A flow variable in economics is a quantity that is measured over a time period. This is in contrast to stock variables, which are measured at a specific time. Flow variables can be measured over any time period but the most common include day, week, month, quarter and year.
Flow variables describe many types of services and goods. A flow variable is useful as it allows economists to compare performance over time periods. Some common examples of flow variables include a person's yearly salary, interest, exports and rent. Any quantity that includes a time reference such as "per hour" or "per month" is a flow variable.
An important example of a flow variable is a country's national income. This is a measure of all the goods and services that are sold during a calendar year. By describing this quantity as a flow variable with a fixed interval of a year, economists can quickly compare how a country's national income is changing over time.
Stocks are another type of variable in economics. A stock does not have an element that is measured over time like flow variables. Instead, economists take a specific measurement of a stock at a fixed moment. Examples of stock variables include loan quantity and population.