Shouldn't
we give both, then?
Why? If the reader really needs non-metric measurements, they can consult
[[conversion of units]] (which, actually, may need a bit of expanding for
ease of use as a conversion table... I might look into that in a bit). That
said,
http://www.wikipedia.org/wiki/Wikipedia%3AManual_of_Style currently
reads, in part, "For now, if using American or Imperial, give metric as a
courtesy. If using metric, remember that many readers will not know what you
mean and will be aided by the equivalent.", so yes, provide both, I'm afraid.