Volume of a rectangular prism is length times width times height. A 4-by-3-by-5 box has volume 60. The ACT does not hand you a formula sheet. Volume as a set is in area and volume.
Fill, not wrap
Volume counts cubic units inside. Surface area counts the wrapping, in surface area. If the question asks how much water the tank holds, you want volume. If it asks how much paint covers the outside, you want surface area.
A space diagonal through the box is a different formula, in space diagonal. The short list is in ACT Math formulas.
The bottom line
Length times width times height. Cubic units, not square units.
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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.