What is the difference between stocks and flows?

Data on all categories of migrants can be collected as stock or flow data. International migrant stock is the total number of international migrants in a country at a particular point in time (usually mid-year or end-year) while international migrant flow is the number of international migrants entering or leaving a country over the course of a specific period, usually one calendar year. Migration flows are crucial for understanding migration patterns and trends or how different factors and policies are affecting international migration (IOM, 2020)). However, in 2015 only 45 countries were reporting flow data to the United Nations, while data on migrant stock are available for around 200 countries. Furthermore, of the 45 countries with data on immigration flows, only 24 identify international migrants as persons who plan to stay for at least one year (UNDESA, 2015). Stock and flow data reported by countries are being published by organizations such as UNDESA (2015), Eurostat (2023), The International Migration Institute (2013) and The World Bank (2023) (see references for links).