The College Board and ACT have created concordance tables to compare ACT and SAT test scores. The tables are based on a group of students who have taken both tests.
If you use the tables, you should be aware of their limitations. There may be differences in students who take both tests and those who take only one of the tests. Students who can afford to take both tests or who take both tests to see on which they score better may have a different academic background than those who take only one test.
According to the methodology used to develop the tables, there is a significant difference between students who take the ACT Plus Writing compared to the total group of ACT test takers.
Furthermore, since the two tests are structured differently, some students may do better on one test than the other. For example, one student scored a 25 ACT Composite which would be an 1150 SAT score on the concordance table. When he took the SAT, he scored a 1340 which was a ACT Composite score of 30 on the concordance table.