Converting a Quick Table Calculation into a Calculated Field

Option 1

1. Create the quick table calculation. For information on how to create a quick table calculation, see Quick Table Calculations in the Desktop Help.
2. Drag the Quick Table Calculation pill to the Dimensions or Measures pane and rename the calculated field as desired.

Option 2

1. Create the quick table calculation. For information on how to create a quick table calculation, see Quick Table Calculations in the Desktop Help.
2. Double-click the Table Calculation in the view and copy the formula. 
3. Select Analysis > Create Calculated Field... 
4. Rename the calculated field as desired and paste the formula. 
5. Click OK.

To view the above steps in action, see the video below: 
How to convert a quick table calculation into a calculated field in Tableau