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[[:en:SI derived unit|Derived units]] are formed from multiplication and division of the seven base units and other derived units<ref name=SI>Ambler Thompson and Barry N. Taylor, (2008), [http://physics.nist.gov/cuu/pdf/sp811.pdf ''Guide for the Use of the International System of Units (SI)''], (Special publication 811), Gaithersburg, MD: [[National Institute of Standards and Technology]], p.&nbsp;3.</ref> and are unlimited in number;<ref>{{SIBrochure8th|page=103}}</ref> for example, the SI derived unit of speed is metre per second, m/s. Some derived units have special names; for example, the unit of resistance, the ohm, symbol Ω, is uniquely defined by the relation Ω&nbsp;=&nbsp;m²·kg·s⁻³·A⁻², which follows from the definition of the quantity [[:en:electrical resistance|electrical resistance]]. The [[:en:radian|radian]] and [[:en:steradian|steradian]], once given special status, are now considered dimensionless derived units.<ref name=SI/>

A [[:en:metric prefix|prefix]] may be added to a unit to produce a multiple of the original unit. All multiples are integer powers of ten, and beyond a hundred(th) all are integer powers of a thousand. For example, ''kilo-'' denotes a multiple of a thousand and ''milli-'' denotes a multiple of a thousandth; hence there are one thousand millimetres to the metre and one thousand metres to the kilometre. The prefixes are never combined, and multiples of the kilogram are named as if the gram was the base unit. Thus a millionth of a metre is a ''micrometre'', not a millimillimetre, and a millionth of a kilogram is a ''milligram'', not a microkilogram.

In addition to the SI units, there is also a set of [[non-SI units accepted for use with SI]], which includes some commonly used non-coherent units such as the [[:en:litre:litre]].