The question Why use a package to typeset units? made me think about something else: In non-scientific context one may have to provide data for persons using ... imperial units (shiver). But there may be two versions of a report, or the desire to put a secondary unit in parentheses following the primary unit value. Is there any package that treats this? So I'd write something like
And the output could be chosen to be any of e.g.
5 m/s 5 m/s (11.18 mph) 11.18 mph (18 km/h)
depending on a definition in the preamble?