Studies of periodically modulated linear-ramp voltammetry have previously been undertaken by simulation coupled with Fourier analysis. Here, we develop a simple formula, based upon neither of these approaches, which permits the prediction of the magnitude of the ac current resulting from a reversible faradaic process. The formula is applicable to any periodic modulation, of any amplitude, and of any frequency, provided that the frequency/ramp rate quotient exceeds 4piF/RT. The formula has been validated using known results for sinusoidal and square-wave modulation.