Inventory availability

Hi,

I need help to determine how to use formula.
Attached is my excel example file, I am doing inventory, where I need to be able to know the total amount items on hand and on project.

I need to be able to know how many inventory on hand and on project which depends on the item that is check-out and check-in, when the items havent check-in it means it still on the project?

So can you help with that?

I really appreciate it.