A recent thread at AUGI and then an echo of the question at a client office prompts me to write this one. The question is, "Can we show both metric and imperial units in the same dimension string?" More specifically the client request was to provide for "Hard Metric" and the imperial value at the same time.
Regardless, sorry, the answer is no. You cannot automatically show both imperial and metric dimensions in the same dimension string. Even if you could show them both Revit could not provide the "Hard metric" value because this is not a consistent conversion.
In an attempt to be clearer, the concept of "Hard" metric values is the alteration of actual dimensions to a "normalized" or nominal metric equivalent, for example: 36 inches = 900 mm or 8 inches = 200 mm. Conversely "Soft" metric is the literal mathematical equivalent dimension or as in the previous example: 36 inches = 914.4 mm or 8" = 203.2 mm.
If it helps you could think of the normalized metric value as "hard" to do while the mathematical conversion is "easy" or..."soft". My interpretation of this is based on reading a number of governmental sites that discuss the use of this practice. Fwiw, I have heard it described exactly the opposite way, hard is soft and soft is hard, hmmm isn't that a Zen thing? Regardless there is a different approach for each.
With regard to Metric versus Imperial project templates and choosing which to start with...consider that an imperial project can be switched to metric easily...to soft metric. Easy, however, the content will STILL refer to imperially named families. As a practical matter a truly metric project should begin with a metric template and use metric content so that schedules report 200 mm walls not 8" walls or Single Flush 900x2100 doors instead of 36x84 doors, for example.
As for the display of either unit of measure in documents...my cynical side wonders who really wants or needs this? In my North American tunnel vision I don't see a real need to show both even though a local jurisdiction may require it. Despite being told since I was a young lad that we are going to "go to metric"...we still haven't. In a practical way our trades don't really need it and in fact may just be confused by it. An exception might be where the actual material involved is truly specified and supplied according to metric specification. Surely this doesn't require an entire set of documents to adhere to such requirements?
Well enough assumption...what to do if you must? You could consider metric plans and imperial plans, completely separate views. Document it fully as if metric were the only appropriate unit of measure, do the same for imperial.
or...you can place a string using one and then place another string either above or directly on top of the previous string and then move the text off to the side "next" to the other. Sure looks like it is the same string when it is printed out but "we" know better. There are a few ways to replicate the dimensions quickly, such as copy/paste aligned. Still quirky and definitely extra work.
Personally I kind of like the idea of duplicating a view with detailing and then switching all the dimensions to a metric formatted version. Either way just keep in mind that you won't get Hard metric, just Soft.
What I like better still is the ability to provide alternate dimension unit display, much like the "step-brother" AutoCAD perhaps? It probably isn't all that hard to do, just needs doing? If it gets done this post will be pointless eh? Here's to pointless posts!