Quantifying custom lines on invoices( added/ multiplied)

Say if I had a custom line on a invoice which I need to multiply the quantity and the unit price to get the amount , can this be done?

Calculation within fields does not work in custom fields. It works in other number fields. See this Guide: https://www.manager.io/guides/18222.

But why would you want to multiply quantity and price except in a line item? What exactly are you referring to as a “custom line?”