On the paper ACT, the edge of your answer sheet can stand in for a ruler. The digital test has no answer sheet, so this does not exist there. Format differences are in digital or paper ACT.
What you can actually do
Line the sheet up with a figure to compare lengths, to see whether a line looks parallel to another, or to extend a segment to where it meets an axis. You are not measuring centimeters. You are comparing. Angle and line facts you still have to know are in lines and angles.
Do not bring a real ruler, a protractor, or any measuring tool. Those are not on the permitted list. What you may carry is in what to bring.
Digital testing
There is no bubble sheet to borrow. Use the figure on the screen, and use scratch paper if you need a sketch. Scratch paper on that format is in scratch paper on the digital ACT.
The bottom line
Paper only: the answer sheet is a straightedge for comparing, not a ruler you were supposed to pack.
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.