Interpreting a score on a standardized test is pretty straightforward: the closer you got to 1600 (for the SAT) or 36 (for the ACT), the better you did.
However, those numbers are arbitrary, meaning there’s no particular reason that 1600 or 36 represent a perfect score on these tests. They don’t mean anything and they were chosen independently of each other, which makes comparing scores from the two tests a bit unintuitive. But you’ll definitely want to compare scores–if you made a 27 on the ACT and a 1350 on the SAT, you should send the more impressive score to colleges when the time comes.
To weigh scores against each other, you’ll want to use an official concordance table. A concordance table organizes SAT and ACT scores by their percentile ranking, or the percentage of test takers who earned lower than a given score. Based on the table provided here, you can see that about 93% of test-takers scored lower than a 1350 on the SAT, but only about 86% of test takers scored lower than a 27 on the ACT. That means the 1350 is the better score, so that’s the one you want to prioritize when applying to colleges.
When in doubt, though, send both! If you attempted both tests, that’s impressive in itself, and colleges like to see students going the extra mile.