Official ACT practice tests should form the core of any ACT prep plan. Take each test timed and review missed questions carefully before moving on to a new test. Keep a record of the questions you’ve missed and how to solve them so that you can go back and review them over time.
There are a total of 12 official practice tests available. The ACT website also contains a series of practice questions roughly equal in length to one practice test. Here is where you can find all this material:
The Real ACT Prep Guide, Third Edition
This contains five tests. These include answer explanations, although they’re of mixed quality.
This online software from the ACT contains two tests. While the tests contain answer explanations (of mixed quality again), you are unable to print the tests and must take them on the computer.
Over the years, the ACT has released 5 additional practice exams as PDFs. These contain answer and scoring information but no answer explanations.
While not technically an exam, this set of official questions roughly matches the length of a full test. Explanations are included.