It seems a fair and simple question, and an important for many users: how much free space do you need on your Mac’s startup volume to be able to update macOS Ventura, say from 13.0 to 13.1?

The obvious place to check first is Apple’s support information about the 13.x updates. Although that eventually links to advice on how to free up disk space when the update fails because there’s insufficient free space, nowhere can I see any figures indicating what is sufficient.

The download itself consistently declares that it’s 2.53 GB; at least that’s a start, if not a particularly helpful one. It appears the only way to discover how much space is required is by trial and error, with a series of Virtual Machines. Although the details are a bit noisier, the numbers work out reasonably consistent.

Successes

For a basic Ventura 13.0 system, the typical total space taken on disk by the boot volume group is around 15.14 GB, although variation in individual volumes is greater. Space taken by the SSV ranged from 8.83-10.14 GB, but that was offset by differences in the “other volumes”, making me suspicious that Disk Utility was still struggling with arithmetic. Each VM boot disk lost a further 2.95-3.68 GB, presumably in hidden containers. Over the range of 30-40 GB total VM boot disk sizes that I tested, free space therefore ranged from 11.19-21.91 GB.

The smallest VM boot disk that I was able to get to update to macOS 13.1 was 33 GB in total size, and provided 14.42 GB of free space for the download and update.

Failures

All three failed updates were on VM boot disks of 30 and 32 GB total size, but the error messages given weren’t entirely helpful or consistent.

With a VM boot disk of 30 GB, there was only 11.19 GB of free space before downloading the update. That was cancelled before it began, with the message that “updating requires 12.97 GB of disk space”. I was foolish enough to believe that, so in the next run I tried a boot disk size of 32 GB, providing 13.32 GB of free disk space, as Disk Utility assured me.

I then had two failures on 32 GB boot disks, the first paradoxically complaining that “updating requires 13.22 GB of disk space” when Disk Utility had just assured me that there was 13.32 GB available. The second attempt failed with an indicated free space of 13.22 GB, this time with the reason that “an additional 340.9 MB of space is needed to install on this disk.” I don’t understand why that message went on to advise me to check my Internet connection either, given that it was Software Update that cancelled the download.

The good news is that each of the updates either worked fully and correctly, or was cancelled by Software Update before downloading had started. No update went horribly wrong because it failed somewhere in the middle.

How much free space?

So depending on when you run it, the installer might claim it needs 12.97 GB, 13.22 GB, or 13.56 GB of free disk space, but really wants around 14 GB.

In practice, even for the modest needs of a basic Ventura 13.0 installation in a VM, the smallest disk size you’ll be able to update from 13.0 to 13.1 is 33 GB, providing at least 14 GB of free space. To have any degree of comfort, make that a 40 GB disk with at least 20 GB free.

That’s only a start, though. Updating to 13.1 also has a long-term cost in terms of free space. Once happily running macOS 13.1, free space was around 0.5 GB less than it had been in 13.0. By the time that we reach 13.6 next summer, even that 40 GB disk with 22 GB of free space in 13.0 could well have lost sufficient free space to make further updates tight for free space.

Conclusions

My results therefore read:

• updating from 13.0 to 13.1 requires a minimum of 14 GB free space in the boot volume group;
• to be more comfortable, the smallest disk (virtual or physical) to accommodate that update is 40 GB;
• to be able to continue to update macOS Ventura, a minimum disk size of 50 GB with at least 25 GB free is wise;
• macOS and APFS don’t like being short of space;
• more work is needed to ensure users are better informed, both in information provided prior to starting an update, and in failure reports when an update is cancelled for the user.