

Parkinson’s Law can be generalized to suggest demand always expands to match supply. For example, upgrading a MacBook Air via System Preferences required ~38 GB of free space, upgrading via Online Recovery required only ~27 GB free.

Note 2: If your goal is to free up space to perform a MacOS upgrade, performing the same upgrade via Online Recovery mode (boot with Command-Option-R held) requires less space to be freed up. Note: Under MacOS 12 Monterey, the category "Other" has been changed to "System Data" but the following information still applies.
