As you might already know, the SAT has changed quite significantly over the last few years, and so has the ACT. The percentiles are constantly shifting from year to year, so I've created a helpful table to help you compare SAT and ACT scores for the 2017-18 school year.
Click on the chart below for a larger, printable version.
You might also find it helpful to check out this SAT/ACT concordance table from the College Board, which shows how students who take both tests tend to perform. Please note that percentiles and concordance tables are not the same thing--there are different pools of students who take each test, so percentiles cannot be compared directly.