The optimal term of copyright has been a matter for extensive debate over the last decade. Based on a novel approach we derive an explicit formula which characterises the optimal term as a function of a few key and, most importantly, empirically-estimable parameters. Using existing data on recordings and books we obtain a point estimate of around 15 years for optimal copyright term with a 99% condence interval extending up to 38 years. This is substantially shorter than any current copyright term and implies that existing terms are too long.