Why do my backed up files appear smaller on Backblaze than on my Mac?

You may notice that Backblaze's file browser reports your backed files as smaller than they appear on your Mac. This is not necessarily indicative of an issue. It's simply a difference in how Mac OS X 10.6 and later calculates storage space versus how Backblaze, Windows and Mac OS X 10.5 and earlier calculates.

For example, these three files are reported at 1 GB, 100 MB, and 100 KB by Mac OS X:

On Backblaze's file browser, these three files are reported at 953.67 MB, 95.37 MB and 97.66 KB:

Why the difference?

Starting with Mac OS X 10.6, Apple calculates disk storage using decimal (base 10) math:

• 1 KB is 1,000 bytes

• 1 MB is 1,000,000 bytes or 1,000 KB

• 1 GB is 1,000,000,000 bytes or 1,000 MB

Backblaze's restore browser, Windows, and Mac OS X 10.5 and earlier all use binary (base 2) math:

• 1 KB is 1,024 bytes

• 1 MB is 1,048,576 bytes or 1,024 KB

• 1 GB is 1,073,741,824 bytes or 1,024 MB

What does this mean?

For the actual files, nothing. Files are backed up and restored with actual bytes and not modified in any way. KB, MB, GB are simply terms that are displayed to avoid large numbers of raw bytes displayed to a user (i.e. it's far easier for a human to read "1.5 GB" than "1610612736 bytes")

If you're attempting to compare two file sizes, it can be a little difficult to relate the two methods of calculation.

The easiest way to check if files are backed up accurately is to select a few for restore, download them to the computer in question and compare them.

Alternately, some math can be applied to convert between the two methods of calculation:

• KB - Take the number of KB that your Mac reports, multiply by 0.9765 to get the number of KB that Backblaze will report

• MB - Take the number of MB that your Mac reports, multiply by 0.9536 to get the number of MB that Backblaze will report

• GB - Take the number of GB that your Mac reports, multiply by 0.9313 to get the number of GB that Backblaze will report

Links with further information on this topic:

http://support.apple.com/kb/TS2419

http://wintelguy.com/gb2gib.html

http://physics.nist.gov/cuu/Units/binary.html