Adding up megabytes

I have a folder "A", with 24 folders "B" under it, each of those 24 folders having anywhere from 1 to 21 folders "C" under it. Each of the final layer of folders (the "C" folders) contains anywhere between one and 26 files, for a total of 83 folders (84 counting the top level "A" folder) and 577 files.

In Windows Explorer, I carefully hovered the mouse over each of the "C" folders to get the size of the folder and entered the number of megabytes into an Excel spreadsheet and summed them. The total in the Excel sheet differs by about 2.5% from what I get if I hover the mouse over the "A" folder. (Excel says 28.2 GB, Windows says 27.5 GB.) While 2.5% isn't a lot, it still amounts to 700 megabytes, more disk space than my first four or five computers had -- COMBINED.

I know that the more files I have, the more partially empty sectors there will be, but I thought that hovering over the "C" folders would take that into account.

Why doesn't the sum of the individual "C" folders add up to the total shown for the "top" "A" folder in the tree?

tanstaafl.

Because the larger numbers you're adding together are ROUNDED NUMBERS, the rounding errors can tend to pile up a bit.

And yes, the entire disk size of your old computer falls within the rounding error margin of your new computer. That's the disk version of Moore's law for ya.

2.5% is suspiciously the difference between 1000 and 1024. Windows counts in binary MB and GB. How did you convert your Excel total in MB into GB?

Peter

Out of curiosity, what led you to compare these numbers?

Don't forget the difference between filesize and the filesize rounded up to the next allocation unit.

10 files times 1 byte each = 10 bytes
10 files times 4096 byte allocation unit = 40.960 Kbytes

Well he did say megabytes and then later compared in gigabytes so I think you're right.

As long as the "size" or "size on disk" quantities weren't interchanged that shouldn't make a difference.

That's it, I think. I added up all those megabytes that Windows said each folder contained, and figured that 28,175 megabytes must be 28.2 gigabytes.

I knew better than that... I just had it in my head that since Windows was providing me with all the number that they would add up, which they do, of course. What caught me out was not thinking through the implications of Windows displaying the grand total in gigabytes but the subtotals in megabytes.

Doh!

tanstaafl.

Figuring how many DVD's it would take to back up the files, and how many (and which) files would go on each DVD.

Even though the DVD nominally holds 4489 MB, I have found that (with my burner, at least) anything over 4000 MB greatly increases the chances of creating coasters, so I allocate the files so as to fill up about 3900 MB.

tanstaafl.