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I'm thinking of installing Catalina on a Volume to test and wondered how much space this new OS will take up on my Mac.

I'm planning on using APFS Volume so both my main Macintosh HD and User data would stay untouched and just install Catalina to look for now, so please assume I don't need to worry about two copies of my user data or Applications - just the bare OS to install and try booting to look around without running the migration assistant.

What space needs should I plan on needing to install a clean macOS Catalina install with no user data or apps?

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The installer takes 8.1 GB of space in your Mojave /Applications folder when it downloads the installer to your Mac.

In the case where you install to a brand new / empty volume, you will end up with two APFS container volumes - one for the read only system files and another for the user data.

APFS Volume Catalina - Data         3.7 GB     disk1s5
APFS Volume Catalina                10.8 GB    disk1s6

Doing the math, that's 14 GB of space for a totally empty installation, no third party apps, no user data migrated, no iCloud and just an admin account home folder.

Add an additional 8.1 GB of space for the installer and you're able to start testing with as little as 25 GB of free space on Mojave.

If you're interested in how this split works in practice - I recommend this article highly.

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  • It wasn't in the question, but I strongly recommend doing your first install to a new partition. I'm seeing 15 minute installs to empty Volumes and hour plus installs when the installer has to run all the migration steps to update documents, apps, settings, etc... Also, having a clean install gives you confidence your installer is good, your hardware is ready and you can see all the default settings. All of these are hard to test if you jump in and migrate a running system with all sorts of setup and legacy apps and data as part of the first test of a new OS.
    – bmike ♦
    Commented Oct 7, 2019 at 20:28

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