You can usually add optional Science or Writing in MyACT after you register, for a fee, until that date's change deadline. Adding a section is not the same as moving the date or the center.
When adding Science makes sense
A college on the list still requires it or will read a STEM score. Colleges that require Science, what the STEM score is. If nobody will look, do not add 40 minutes for sport.
When adding Writing makes sense
A campus or scholarship still lists the essay. Should you take Writing.
How to do it
Log into MyACT, open the registration, choose add Science and/or Writing, pay, and check that the admission ticket updates. If the change window has closed, you sit the core you already bought.
The bottom line
Add a section only when a named school will read it, and only before the change deadline. Otherwise protect the three-section Composite.
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.