The following instructions affect all units in the file you are working with. This includes all previously created units as well.
1. Go to Options > Document Properties
2. Select Units. Then change the Unit System over to Custom. (This option then allows the Length option from Basic Units to change.) Select microinches, mils, inches, feet, or feet & inches in order to be able to use the Fractions selection.
3. Select the box under Fractions to decide on a fraction denominator from 2 to 256. (In the example below, the number "8" is entered. Entering "16", for example, will allow precision of up to 1/16th of an inch.)
4. Select the box under More that shows “…” Then select "Round to Nearest Fraction" to have the inches round to the nearest fraction. (Note that if this selection is not made, then only exact fractions will show.)