Spectrum demand is a key input to decisions over spectrum allocation and assignment. Yet estimates of spectrum demand are sensitive to assumptions regarding future mobile traffic, which are uncertain. In this Insight we consider an alternative approach to modelling the demand for spectrum which dispenses with the need for a data traffic forecast. Instead, the approach iteratively solves an economic supply-demand balance based on assumptions regarding network costs and consumer willingness to pay for data. The approach offers fresh insights into mobile and spectrum demand, and provides a foundation for a deeper understanding of these core questions for policy makers and industry.