Inline Calculator is a plugin for Brackets which lets you do inline calculations with a few keypresses.
All you need to do is select the mathematical expression and press Alt+C.
By default, Inline Calculator rounds the result down to two decimals. However, you can specify the number of decimals you would like in the preferences file. Only create property "inline-calc.precision": (the number of decimals) in the root of the settings tree.
If you want to change the default keybinding, add a new keybinding with command "inline-calc".
Example:
"Ctrl-D": "inline-calc"