Since UNSCR 1325, the number of female peacekeepers has steadily been growing. We assess the variation of the proportion of female peacekeepers across missions. Specifically, we ask: why do some missions receive more female personnel than others? And, why do some countries send more female peacekeepers than others? We argue and find support for three factors that influence the variation: the availability of female personnel, a gendered protection norm in contributing countries and gender equality within contributing countries. Out of the three factors, the gendered protection norm especially stands out as an important limitation on the female ratio balance in peacekeeping operations.