We present a new method that allows paleomagnitude and paleorupture length to be estimated quantitatively given a measurement of earthquake rupture displacement at a point along a fault. Rupture displacement typically varies along a rupture profile such that a point paleoseismic displacement measurement constrains the average displacement only to within a factor of three or so. We used previously published results summarizing rupture variability and then applied a graphical method of identifying the relative likelihoods among a suite of magnitudes, one of which must have caused the measured displacement. Results were developed for displacement observations from 1 to 6 m using a magnitude range of 6.0 ≤ M ≤ 8.0. Probabilities of rupture lengths for a given displacement were developed at the same time. Although smaller earthquakes can cause ground rupture, we show that they would not strongly influence likelihoods for 1 m and larger observed displacements. Displacements less than 1 m are also of potential interest but will require extension of the method to include the declining probability that smaller magnitudes produce ground rupture. We also consider application of length distributions inferred from a displacement measurement to correlation of rupture evidence between sites. Dating evidence alone, even when excellent, does not provide a physical basis to relate rupture at one site to rupture at another. Ruptures, however, have an expected length, and thus do provide a physical basis for correlation. We present probability of correlation curves for given rupture lengths, which may be combined with probabilities of rupture length to obtain a probability of correlation given a point displacement. Applications for quantitative probabilities of magnitude and length given a paleoseismic displacement measurement include probabilistic seismic hazard analyses, where probabilities of magnitude and length must be assigned to branches in the analysis.