For engineering, CS, and pre-med, pick the test where your math score is stronger—then take ACT Science if you chose the ACT and the colleges might use a STEM score. Neither exam is "the STEM test." Math weight and whether you can show science reasoning are the real differences.
Math
Digital SAT is very math-heavy in the composite. ACT Math is one of three Composite sections and always allows a calculator. If SAT Math modules crush you and ACT Math does not, that can decide the whole application testing plan. Improve ACT Math.
Science (ACT only)
Optional, not in the Composite, but it creates a Science score and a STEM score (Math + Science). Engineering colleges often still glance at it. Do engineering colleges care about Science, is Science optional.
When SAT can still win for STEM
You already have a strong Digital SAT, your ACT Reading is the hole, or every target is SAT-fluent and test-required. Do not add a weak ACT Science sitting just because a blog said "STEM takes Science."
The bottom line
Diagnose both. Commit to the higher math story. On the ACT, default to taking Science for STEM unless a college tells you not to.
Start practicing
Start with a free diagnostic, then drill your weak spots with 15-question quizzes and track how you're doing across Reading, English, and Math. Compare plans whenever you're ready to go further.
This article offers general ACT prep guidance. The ACT can change from year to year, including its format, scoring, policies, test dates, and fees, so always confirm the latest details on the official ACT website at act.org before you make decisions. ACT® is a registered trademark of ACT, Inc. thirty-six is not affiliated with, endorsed by, or sponsored by ACT.