SI units (the International System of Units) form the coherent, standardised basis of every physical measurement: the metre (m), the kilogram (kg), the second (s), the ampere (A) and their derived units (volt, watt, newton and so on), together with standard prefixes (milli, kilo, mega and so on).
In the project
What is at stake here is not theory but engineering rigour: always write a value with its unit, use the standard prefixes, and never mix scales (a centre distance in mm, a cable length in m, a current given sometimes in mA and sometimes in A). A specification or a measurement with no unit, or with an ambiguous one, is a classic source of error, from the conversion bug to the undersized component. Unit consistency holds across every deliverable: the specification, the technical design file, the measurement records.
See also
- Reading a datasheet — where quantities and their units are read
- Technical specification — expressing numerical requirements with their units
- Characterising a requirement — the discipline of a number plus its unit in requirements
- Technical design file — recording values that are consistent with one another