I would like to change the below dimension to be calculated from an equation (be based on global variables):

Expected:

- I right-click on the dimension, and there would be some way.to convert it to this:

Actual:

- I can only derive a sketch dimension from an equation by _creating_ one with such. Thus: 1. delete existing dimension, 2. create a new one using global variables, 3. retype the comments etc. that were lost in this process

Is this so (making this a feature request)?

Suggestion:

When right clicking on an equation-driven dimension, there is "Edit Equation..." menu item. I feel this could be there also for constant dimensions, showing the user that there's a possibility of dimensions to be equation driven. It's best if the context menus are relatively uniform - if the state of a drawing matters for a certain menu to exist or not, it takes some time before we users learn the logic. This seeming randomness is likely one of the biggest hurdles for newcomers to SolidWorks.

SolidWorks 2018 SP 2.0

In addition to what Rob Edwards says, you can also get at it this way:

Then you can use the same = sign to get your equation.