"Resolving power" is defined as the 95% confidence interval for quantitative trait locus (QTL) map location that would be obtained when scoring an infinite number of markers in a given constellation of a marker-QTL mapping experiment. Resolving power can serve as a close estimate of the confidence interval of QTL map location, as well as a guide to the lower efficient limit of marker spacing in an initial marker-QTL mapping experiment. In the present study, an extensive series of simulations was carried out to provide estimates of resolving power, for backcross (BC) and F2 designs, over a wide range of experimental sizes and of gene effects and dominance at the QTL. From the simulation results, the remarkably simple expressions, 3000/(mNd2) (where m = 1 for BC and m = 2 for F2; N = population size, and d = allele substitution effect) and 530/Nv (in terms of v, the proportion of variance explained), were obtained for estimating resolving power. These expressions can provide a convenient guide to planning marker spacing in BC and F2 marker-QTL linkage experiments and for placing confidence intervals about QTL map location obtained in such experiments.