The world’s Muslim population grew by an estimated 347 million from 2010 to 2020, reaching about 2 billion. Pew Research Center ranked Islam as the fastest-growing major religion over that decade. The report was released in 2025 and compares population estimates for two years, 2010 and 2020. It cannot establish which religion grew fastest during the single calendar year of 2022. Understanding the decade’s result starts with what the researchers counted and how population growth occurs. Pew’s global population estimates Pew’s summary of Muslim population growth
What exactly grew fastest?
One measure is the change in the number of people who identify with a religion. Another is the rate of growth relative to the group’s initial size. The estimated Muslim population increased by 347 million, or about 21 percent of its starting count. A third measure is the group’s share of the entire world population. For Muslims, that share increased by about 1.8 percentage points, to 25.6 percent. Each measure describes the same decade with a different denominator. Pew’s global population estimates Muslim population estimates by region
Compare the Christian figures in the same report. The number of Christians rose by 122 million, reaching roughly 2.3 billion. Their share of the global population still declined because the total population grew faster. A reader can therefore encounter three accurate descriptions in the same report. Christianity remained the largest religion, it gained people in absolute terms, and it accounted for a smaller proportion of the world at the end of the decade. The rate at which a group grows and its rank by size are separate results. Pew’s global population estimates
How age and births affect a population
Natural increase is the balance between births and deaths. Age structure matters because it affects the number of people who are likely to have children or die during a given interval. Consider two populations with the same total number of people and similar fertility rates. If one has many more adults of childbearing age, it can record more births even when the average number of children per woman is similar. A group with more people at older ages will generally also have more deaths over the same interval. Pew’s analysis of demographic factors
Pew identified a relatively young population and higher fertility as the main reasons for rapid global Muslim population growth in the decade it studied. The researchers estimated, using 2015–2020 data, that a Muslim woman would have an average of 2.9 children over her lifetime, compared with 2.2 for women outside the group. Those are estimates based on fertility patterns for a particular period. Individual families vary, as do age profiles and fertility patterns across countries. Pew’s analysis of demographic factors Pew’s summary of Muslim population growth
Age also presents a measurement problem. Many censuses cover all ages, but surveys often ask adults about their religion and say little about children. For some countries Pew estimated the share of children in each group using fertility and age data and assumed that children shared their mother’s religion. This fills a gap in a population estimate. It is distinct from asking those children how they identify when they grow up. Research methods and data sources
What a survey can tell us about switching
Pew estimates religious switching by comparing the group adults say they were raised in with the one they report now. This can capture someone who became Muslim as an adult or who grew up Muslim and now gives a different answer. It does not measure every change in religious practice, and a person who moved between traditions within the Muslim category may still give the same broad affiliation in both answers. The method concerns the way an adult reports identity at two stages of life. Pew’s analysis of demographic factors Research methods and data sources
The researchers found that movement out of Islam and movement into it were small relative to the group’s size and roughly offset each other at the global level. A local community can have a different experience. The figures also leave room for the experiences of individual converts, for whom a change in affiliation can be central to their lives. Those individual changes answer a different question from what accounts for hundreds of millions of people in the worldwide increase. Pew’s analysis of demographic factors
How to read the dates
The worldwide comparison draws on more than 2,700 sources for 201 countries and territories. Pew compared the same two endpoints across the groups, using censuses, surveys and other evidence. It revised its earlier estimates for 2010 as part of the study. Comparing a figure from an older publication to a figure from the 2025 series can mix different measurement decisions. The two endpoints in the same report were constructed to be more comparable. They also describe net change over ten years, with no annual series between them. Pew’s global population estimates Research methods and data sources Explanation of revisions to the 2010 estimates
Conclusion
The evidence supports a specific claim about Islam’s global growth from 2010 to 2020. A young population and higher fertility explain much of the rise, while estimated movement into and out of the religion largely balanced at the worldwide level. The result is easiest to interpret when the interval, the measure of growth and the kind of affiliation being counted are all stated. Pew’s global population estimates Pew’s analysis of demographic factors Research methods and data sources
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