Diameter dimensions can be modified to display as linear dimensions if needed.
To convert the diameter dimension into linear follow these steps:
Right click the Diameter dimension > Display options > Display as linear
OR this can also be done by selecting the dimension > property manager launches on left > go to Leaders tab > select the linear dimension icon.
Now, move the dimension in such a way that value centers in between the extension lines.
Drag the little dot (handle) right beside the value and re-position the dimension as a linear dimension.