Abstract: The bracelet for assessing acute malnutrition (MUAC Strip) has become the signature tool of humanitarian aid: it is widely used for screening children into feeding programs, for producing statistics on nutritional status and for mapping emergencies. This article takes this tool as an entry point into the history of humanitarian expertise, following the medical doctors who invented the strip from the 1960s until today. Humanitarian organizations often argue that they address needs all over the world because human needs are universal per se. However, the history of humanitarian expertise shows that needs—in this case acute malnutrition—had to be made commensurable in the first place. Thus, this article is an invitation to historicize the production of universalisms.