I have a Retina MBP with a 512 GB SSD. Today, Time Machine shows me this:

enter image description here

Does anybody know how Time Machine can be backing more data than I can fit on my hard disk? For what its worth, I still have about 40 GB free on the internal hard drive.

Time Machine is setup on a 2TB Time Capsule.

  • So you think you have 512-40 = 472 Gig to back up ?
    – Ruskes
    Aug 18, 2013 at 21:55
  • @Buscar웃 I'm not sure of the exact amount. Time Machine's backed up stuff before, so I should have fairly less than 512 gigs based on their incremental backup system. I'm just a little confused as to how it even found 520 gigs of data on a 512 gig machine. Aug 18, 2013 at 21:58
  • Do you have any other external drives or devices attached that it might be trying to back up?
    – tubedogg
    Aug 18, 2013 at 22:10
  • How much do you have on your SSD ?
    – Ruskes
    Aug 18, 2013 at 22:21
  • I think this might just be Time Machine calculating the amount of backed up data in an approximate way. If you go to Finder and open the Info Window (Cmd+I) on your Hard Disk, what values do you get there? Aug 18, 2013 at 22:25

4 Answers 4


The 520 GB in your screen shot is just an estimate and since in some cases the estimate can be high somewhat substantially, you won't know if this is actually a problem until the backup completes. At that point you could use tmutil compare or a tool like Backuploupe to see the actual payload size for that specific backup and feel better about whether this is an issue with directory corruption or just an estimate algorithm that isn't highly accurate for your storage details.

Due to how files are stored on drives, you could have a lot of files that fit efficiently on the SSD but end up wasting more space when they sit on a larger volume like the 2 TB Time Capsule so it could also be literally correct and no over estimate. HFS is fairly sophisticated on how it allocates blocks and stores files, but to simplify things - there is potential for huge inefficiency in file system storage.

For example, take this contact card I dragged onto my desktop:


The file size is 536 bytes, yet it takes well over 600% more space than needed since that one file takes up a 4 KB block of space on the filesystem. As the software is likely glossing over these details or making overly conservative estimates on how much slack space is going to be needed on the destination.

My experience is 20 GB of storage overhead is extremely unlikely so my hunch is it is an overly cautious estimate where the designers wanted to be "better safe than sorry" with making sure the actual backup used less space than estimated.


I'd say that the Info Window on your Hard Drive that you posted in the comments is completely correct. It uses the new base 10 system for calculating your Hard Drive space.

I guess Time Machine is misreporting the amount of data backed up. This might be due to either Time Machine still using the old base 2 method for calculating HD space, or by Time Machine getting confused by the use of symlinks and adding up the size of some files multiple times.

It also could be that somehow the backed up data is bigger than the original data. This could be due to metadata, error recovery, or other factors. But I don't think this is the case, because the difference that you report is quite big.

  • The Symlinks might be the cause. I have a few to backup some Eclipse workspaces within Dropbox. I still don't think they'd account for such a big difference though. If no one has a better answer in the next couple of hours, I'll accept this. Aug 18, 2013 at 22:50

Multiple sources confirm that Time Machine requires approximately 20-25% overhead space. Time Machine seems to be reporting a slightly lower figure than 20% over used space, but it's fairly close.

I don't have an "official" source to point you to, but this figure is repeated over and over in the Apple Discussion Forums by reputable posters.

https://discussions.apple.com/message/22468774#22468774 https://discussions.apple.com/message/9815174#9815174


I have noticed in Mavericks that if a file e.g. a VM virtual disk is in use when Time Machine is backing up it seems to notice if that file changed during the backup and will try to back it up again but when it does it treats it as the same session so increases the total size of the backup reported for that session. I have a xGB of 70GB VM hard disk and, if in use during a backup, TimeMachine reports, over a period, xGB of 70GB then xGB of 140GB then xGB of 210GB... until I quit Parallels or the VM.

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