Young children's integration of weight and distance information was examined using a new methodology that combines a single-armed apparatus with functional measurement. Weight and distance values were varied factorially across the item set. Children estimated how far the beam would tilt when different numbers of weights were placed at different distances from the pivot. There was a developmental progression from non-systematic responding (3-year-olds) to responding based on a single variable, usually weight (4-year-olds) to responding based on integration of weight and distance (5-, 6- and 7-year-olds). Individual analyses revealed additive and multiplicative integration rules in children aged from 5 years. Weight-distance strategy complexity increased with age and was associated with better performance on independent measures of relational processing, especially the more complex items. Thus weight-distance integration involves processing of complex relational information. The findings enhance the explanatory power of Relational Complexity theory as a domain-general approach to cognitive development.