Child or early marriage is recognized as an important development and human rights issue that affects girls especially in many developing countries. The practice has been linked to psychological, health, and education risks. These negative impacts explain why in many countries child marriage has been prohibited by law but often with little effect. While child marriage has been recognized as a major issue, its measurement has remained unsophisticated. Existing studies tend to simply report the incidence of child marriage, that is the share of girls who marry early within a population. Yet, the consequences of child marriage are not the same whether one marries at the age of 12 versus 18. Typically, the cost of child marriage for health, education, and well-being is much larger when the girl marries very early. This paper suggests that it would be straightforward to use the techniques of poverty measurement in order to provide richer information on the extent of child marriage, including its depth and severity apart from its incidence, and to test for the robustness of child marriage comparisons between groups or over time to the age threshold used to identify child marriage. An illustration is provided for Nigeria.