“Doubling up” (living with relatives or nonkin) is a common source of support for low-income families, yet no study to date has estimated its economic value relative to other types of public and private support. Using longitudinal data from the Fragile Families and Child Wellbeing Study, we examine the prevalence and economic value of doubling up among families with young children living in large American cities. We find that doubling up is a very important part of the private safety net in the first few years of a child's life, with nearly 50 % of mothers reporting at least one instance of doubling up by the time their child is 9 years old. The estimated rental savings from doubling up is significant and comparable in magnitude to other public and private transfers.