A nonlinear design technique is described for predicting the high power sound output and efficiency of a magnetostrictor. The method is based upon the assumption that the magnetic permeability of the active material is strain invariant. An experimental perturbation technique, and an alternative method involving the measured strain field, are used to determine the complex transformation ratio of a Ni‐Co ferrite at various values of the field, induction and strain. The results are plotted as sets of nonlinear design characteristics. The transformation ratio is used in conjunction with the Van Dyke equivalent circuit to predict the nonlinear sound output and efficiency of a magnetostrictor formed from the ferrite. The magnetostrictor is a slotted bar transducer resonating at 24.8 kHZ in the fundamental‐length mode. A novel feature of the experiments is the use of a piezoelectric transducer (dummy load) to backdrive the magnetostrictor to vary the strain independently. The magnetostrictor sound output is predicted to an accuracy greater than 15% at induction swings as large as one‐third of saturation. The shape of a skewed resonance curve is successfully predicted indicating that nonlinearity is a cause of the resonance curve distortion.