Child marriage has significant negative impacts, not only for girls, but also for a range of development outcomes. This study aimed to assess, in a more detailed way than done so far, the magnitude of the relationship between child marriage and total fertility in multiple countries representing diverse settings. Data from the most recent Demographic and Health Surveys in the fifteen countries of interest were used. Analysis was restricted to a subsample of women aged 35–49 years in order to capture completed fertility. Poisson regression was conducted to estimate the impact of each additional year of early marriage on the total number of births women have, controlling for selected sociodemographic characteristics. Counterfactual analyses were carried out to estimate the reduction in the number of children that women would have over their lifetime in the absence of child marriage. Controlling for socioeconomic and other characteristics, girls who marry as children have more children over their lifetime than women marrying after the age of 18. Nationally, across fifteen countries, the reduction in total fertility from ending child marriage ranges from 0.24 to 1.06 children per woman. The simulated change in total fertility that would result from ending child marriage tends to be higher in countries that have a higher incidence of child marriage.