Finder's weird behavior when reporting file sizes.

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Level 1

4 points

Finder's weird behavior when reporting file sizes.

I'm not sure if this is actually a problem or just some weird way that Finder works, but I'm having serious issues with the way it reports file sizes.

Let me give you an example. I have a file that Finder reports as being 723 megs. I needed up to upload this file to a friend of mine using an FTP client. When I initiated the transfer, the FTP reported the file as being only 690 megs. The transfer wasn't broken or anything, the entire file arrived intact. I also had to send this same file to another friend and I used DCC file transfer via IRC. There I also had the same issue. IRC reported the file as being 690 megs.

I can also give you an example on a smaller scale. I have comic books that I import into an application called comicbooklover. I just imported about 60 of these. In Finder, these comics are listed as being 8.7 megs. After being imported into the application, comicbooklover reports their size as 8.0 megs. Small difference, I know but you can see with the big file what ends up happening when the megs add up.

I can give you tons more examples of stuff like this where Finder reports files as a certain size while various apps report them as something different. I'm curious as to why this is happening. I don't remember this being an issue before Snow Leopard, but to be honest, I might have not noticed it. If anyone knows what's going on and why, I'd love to know.

Macbook, Mac OS X (10.6.2)

Posted on Mar 18, 2010 9:54 PM

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Answer 1

Camelot

Level 9

58,527 points

Mar 18, 2010 10:19 PM in response to Mr_Lyle

The answer may lie in the specifics of the file(s) in question.

Despite looking like a 'file', there are several file types that are really collections of files - somewhat like a folder. Most applications are really like this - they look like a single file but really are a special kind of folder that contains many elements of the application.

If that's the case here, then the difference could be due to the common issue of block sizes. You see, each disk is divided into a number of logical blocks. Each block might be something like 4KB in size. The issue is that no block can contain more than one file, so a file of just 1 byte would take up 4KB on disk - the other 3.99KB is just blank space.

That's not a big deal on a single file, but if you add that rounding up across many files it can amount to a lot.

Now, that covers how much space the file takes on disk (the 723MB reported by the Finder), but when you transfer a file over the network it only sends the actual bytes of the file(s), not the entire block (with all the blank space). That could account for the smaller byte transfer reported by your FTP client.

You can see the difference in bytes vs. blocks if you Get Info on the file in the Finder. You'll see something like:

Size: 64.4 MB on disk (56,871,278 bytes)

Here you can see this file (an application in this case), takes 64.4MB of disk space (all those blocks, including the padding/blank space for each file), but there's only 56,871,278 actual used bytes of data.

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Answer 2

Barney-15E

Level 10

122,252 points

Mar 18, 2010 10:33 PM in response to Mr_Lyle

As of Snow Leopard, Mac OS X reports file and disk sizes based on 1kb=1000 bytes, not 1024 bytes. This matches what drive manufacturers have used for a while. However, that would only bring the 690 MB file up to 707MB, unless I'm doing the math wrong.

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Answer 3

Mr_Lyle Author

Level 1

4 points

Mar 18, 2010 10:49 PM in response to Camelot

I understand how .app files work (more or less). They're basically a folder full of files that come together to make a single file that you use to launch the app in question.

Now this might be the case for the comic files as they're basically a .rar or .zip archives containing multiple .jpg files in a particular sequence. But the file that's 723 megs is a movie. It's an .mov file. Using Get File info, this is what it says:

722.9 MB on disk (722,886,229 bytes)

Those figures are exactly what they should be. So I'm still not sure why Finder says 723 (or 722.9, whatever) while other apps report it as 690. the 1MB = 1000 bytes thing might make sense. But if his math is right, something is still odd.

Message was edited by: Mr_Lyle

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Answer 4

Mr_Lyle Author

Level 1

4 points

Mar 18, 2010 11:19 PM in response to Mr_Lyle

Ok, problem solved. It was indeed the 1kb = 1000 bytes, here's the math with the exact file sizes:

698.3MB x 1024KB x 1024B <---- Windows
722.9MB x 1000KB X 1000B <---- Mac

Both formulas come to: 722,886,229 bytes
This is what both Mac and Windows report the size to be in bytes. It won't be the answer if you try yourself because 1 decimal point isn't accurate enough but anyway. Yeah, in bytes, both files are identical.

Thanks, this has been driving me absolutely nuts.

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