¼-to ½-inch, 27- gauge needle

18:33 Sep 7, 2018 

Sometimes the manufacturer will provide equivalent in mm. You are right, conversion is not always accurate.

18:28 Sep 7, 2018 

The danger with changing measurements from inches to metric is that they don't always exactly match up. (Which is why I've thrown out an old set of English spanners.) Also, I was surprised to find, when buying a plumbing fitting in a French DIY store, that the sizing was in inches, not mm.

18:18 Sep 7, 2018 

Tony is right on that one, inch fraction may need to be written in mm for the European market. If it is for Canada, however we normally write both mm and (inches).

18:00 Sep 7, 2018 

To some extent, it depends on the purpose of your translation — if the needle length as expressed here is simply a guide, then there might be an argument for converting to metric. However, if as I suspect this is more a case of instructing to use certain standard-size needles that are identified by their length, it is probably best to keep them in EN only. I would imagine these are possibly made by a US manufacturer, hence the use of imperial units. Or is it just that the document originates from the US?
Regarding 'gauge', the diameters of many things in medicine are expressed in units of 'French', whereas diameters of other things are often in AWG, which is an inverse scale (it represents something like 'so many to the inch'. 27 AWG wouldn't be impossible for the size of a (quite fat) needle, but not being a medical expert, I can't tell you which measurement system is normally used; in any event, as Lorraine rightly says, whatever units it is in, it certainly represents the 'calibre'.