Various normalization techniques to transform journal impact factors (JIFs) into a standard scale or range of values have been reported a number of times in the literature, but have seldom been part of collection development librarians' tool kits. In this paper, JIFs as reported in the Journal Citation Reports (JCR) database are converted to percentiles (0%-100%) using Microsoft Excel's PERCENTRANK function. This permits a more intuitive evaluation of journal ranking within JCR subject categories and broader disciplines. The top journal by impact factor in any category is set to 100% while the lowest impact factor journal is set to 0% with all other journals on the list scaled according to their rank. Percentile-based impact factors (PIFs) also allow valid cross-disciplinary comparisons of impact factors, something not possible using JIFs. Finally, since a given journal title is often assigned to multiple subject categories, the relative impact of the same journal title can be compared and evaluated across each of those categories. This paper argues that PIFs should become a standard component of journal collection evaluation projects. The history, use, and misuse of impact factors are also discussed. (Contains 3 tables.)