An inventory monitoring system measures changes in weight of an inventory stored on one or more scales. The inventory items are divided into classes by weight, and the system determines the maximum number of items that can be in a combination while maintaining a distinct combined weight. Items are then removed or added, and the change in weight on the scales is correlated to a change in the number of inventory items to track the number of items removed or added over time. In a combined system, scanners are employed to track inventory received or sold, and data from the scanners relating to the items in the inventory is filtered out for comparison to the inventory changes indicated by changes in weight. The system has particular utility for monitoring inventories of cigarettes.