WorkGuru uses the FIFO (first in first out) perpetual inventory method.  Perpetual inventory tracks inventory changes in real time, and knows the value of the stock, from which transaction created it, and which transaction used it.  WorkGuru tracks your stock on a FIFO, Serial or Batch basis. When Purchase orders are received, the stock on hand is increased, and it is reduced at the value of the oldest when you use stock on a project, dispatch a stock sale, or do a stock count / adjustment