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I have noticed this mainly with torrents, but also with other things.

For example, if I download a ~700MB movie, it'll download and take up ~760MB on my mac. The exact same torrent will take up the ~700MB listed on the download page on my Nexus 7, or on a Windows 7 desktop.

I've also seen this space increase with photos from a digital camera (JPG format), movie files from Final Cut Pro, downloaded files like images, word documents, PDFs etc. Mostly with small files the difference is negligible, usually amounting to a few kB. However, when the files are bigger, the difference is harder to ignore.

Does anybody know why this happens? Is it because of the Mac OS X filesystem as opposed to NTFS and FAT on Windows or YAFFS2 and vFAT on Android?

Or does Mac store files in a different way?

I would prefer a canonical answer if possible.

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Yes Raghav, i think it's because of File System structure only, i have 8 GB Pen Drive with FAT32 System, and i can't copy 5.5 GB's file which is in Linux System, it gives error as no more space in pen drive, however pen drive is just empty, all 8GB is available. – Lucifer Oct 27 '12 at 9:28
@Lucifer That's because FAT32 has a maximum file size of 4GB minus 1 byte. However, I'm not asking about the maximum file size. I'm asking why it is like this, and I think it's because of the file system structure as well. – Raghav Sood Oct 27 '12 at 9:30
that's what i am saying, it's because of File Structure of the Operating System. – Lucifer Oct 27 '12 at 9:31
In windows, when you right click and bring up file properties, you get 2 numbers, "size" and "size on disk". Can you give us those full numbers. And the equivilant full number(s) for mac OS? – weston Oct 27 '12 at 9:36
Closely related, but not exactly the same issue: How much storage am I using? – slhck Oct 27 '12 at 10:01
up vote 15 down vote accepted

Are you sure that you are always using the same unit ?

Some programs count bytes by multiples of 1000, others by multiples of 1024. This way, a file of one billion bytes will be 1000 000 000 bytes, or 1000 MB (megabyte), but only 954 MiB (mebibyte).


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A few years ago, Mac OS X Snow Leopard changed the size of a MB to the standard that mega is 1000000 and not the historical 1048576 [1].

So if you compare the actual number of bytes used by the files, it should be the same.


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...and I guess they did it so people would finally stop complaining "My new 3 TB drive only has 2.7 TB! I'm being robbed!" – Tim Pietzcker Oct 28 '12 at 7:38
That, and using 1000 bytes == 1 KB is correct according to the standards. – Mike Weller Nov 20 '12 at 15:59
Of course, everyone knows the "standards" weren't designed out of good heart to get the binary prefixes SI-compatible, but because of disk drive industry lobbying. I mean, "kibibyte"? Come on. The metric system shouldn't even apply here, for many reasons. But hey, it's the standard... – Thomas Feb 23 '13 at 13:27
@Thomas It had almost nothing to do with disk industry lobbying but actual inconsistencies in usage. For example, "gigabit Ethernet", has a line speed of 1 billion bits per second. So should that be ".93 gigabit Ethernet"? Or are you going to blame Ethernet industry lobbying? – David Schwartz Nov 6 '14 at 19:24

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