That all depends on the correct database design and implementation. You have given FAR too few details to understand where the problem may be. I would suggest carefully studying inventory applications: how are they designed. How is stock taken. How do I
run a query to get the current quantity? You do NOT want to reinvent the wheel here.
One of the good resources is the Inventory sample application by Ken Sheridan here: https://onedrive.live.com/?cid=44CC60D7FEA42912&id=44CC60D7FEA42912!169
-Tom. Microsoft Access MVP