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df -h shows: 

Filesystem             Size   Used  Avail Capacity iused      ifree
%iused  Mounted on /dev/disk1            233Gi  150Gi   83Gi    65%
3350360 4291616919    0%   /

From Finder, Info on the HDD shows:

249.68 GB
101.57 GB (12.78 GB purgeable)
148,108,524,404 bytes (148.11 GB on disk)

Why are 83 GB available from df, but 101.57 GB available in the other view of the same HDD?

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You're seeing two different values because macOS's Finder reports GB, while df reports GiB. Both numbers refer to the same actual number of Bytes.

You can use different bases to specify a large number of Bytes:

  • 1 GB = 1 Gigabyte = 1000^3 Bytes (“Giga” is a so-called SI prefix)
  • 1 GiB = 1 Gibibyte = 1024^3 Bytes (“Gibi” is a so-called binary prefix)

The former has been primarily used by hard drive vendors, and Apple has chosen to report the disk size in Finder in the same way as you'd get when you buy a drive. This is technically correct, as drives are sold in “GB” and “TB”.

The df shipped with macOS instead shows your drive with binary prefixes, thus with a total capacity of 233 GiB (which is roughly equivalent to 250 GB).

PS: The GNU df shipped with Linux will show binary prefixes when using df -h, but labeling them as G. If you want to use SI prefixes there, you have to use df -H.

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  • The difference reported is too large to be explained by the unit difference. 83GiB is a bit more than 89GB.
    – DarrenPMeyer
    Commented Aug 4, 2018 at 4:27
  • Agree with @DarrenPMeyer, this doesn't fully explain it. Since 1 GiB = 1.074 GB, then the 83 GiB from df is only 89 GB from Finder. There must be something else to account of the remainder.
    – user1334007
    Commented Aug 16, 2018 at 16:10
  • @majkelx's answer below explains the remainder of the issue; in short, df will report purgeable space (which will be auto-freed if needed) as in use, while Finder will not
    – DarrenPMeyer
    Commented Apr 14, 2020 at 2:06
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The Finder does not report "purgable space", taken be the Timemachine local backups and system caches, because MacOS will free that space if necessary. df does not know about that and includes disk space taken for those purposes.

You can read more about that here: https://daisydiskapp.com/manual/4/en/Topics/PurgeableSpace.html

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