Severe Acute Malnutrition (SAM) and its complications are major causes of death in under 5s in the developing world. A simple, reproducible, scalable way to recognise acute malnutrition and target early intervention would be a huge advance. Can mothers with limited training reliably measure and interpret the results of Mid Upper Arm Circumference (MUAC), a measurement performed with a MUAC measuring tape costing $0.06? MUAC contrasts with Z-score (WHZ), which requires both scales and a height gauge, both health worker interventions susceptible to considerable error, sensitive to non-nutritional factors, requiring calculations and growth curves, yet much less accurate in predicting risk of death compared with MUAC.