Economics, business, accounting, and related fields often distinguish between quantities that are stocks and those that are flows. These differ in their units of measurement. A stock
variable is measured at one specific time, and represents a quantity
existing at that point in time (say, December 31, 2004), which may have accumulated in the past. A flow variable is measured over an interval of time. Therefore a flow would be measured per unit of time (say a year). Flow is roughly analogous to rate or speed in this sense.
For example, U.S. nominal gross domestic product
refers to a total number of dollars spent over a time period, such as a
year. Therefore it is a flow variable, and has units of dollars/year.
In contrast, the U.S. nominal capital stock
is the total value, in dollars, of equipment, buildings, inventories,
and other real assets in the U.S. economy, and has units of dollars. The
diagram provides an intuitive illustration of how the stock of capital currently available is increased by the flow of new investment and depleted by the flow of depreciation.