Students and parents alike ask this question in hopes of some secret insight. Unfortunately, the two tests cannot be ranked by difficulty that easily.
First, the design specifications of both tests ensure that each year’s cohort scores along a similar bell curve, meaning that approximately the same numbers of SAT test takers earn scores at various percentile benchmarks as ACT test takers. If two million graduating college-bound seniors take both tests, one million will score in the 50th percentile or lower on each. Further, the tests are crafted to maintain comparable difficulty levels from one administration to the next.
Second, the two tests evaluate highly overlapping reading, writing, math, and test taking content and strategy. Sure, some consider the ACT Math section slightly tougher, but students who struggle with certain SAT-specific math concepts might disagree. In the same vein, SAT Reading questions are deemed tougher than ACT Reading questions, but test takers have more time per SAT Reading question.
Third and last, even if one test was widely agreed to be just the slightest bit easier than the other, that determination would be irrelevant to any specific test taker. Difficulty is relative. Use the differences between the two to guide your research, but explore both and use SAT/ACT Concordance Tables to compare scores. The test that is easier for your high schooler is the one she consistently scores higher on in practice or on official tests. That’s the one to go with!