As you can see below, there is so much difference between the Size and Size on disk fields in my folder. Why is that?

Screenshot showing 50,875 files in 1,504 folders, 105 MB being 1.43 GB on disk

I know that Size on disk should be a little more than Size because of allocation units in Windows, but why that much of a difference? Could it be because of the large number of files?

BTW, this folder is on my Android phone’s SD card. Inside this, my maps app stores its cached maps and the app gets its map from Google Maps.

  • I have found something : Question. Could this be my problem also?!
    – vfsoraki
    Commented Jan 20, 2014 at 9:54
  • @Braiam Yes, it is FAT32 formatted. I have to reformat the disk.
    – vfsoraki
    Commented Jan 20, 2014 at 16:32
  • 11
    Looks to me like this app is seriously badly programmed – consider filing a bug report. I'm by no means a professional programmer, but I once hacked something similar together in JavaME, and of course one of the problems I had to solve was how to store all those little map tiles efficiently (storage & access) in a container. I ended up using uncompressed zip files.
    – A. Donda
    Commented Jan 20, 2014 at 18:15
  • 1
    Related: What is the difference between size and size on disk? Commented Nov 10, 2015 at 13:33

9 Answers 9


I will be assuming that you are using the FAT/FAT32 filesystem here, since you mention this is a SD card. NTFS and exFAT behave similarly with regards to allocation units. Other filesystems might be different, but they aren't supported on Windows anyway.

If you have a lot of small files, this is certainly possible. Consider this:

  • 50,000 files.

  • 32 kB cluster size (allocation units), which is the max for FAT32

Ok, now the minimum space taken is 50,000 * 32,000 = 1.6 GB (using SI prefixes, not binary, to simplify the maths). The space each file takes on the disk is always a multiple of the allocation unit size - and here we're assuming each file is actually small enough to fit within a single unit, with some (wasted) space left over.

If each file averaged 2 kB, you'd get about 100 MB total - but you're also wasting 15x that (30 kB per file) on average due to the allocation unit size.

In-depth explanation

Why does this happen? Well, the FAT32 filesystem needs to keep track of where each file is stored. If it were to keep a list of every single byte, the table (like an address book) would grow at the same speed as the data - and waste a lot of space. So what they do is use "allocation units", also known as the "cluster size". The volume is divided into these allocation units, and as far as the filesystem is concerned, they cannot be subdivided - those are the smallest blocks it can address. Much like you have a house number, but your postman doesn't care how many bedrooms you have or who lives in them.

So what happens if you have a very small file? Well, the filesystem doesn't care if the file is 0 kB, 2 kB or even 15 kB, it'll give it the least space it can - in the example above, that's 32 kB. Your file is only using a small amount of this space, and the rest is basically wasted, but still belongs to the file - much like a bedroom you leave unoccupied.

Why are there different allocation unit sizes? Well, it becomes a tradeoff between having a bigger table (address book, e.g. saying John owns a house at 123 Fake Street, 124 Fake Street, 666 Satan Lane, etc.), or more wasted space in each unit (house). If you have larger files, it makes more sense to use larger allocation units - because a file doesn't get a new unit (house) until all others are filled up. If you have lots of small files, well, you're going to have a big table (address book) anyway so may as well give them small units (houses).

Large allocation units, as a general rule, will waste a lot of space if you have lots of small files. There usually isn't a good reason to go above 4 kB for general use.


As for fragmentation, fragmentation shouldn't waste space in this manner. Large files may be fragmented, i.e. split up, into multiple allocation units, but each unit should be filled before the next one is started. Defragging might save a little space in the allocation tables, but this isn't your specific issue.

Possible solutions

As gladiator2345 suggested, your only real options at this point are to live with it or reformat with smaller allocation units.

Your card might be formatted in FAT16, which has a smaller limit on table size and therefore requires much larger allocation units in order to address a larger volume (with an upper limit of 2 GB with 32 kB allocation units). Source courtesy of Braiam. If that is the case, you should be able to safely format as FAT32 anyway.

  • 4
    The wasted space due to minimum allocation sizes is actually technically called "internal fragmentation", so you could say that fragmentation is the culprit. But it's still not something that any "defragment" tool can do anything about.
    – hobbs
    Commented Jan 23, 2014 at 6:21
  • 3
    (Less technically, it's just called "slack".)
    – hobbs
    Commented Jan 23, 2014 at 6:21
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    Cluster sizes also limit the maximum filesystem size. For example, if your address space is 32-bit, you have a total of ~4.29 billion possible total clusters. Now, if you use the smallest cluster size supported by NTFS (512 bytes), you can address a maximum of 512 * 2^32 bytes = 2 GiB. If you need a volume that can store more than 2 GiB of data, you have to increase the cluster size. This is all independent of the actual largest file you attempt to store, granted you cannot store a file larger than 2 GiB that is the least of your problems. Commented Jan 24, 2014 at 20:23
  • 1
    4 KiB clusters will allow you to address files in a volume that is up to 16 TiB in size, which should be enough for the foreseeable future. Commented Jan 24, 2014 at 20:31
  • 2
    Well, he could compress his archive of small files into one large file.
    – einpoklum
    Commented Jan 26, 2014 at 14:55

This is one of those situations where compressing/archiving into a single file may help. What Bob said in his answer is true but the solution may be easier than reformating the disk as other answers suggests. If you compress or archive the directory (using zip, tar, or any other method) the file system will see that you have a single big file, instead of several smaller ones. Even without compressing you will be getting back almost 1.4 GiB of space back, because all those "small files" will be counted as a single big file.

Inside this, my maps app stores its cached maps and the app gets its map from Google Maps

Maybe you should discuss with the developer to use an archive or a database instead of multiple files. This probably will also help to have the disk less fragmented and will surely save space especially if it's a NAND flash drive. If you explain the ridiculous situation where 100MB of payload/useful data becomes 1.4GiB, there's something wrong with how the data is stored, and the developers should bring a nicer solution.

  • 1
    > Inside this, my maps app stores its cached maps and the app gets its map from Google Maps. - unfortunately, in this case, compression (which is effectively a file system above the base one) would require support from this mapping app.
    – Bob
    Commented Jan 20, 2014 at 15:04
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    @Braiam It's not tricking the file system into thinking there's only one file; there is only one file. As to why the developers don't store the cache information in an archive, it's probably because most archive formats are not designed for fast random writes, which a cache certainly needs. A better alternative might be to use a lightweight database library like SQLite.
    – bcrist
    Commented Jan 21, 2014 at 1:30
  • Maybe there also exists or should be added option to that APP to clear cache by some timed characteristic (delete OLD files)
    – ALZ
    Commented Jan 25, 2014 at 18:39
  • +1: It is refreshing to see at least one person understands the IEEE 1541-2002 standard for binary units of measure. Everyone else has been discussing sizes incorrectly. A 4 kB cluster is 4,000 bytes, which means you lose up to ~0.5 TiB worth of storage if you create a volume with a 32-bit address space using 4 kB clusters. 4 kB clusters can only address a maximum of 15.625 TiB of data, whereas 4 KiB clusters can address 16 TiB. That situation is far harder to fix than internal fragmentation. Commented Jan 25, 2014 at 21:41
  • Since it's Android, SQLite is guaranteed to be available. In case it helps convince the developers, the SQLite documentation has some performance benchmarks comparing BLOBs stored in an SQLite database (what should be done) to ones stored outside it (current state of affairs). It was done using a SATA disk which probably wasn't an SSD, but they do provide enough information to replicate it on flash-backed storage if necessary.
    – ssokolow
    Commented Jan 26, 2014 at 13:25

As already explained the most common reason for the size difference is used space vs. allocated space. But it's not the only possible one, NTFS has a feature to add hidden data to files. This possibility was the one exploited by the healthcare industry ransomware late 2019.

File fork and alternate data stream

"Resource fork" has been used by Apple since 1984 (Macintosh) to store the main content of a program (instructions) and the associated resources (like icons and menus) in the same file. Embedding resources in executable files is a common technique, but doing it with forks isn't.

Apple consistently designed the Macintosh file systems to support file forking, and when Microsoft designed NTFS to replace FAT, fork was also introduced under the name of "alternate data stream" (ADS).

In NTFS, a file contains:

  • The mandatory unnamed data stream (UDS)
  • One or more optional alternate data stream(s) (ADS).

Hidden in plain sight

File forking isn't bad, except NTFS ADS are not supported by common tools, including Windows Explorer, ADS is de facto a hidden feature, an unexpected gift for hackers. From Wikipedia:

Alternate streams are not listed in Windows Explorer, and their size is not included in the file's size.

While the file size, which reports only the UDS size, isn't changed by ADS existence, the allocated size (clusters allocated to the file by the file system) reports the actual size of the file, all streams included.

Windows explorer doesn't report ADS, neither the CMD command dir. However ADS are visible with:

Note it's still possible to hide ADS from some of these tools by using file system reserved keywords (see Pierce's document linked below).

  • Windows uses ADS to tag a file as downloaded from Internet and to store other metadata.

  • Hackers use ADS to hide data and code for malicious activities.

Comprehensive description of ADS worth reading:

Malware use of ADS

Serious anti-malware tools watch for ADS, but malware still uses ADS, at large scale, because:

  • Some security suites are not even ADS aware, or can't identify malicious uses of ADS.
  • It's easy to redirect the execution of a legitimate file to an ADS (e.g. using a shortcut).


The ransomware BitPaymer enters the computer as a normal and visible file, but when executed copies itself in a legitimate file as an ADS, then delete the initial file. As this doesn't change the size of the legitimate file, and ADS are not listed by common tools, the malware is now virtually hidden.

Operation Cobalt Kitty

Also hides using ADS.

My point is: In case of big file size difference observed (more than a cluster size: 4KB), don't overlook the possibility of ADS, and hidden malware.

Experiment ADS yourself

To safely experiment with ADS, try this at DOS/CMD level...

Create and then display the content of a file in the root of C:

C:\> echo The main data stream> test.txt
C:\> type test.txt


C:\> The main data stream

Now add an ADS with the same method, just specify the ADS name in addition of the file name:

C:\> echo The secret message> test.txt:secret

You have just hidden the secret message in the file. Note that the file size in Explorer has not changed in spite we added bytes in the ADS "secret".

Try to display the ADS content:

C:\> type test.txt:secret


The filename, directory name, or volume label syntax is incorrect.

CMD type is not able to display the content of the ADS. We will use Notepad instead:

notepad test.txt:secret

In Notepad we can see the content of the ADS:

The secret message

You can also hide a full executable in an ADS of an innocent text file, and run it at any time. Wealth does not harm for hackers :-)

  • 4
    It's worth using a tool such as Streams from Sysinternals to check for ADS usage. For instance files downloaded on a Windows system may be tagged with a source in ADS, though this is tiny and shouldn't take up space. It won't show in dir or Explorer output ordinarily. It may take up blocks and aggravate the disk usage problem you are investigating. .
    – adric
    Commented Jan 21, 2014 at 13:38

The problem may be because of the cluster size.

According to Microsoft:

If you are not using NTFS compression for any files or folders contained on the volume, the difference between SIZE and SIZE ON DISK is wasted space because of a larger-than-necessary cluster size. You should attempt to use an optimal cluster size so that the SIZE ON DISK value is as close to the SIZE value as possible. An excessive discrepancy between the SIZE ON DISK and the SIZE value is an indication that the default cluster size is too large for the average file size that you are storing on the volume, and that it should be decreased. This can be done only by backing up the volume and then reformatting the volume by using the format command and the /a switch to specify the appropriate allocation size: IE: format D: /a:2048 (This example uses a 2-KB cluster size).

Try formatting your drive with smaller cluster size.

  • 4
    That been said, one shouldn't make cluster size less than 4096 bytes or just not multiple of this number. 32 bit OS works with pages which (in non-PAE case) are of 4096 bytes, so using non-multiple clusters may negatively affect file system performance. This is why default size is set to 4096 bytes.
    – Ruslan
    Commented Jan 20, 2014 at 14:17
  • 2
    To add on to what @Ruslan said, newer hard drives now have a 4 kB sector size, and it would be optimal to align the filesystem to the physical sectors, and have a multiple of the physical sector size as the allocation unit size.
    – Bob
    Commented Jan 20, 2014 at 15:22
  • 1
    @Ruslan I believe you mean to say that it should be a power of two times 4096. 12288 (3×4096) and 20480 (5×4096) are not great choices. Commented Jan 23, 2014 at 22:57

I see many people recommending to reformat your drive with a smaller cluster size. Since this is an SD card, note that many vendors pre-format the card to the recommended cluster size to match the size of the NAND's cluster size (keeping both in sync is very important for optimum read/write performance and reducing wear-out)

You can't change the NAND's cluster size (it's a physical attribute of your SD card's hardware).

First run scandisk/chkdsk on your SD card to be sure the size report problem doesn't lie within a corrupted filesystem.

Second, I'd suggest you report the bug to Google Map devs, for them being the one to blame here. They should be using a superior storage method. Fixing it should also make the app to run faster on many devices due to less I/O and file system's driver activity.

  • Actually, it was not Google Maps, but another app using Google's maps. I informed the developer, and just removed those files from my SD.
    – vfsoraki
    Commented Jan 21, 2014 at 19:33

This is a general issue with many filesystems. There are two factors at work here, the maximum number of "blocks" a filesystem can handle per logical volume and physical restrictions of the storage medium. Only 1 file can be allocated to any given block (files generally take as many blocks as they need). So a text file with 64 bytes can often take anything from 4k to 32k, depending on the block size of the filesystem it resides on.

One way to think about this is think of each block in the filesystem as a box, and the filesystem as a room. All your boxes are the same size, and you try to fit as many as you can in a room. If you fit them all in with more room left over, you have to get bigger boxes so that the room is filled completely with boxes.

One of the rules for putting things in boxes is that you can't put two unrelated things in a box. They have to be part of the same document. So if I were to type up a page of text, it would have it's own box. If my typed text had so many pages I couldn't fit it all in one box, I'd simply find another box and continue putting pages in there instead, repeating until I'd filed all my pages. I'd also have written down the boxes I'd used for that document and the order of the boxes to read it in sequence.

Depending on how I'd organize the boxes, I may only have enough room in my manifest for a certain number of boxes. So if I had a big room to fill, but only a small number of boxes I'd have to use very large boxes to reach the room capacity.

So in that case my one page document would still occupy a single box, with nothing else sharing it.

The same situations play out amongst various storage solutions. FAT32 can only manage what is considered a low number of "boxes" on today's huge hard drives, so it ends up with very large "boxes" to compensate for this.


Aside from cluster sizes, you can also have a discrepancy due to the following conditions:

  • Compressed or encrypted files can use up a different space than that of the logical file size.
  • Linked files will report n times the number of links times the size of the file for the logical file size, but the physical space used is usually less.
  • Generally, that could be true. But in my case, high allocation unit was the problem.
    – vfsoraki
    Commented Jan 21, 2014 at 6:29

You should have a look at Block Suballocation entry in Wikipedia. That is exactly what's happening to you. Using a file system with support for Tail Packaging is a file system level solution for this problem besides changing allocation cluster size.

All have the inconvenient of needing to reformat the disk.

In some case merely storing those files in an archive would fix the problem (and the small files would also be compressed beside stopping loosing space at end of files). This has the inconvenient of spending some time for decompression.

Another option if you have so many small files because of some specific application related problem is store your software data using another method (may be in a database). But of course it's a solution for programmers, not end users.



I noted huge file size discrepancies in Windows 10 on an individual file, but if I look at the properties of the SAME file from the same location (a network drive), with Windows XP, the large discrepancy is not there; just a small difference, which is what you would expect. I think there is a bug in Windows 10. A file that is 449MB probably doesn't take up 3.99GB, which is what Windows 10 is telling me.

  • 1
    Just an FYI, the question has nothing to do with Windows 10. OP is using windows 7.
    – TheKB
    Commented Jun 15, 2016 at 18:09

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