Unlike plays and television scripts, which explicitly indicate their act breaks, most feature screenplays don’t print act breaks anywhere in the script.
But in everyday discussion, screenwriters generally talk about movies having three acts.
Crap. I’m at page 38 but I’m nowhere near the end of the first act.
Boom! Page 90, worst-of-the-worst, big third act set piece.
With a 120 page screenplay, the first act will be around 30 pages, the second will be 60, and the third will be 30. So the never-actually-included act breaks would come at roughly pages 30 and 90.
This way of talking is largely based on Syd Field’s paradigm, in which acts 1, 2a, 2b, and 3 each take up roughly one quarter of a screenplay. Even screenwriters who dismiss Field’s approach end up using some of the terminology, because it’s pervasive in the industry.