Another thing that's missing from the Line Item object is a Line Item Total field that displays the product of each Line Item's Quantity and Unit Price. In this tutorial, you implement this common business logic by creating a new formula field in the Line Item object, again, without writing any code.
You must complete Quick Tutorial 1: Build a Cloud App in 5 Minutes before continuing with this tutorial.
In this step, add a new custom field called Line Item Total to the Line Item object that automatically calculates field values from using a formula.
Line Item Total
Accept the defaults on the following screens by clicking Next until you can Save.
To see the new Line Item Total field in action, click the Invoices tab and click an existing invoice. You haven't added the new fields to the Invoice page layout (yet), but if you load a specific Line Item, notice there's now a Line Item Total and it's automatically populated.