A new curve of seawater 87Sr/86Sr variation through the past ∼450 m.y. enables dating and correlation of marine sedimentary rocks with quantifiable uncertainties. We discuss the method by which this curve was constructed. Strictly standardized 87Sr/86Sr ratios and biostrat igraphic data for ∼1300 87Sr/86Sr analyses of marine minerals were input into a data base. Objective criteria were used to assign each data point a reliability score. These scores were used as initial weights in a robust nonparametric regression (LOWESS) to obtain a curve of seawater 87Sr/86Sr variation with rigorous 95% confidence intervals for prediction of age uncertainty. Precise dating and stratigraphic correlation are feasible for much of the Phanerozoic. The main problem with future Sr isotope stratigraphy is likely to be sample quality; we discuss guidelines for sample selection.