The ACT and SAT are different tests that measure similar but distinct constructs. The ACT measures achievement related to high school curricula, while the SAT measures general verbal and quantitative reasoning.
ACT and the College Board (producers of the SAT) have completed a concordance study that is designed to examine the relationship between two scores on the AC and SAT. These concordance tables do not equate scores, but rather provide a tool for finding comparable scores.
You can also find the concordance tables and guidelines for proper use by clicking Concordance.