Using data from the International Social Survey Program and the World Values Survey about current and former religious adherence, we calculate country-level religious-conversion rates for 40 countries. Drawing upon a theoretical model based on rational individual choice, we posit that the frequency of religious conversion depends on the cost of switching and the cost of having the “wrong” religion. Findings accord with several hypotheses: religious-conversion rates are positively related to religious pluralism, gauged by adherence shares; negatively related to government restrictions on religious conversion; positively related to levels of education; and negatively related to a history of Communism. Conversion rates are not related to per capita GDP, the presence of state religion, and the extent of religiosity. Effects from the types of religious adherence are minor, except for the negative effect of Muslim adherence.