Microsoft Word will auto-correct many common fractions into their fraction symbol, but it does not auto-correct all of them by default.  Less common fractions, such as 1/8 or 5/8, will not auto correct. Fortunately, it is very easy to add custom fractions to your auto-corrections and Word will auto-correct all future instances. Below is a link to Microsoft which explains this process in detail.

support.microsoft.com/kb/137734