Custom field multiplication and track

i Have created 2 new custom fields before Qty
i want them to multiply both and autofill quantity also both fields should add and minus sale and purchase in inventory

You can’t

The closest is to have a field in which you enter information in a format which is also a valid number calculation in Manager then manually past that into Manager’s quantity, unit price or amount field.

Eg if an item is sold by area and you want to enter length & width. For a length 6 & width 7 enter in an interim field (custom or description)
6 * 7

Then manually paste that into quantity