to one newton-metre (1 N⋅m).
• The watt (W) is equal to one joule per second (1 J⋅s⁻¹).
• The coulomb (C) is equal to one ampere second (1 A⋅s).
• The volt (V) is equal to one joule per coulomb (1 J⋅C⁻¹).
• The weber (Wb) is equal to one volt-second (1 V⋅s).
• The tesla (T) is equal to one weber per square metre (1 Wb⋅m⁻²).
• The farad (F) is equal to one coulomb per volt (1 C⋅V⁻¹).
• The ohm (Ω) is equal to one volt per ampere (1 V⋅A⁻¹).
• The siemens (S) is equal to one ampere per volt (1 A⋅V⁻¹).
• The henry (H) is equal to one volt-second per ampere (1 V⋅s⋅A⁻¹).
• The degree Celsius (°C) is equal to one kelvin (1 K).
• The lumen (lm) is equal to one candela-steradian (1 cd⋅sr).
• The lux (lx) is equal to one lumen per square metre (1 lm⋅m⁻²).
• The becquerel (Bq) is equal to one reciprocal second (1 s⁻¹).
• The gray (Gy) is equal to one joule per kilogram (1 J⋅kg⁻¹).
• The sievert (Sv) is equal to one joule per kilogram (1 J⋅kg⁻¹).
• The katal (kat) is equal to one mole per second (1 mol⋅s⁻¹).

Furthermore, there are twenty-four metric prefixes that can be combined with any of these units except one (1) and kilogram (kg) to form further units of the SI. For mass, the same prefixes are applied to the gram (g) instead of the kilogram.

There are several metric systems, most of which have become disused or are still used in only niche disciplines. Systems are listed with named units that are associated with them.

• Electrostatic units
• Gaussian units
• Gravitational metric system
• History of the metric system
• Metric system
• Outline of the metric system
• RKM code
• Unified Code for Units of Measure