The tradeoff between saturation flux density and resistivity in soft magnetic materials for thin film inductors is examined quantitatively. We use an optimization routine to evaluate the achievable efficiency and power density using designs individually optimized to make best use of particular materials. The optimization selects switching frequency, ripple ratio, inductor dimensions and MOSFET sizes. For the example application of a 3.3- to 1.1-V, 7-A output dc-to-dc converter, Co-Zr-O, Fe-Al-O and Co-Fe-B-Si-O materials are considered. Predicted efficiencies are in the range of 80% to 90% at power densities of 20 W/cm2 to 1000 W/cm2. Different materials show advantages in different power-density or efficiency ranges.