Current estimates of the global Muslim population cited in the academic literature range between
one billion and nearly two billion. In this article, we describe the most rigorous effort to date to estimate
the size of the Muslim population in each country of the world, resulting in a global estimate
of 1.57 billion Muslims in 2009, or nearly one in four of the world‘s 6.8 billion inhabitants.
We describe a demographic data quality index that we developed to assess the sources used in our
estimate. We discuss the limitations of censuses, surveys, and other data sources, and we argue
that the best strategy is to select the strongest data source for estimating the size of the Muslim
population for a particular country rather than using the index to create a weighted average. We
also present other innovations, including a methodology for combining separate male and female
demographic and health survey datasets to arrive at country estimates for religious affiliation.