A system and method for generating a probability distribution indicating the probability of a range of performance ratios between computer systems. Benchmark and/or other testing results are obtained for both computer systems. For each test, a ratio of one system's performance to the other is calculated. The ratios and/or testing results may be weighted. From the performance ratios, a histogram is produced to indicate the probabilities of achieving the various ratios. Based on the distribution of probabilities, a particular performance ratio can be predicted with a corresponding level of confidence, and used for capacity planning or configuring a computing environment.