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I'm interested in determining the size of a given Time Machine Backup. This seems possible with tmutil using either uniquesize, compare, or calculatedrift, but I've been unable to figure out the precise command to find the size of a given backup image. Can anyone help? Thanks in advance.

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There is no such thing as a given backup image in Time Machine. Practically speaking only changed files get backed up each time but the snapshot volume also contains links to the most recent version of all unchanged files on the backup drive. So each snapshot volume represents a backup of your whole hard drive (also summing up the size of all snapshot volumes together will give you a number significantly higher than the size of your whole backup disk).

To find out how much data was added/changed/removed during one Time Machine backup, tmutil calculatedrift /Volumes/Time\ Machine/Backups.backupdb/NameOfYourMac is the way to go:

pse@Fourecks:~$ sudo tmutil calculatedrift /Volumes/Time\ Machine/Backups.backupdb/Fourecks
[...]

2013-06-02-131853 - 2013-06-02-164246
-------------------------------------
Added:         807.2K
Removed:       5.0M
Changed:       100.7M

2013-06-02-164246 - 2013-06-02-174230
-------------------------------------
Added:         9.9K
Removed:       0B
Changed:       43.8M

Drift Averages
-------------------------------------
Added:         715.6M
Removed:       535.5M
Changed:       571.5M
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  • In both commands (answers) my terminal just stucks and never show the results, just the first date "2018-04-15-151230 - 2018-07-07-151148" – Tiago Gouvêa May 14 '20 at 11:49
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If you want to include linked files you can just use du:

$ sudo du -sh /Volumes/Time\ Machine/Backups.backupdb/iMac/2013-06-03-111535/
Password:
112G    /Volumes/Time Machine/Backups.backupdb/iMac/2013-06-03-111535/

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