Stock and flow
A stock in business and social accounting refers to the value of an asset at a balance date (or point in time), while a flow refers to the total value of transactions (sales or purchases, incomes or expenditures) during an accounting period. If the flow value of an economic activity is divided by the average stock value during an accounting period, we obtain a measure of the number of turnovers (or rotations) of a stock in that accounting period. Some accounting entries are normally always represented as a flow (e.g. profit or income), while others may be represented both as a stock or as a flow (e.g. capital).
However, as accounting and economic concepts have been applied outside the field of business, stocks and flows may also take on other meanings. Thus Stocks and flows are the basic building blocks of system dynamics models. Jay Forrester originally coined them as "levels" (for stocks) and "rates" (for flows).
A stock (or "level variable") in this broader sense is some entity that is accumulated over time by inflows and/or depleted by outflows. Stocks can only be changed via flows. Mathematically a stock can be seen as an accumulation or integration of flows over time - with outflows subtracting from the stock. Stocks typically have a certain value at each moment of time - e.g. the number of population at a certain moment.
A flow (or "rate") changes a stock over time. Usually we can clearly distinguish inflows (adding to the stock) and outflows (subtracting from the stock). Flows typically are measured over a certain interval of time - eg. the number of births over a day or month.
Examples
| Stock | Inflow(s) | Outflow(s) |
| inventory | incoming goods | outgoing goods |
| guests in a hotel | guests arriving | guests leaving |
| population | births immigration | deaths emigration |
| water in bathtub | water pouring in | water leaving through sink |
| waste in disposal site | dumping waste | decay of waste |
| bank balance | paying in | withdrawals |
| fuel tank | refueling | fuel consumption |