Stock and flow

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") 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 account	paying in	withdrawals
fuel tank	refueling	fuel consumption