An SI prefix supplies an exact decimal factor when attached to a unit of measurement. For example, milli means and micro means ; a millimetre and a micrometre are correspondingly scaled metres. The prefix symbol and unit symbol form one unit that can itself be raised to a power.[ref]
The International System of Units (SI), ninth edition, SI Brochure. Chapter 3, Table 7 and rules following the table
For an original conversion example,The area conversion squares the factor as well as the unit. Applying just one factor of to that area would give , a value 1000 times too large. Scientific notation makes the power explicit.
Prefix symbols are case-sensitive and cannot be stacked. For mass, prefixes attach to gram, because the base-unit name kilogram already contains a prefix. SI prefixes denote powers of ten, even in a computing context; they do not silently change into powers of two.[ref]
The International System of Units (SI), ninth edition, SI Brochure. Chapter 3, printed pp.138–139: capitalization, compound prefixes, kilogram and decimal factors
An exact unit conversion scales any stated measurement uncertainty with the value; it does not make an uncertain measurement exact. New to topics? Read the docs here!