How to calculate inventory at different levels in the whole supply chain
Your question does not seem to be well-directed, so as to understand what kind of inventory calculation you are looking for. We would like to tell you that since physical counting is a troublesome task, the industry is equipped with certain IT tools like warehouse management system, the delivery management system, etc. These tools assist in keeping a track of inventory at all the nodes like the warehouse, distribution hubs, delivery centres, etc. Along with the stored inventory, these tools can also keep a check on inventory in-transit.
Stock can be scanned using barcodes, QR codes, RFIDs, etc. and the count can be automatically maintained by the system. The systems also are equipped with different analytics reports to guide the management over decisions and keep them informed.