Why is the source size of the backup smaller than the "real" source?


I’m wondering why duplicati is showing a source size of my backup as 127GB, but Windows is showing me 140GB for the selected folders:


As source files I have selected these folders in the backup options:

  • C:\Users\User\Desktop\
  • C:\Users\User\Documents\
  • C:\Users\User\Dropbox\
  • C:\Users\User\Music\
  • C:\Users\User\Pictures\

Checking the size in windows shows 140GB (sorry for the german screenshot):

So what is missing from my backup? Are these temp files etc. which are automatically excluded by duplicati? (I didn’t specify any filters). But I would doubt that I have 13GB of temp files in these directories…

Or is it possible that duplicate files (same files in different folders) are not counted by duplicati? In my picture collection there might be some duplicate files… Or is the source size even calculated on block level?

Do you have any Idea how to check the mistmatch? I want to make sure everything is backed up…

Welcome to the forum @babelfish

Why is the source size of the backup smaller than the “real” source?

Two possible reasons are the compression and deduplication Features.

You can restore backup to somewhere else on the drive, then compare.
Test restores are best practice. You want to make sure you can restore.
There are tools available to compare folders. Ignore expected changes.

Or if you really want to just rely on file listings, use The FIND command.
You can start with an Export As Command-line then edit that command.
Comparing to original source trees might be hard unless you can script.

For simple comparison of folder and file count, look at backup log. Mine:


which is from backup where File Explorer says source folder has 7 files.

unfortuantely the backup log also only lists 127GB as source files and 51599 instead of 59919 files:

Sounds like you’ll have to give one of the two other methods a try.
Also expand the Filters and Exclude sections to confirm no usage.


I suppose a test option is to check (backup or restore) your five folders separately, if that’s simpler.