What is the Spreadsheet Perpetual Inventory Method? It is a method to use a Spreadsheet to derive a stock value from a continuous list of flow variables. In the Spreadsheet Perpetual Inventory Method the flow inputs are totaled for the specific period of that cohort of flow data to provide the total stock in that period. The Spreadsheet Perpetual Inventory Method not only provides more accurate and dependable results than the Perpetual Inventory Method using mathematical equations, but it is simpler to use. The results are more understandable to the average person. Also, you do not require years of study as an actuary or a demographer to learn and use complex formulae. All you need to understand is the relationship between flows, including depreciation and appreciation when they are used, and stocks at any point in time. The user just has to understand simple arithmetic. Indeed, after the spreadsheet has been set up, the user just needs to know just how to use the spreadsheet and how it works. Yet this spreadsheet produces more accurate results than using mathematical equations. In this book the Spreadsheet Perpetual Inventory Method (SPIM) is described clearly and precisely, and this methodology can be used estimate the total stock or population at any time from the flows of the inputs and outputs. These flows can be continuously depreciated and appreciated at the same time, yet produce accurate totals. SPIM will greatly simplify accounting inventory estimation, actuarial methods, demography, ecology, and any other study that tries to estimate the value of stocks from flow data.