Why files may take up more disk space on Windows

Martin Brinkmann
Jul 31, 2014
Updated • Jul 31, 2014

If you check a folder's properties on Windows, values for the data's size and the data's size on disk are displayed to you.

These values may differ slightly or a lot, depending on the file system used and the type of files stored on the drive.

It can happen that you see a Gigabyte or more of a difference between the two values.

You can test this by right-clicking any folder or drive letter in Windows Explorer and selecting the properties option from the context menu.

It may take a while before the final values are displayed which depends largely on the files stored under the structure.

As you can see on the screenshot below, the values differ by 0.2 Gigabyte which is not much if you take the size into account. There are however situations where the difference may be much larger.

size on disk

So why do the sizes differ?

The answer comes in the form of file systems and cluster sizes. Without going into too many details, each file system that Windows supports, e.g. NTFS or FAT32, uses so-called clusters.

A cluster is the smallest amount of disk space that can be used by a file.  Microsoft notes on the topic:

Cluster size represents the smallest amount of disk space that can be used to hold a file. When file sizes do not come out to an even multiple of the cluster size, additional space must be used to hold the file (up to the next multiple of the cluster size).

Typical default cluster sizes:

  • NTFS 2 GB - 16 TB drives - 4 KB
  • FAT16 1 GB - 2 GB drives - 32 KB
  • FAT16 2 GB - 4 GB drives - 64 KB
  • FAT32 256 MB - 8 GB drives - 4 KB

Imagine this scenario: You have a 1 Gigabyte FAT16 drive connected to your PC. Stored on it is a folder that contains 1000 files that have a size of 1 Kilobyte each.

The size value in the properties dialog would display as 1,000 Kilobyte, while the size on disk value would display as 32,000 Kilobyte (1000x32KB) instead, a difference of 31,000 Kilobyte.

While each file has a size of 1 Kilobyte, each wastes 31 Kilobyte in addition as the cluster size of the drive is 32 Kilobyte.

This has become less of an issue under Fat32 or NTFS file systems as the cluster size is 4 Kilobyte by default on those systems. Some USB Flash Drives or old computer systems may however use FAT16 as the file system.

With increasing storage sizes, it is becoming an issue again. The cluster size of a 64 TB NTFS volume for instance is 32 Kilobyte.

Determine the cluster size of a hard disk

cluster size windows

  1. Tap on the Windows-key on the keyboard and type cmd.
  2. Right-click cmd.exe and select run as administrator from the context menu.
  3. Run the command chkdsk drive letter (e.g. chkdsk d:) and wait for it to finish.
  4. Check the "bytes in each allocation unit" output. It is in bytes, if you want it in Kilobytes, divide the number by 1024. If you take the example above, 4096 bytes become 4 Kilobyte (4096 / 1024 = 4).

What can you do about it?

Depending on how the storage is used, you may be able to reduce the cluster size:

  1. Use Fat32 or NTFS instead of FAT16. This may not always be possible, for instance if the file system needs to be FAT16. If there is no such requirement, you may be able to free up lots of space on disk this way by changing the file system. You can use the command line tool CONVERT for this. To change the file system of drive d: to ntfs, you would run the command CONVERT d: /fs:ntfs on an elevated command prompt. There won't be any data loss.
  2. Partition the drive. If you reduce the size of each partition, you may be able to reduce the cluster size as well. A 512 MB FAT16 partition has a default cluster size of 16 KB for example, while a 1 GB partition one of 32 KB
Why files may take up more disk space on Windows
Article Name
Why files may take up more disk space on Windows
Files on a hard drive in Windows may take up more space than their actual size. Find out how to determine if this is the case, and how to change it.

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  1. Dan Donx said on January 15, 2023 at 10:29 am

    What mental age of reader are you targeting with the first sentence? 10?

    Why not write an article on how to *avoid* upgrading from W10 to W11. Analogous to those like me who avoided upgrading from 7 to 10 for as long as possible.

    If your paymaster Microsoft permits it, of course.

  2. Dexter said on January 15, 2023 at 11:14 am

    5. Rufus
    6. Ventoy

    PS. I hate reading these “SEO optimized” articles.

    1. cdr said on January 15, 2023 at 3:32 pm

      I used Rufus to create an installer for a 6th gen intel i5 that had MBR. It upgraded using Setup. No issues except for Win 11 always prompting me to replace my local account. Still using Win 10 Pro on all my other PCs to avoid the bullying.

  3. sv said on January 15, 2023 at 6:40 pm

    bit pointless to upgrade for the sake of upgrading as you never know when you’ll get locked out because ms might suddenly not provide updates to unsupported systems.

    ps…. time travelling?
    written. Jan 15, 2023
    Updated • Jan 13, 2023

    1. Martin Brinkmann said on January 16, 2023 at 5:49 am

      This happens when you schedule a post in WordPress and update it before setting the publication date.

  4. Anonymous said on January 16, 2023 at 8:24 am

    Anyone willing to downgrade to this awful OS must like inflicting themselves with harm.

  5. basingstoke said on January 16, 2023 at 11:18 am

    I have become convinced now that anybody who has no qualms with using Windows 11/10 must fit into one of the following brackets:

    1) Too young to remember a time before W10 and W11 (doesn’t know better)

    2) Wants to play the latest games on their PC above anything else (or deeply needs some software which already dropped W7 support)

    3) Doesn’t know too much about how computers work, worried that they’d be absolutely lost and in trouble without the “”latest security””

    4) Microsoft apologist that tries to justify that the latest “features” and “changes” are actually a good thing, that improve Windows

    5) Uses their computer to do a bare minimum of like 3 different things, browse web, check emails, etc, so really doesn’t fuss

    Obviously that doesn’t cover everyone, there’s also the category that:

    6) Actually liked W7 more than 10, and held out as long as possible before switching, begrudgingly uses 10 now

    Have I missed any group off this list?

    1. Heinz Strunk said on September 19, 2023 at 3:57 pm

      You have missed in this group just about any professional user that uses business software like CAD programs or ERP Programs which are 99% of all professional users from this list.

      Linux doesn’t help anyone who is not a linux kid and apple is just a fancy facebook machine.

  6. ilev said on August 24, 2023 at 7:34 pm

    Microsoft has removed KB5029351 update

    1. EP said on August 24, 2023 at 9:21 pm

      only from windows update though
      KB5029351 is still available from the ms update catalog site

  7. Anonymous said on August 24, 2023 at 11:05 pm

    1. This update is labaled as PREVIEW if it causes issues to unintelligent people, then they shouldn’t have allowed Preview updates ot install.

    2. I have installed it in a 11 years old computer, and no problems at all.

    3. Making a big drama over a bluescreen for an updated labeled as preview is ridiculous.

    This is probably another BS internet drama where people ran programs and scripts that modified the registry until they broke Windows, just for removing stuff that they weren’t even using just for the sake of it.
    Maybe people should stop playing geeks and actually either use Windows 10 or Windows 11, but don’t try to modify things just for the sake of it.

    Sometimes removing or stopping things (like defender is a perfect example) only need intelligence, not scripts or 3rd party programs that might mess with windows.

  8. john said on August 24, 2023 at 11:17 pm

    Windows 11 was a pointless release, it was just created because some of the Windows team wanted to boost sales with some sort of new and improved Windows 10. Instead, Microsoft cannot support one version well let alone two.

    1. John G. said on August 25, 2023 at 12:08 pm

      Windows 11 is the worst ugly shame by Microsoft ever. They should release with every new W11 version a complete free version of Starallback inside just to make this sh** OS functionally again.

  9. EP said on August 25, 2023 at 3:10 pm

    motherboard maker MSI has recently released a statement regarding the “unsupported processor” blue screen error for their boards using Intel 600/700 series chipsets & to avoid the KB5029351 Win11 update:

  10. EP said on August 29, 2023 at 7:32 pm

    check out the following recent articles:

    Neowin – Microsoft puts little blame on its Windows update after UNSUPPORTED PROCESSOR BSOD bug:

    BleepingComputer – Microsoft blames ‘unsupported processor’ blue screens on OEM vendors:

  11. Leonard Britvolli said on August 30, 2023 at 10:33 pm

    While there may be changes or updates to the Windows 10 Store for Business and Education in the future, it is premature to conclude that it will be discontinued based solely on rumors.

  12. sembrador said on September 5, 2023 at 9:32 pm

    My advice, I left win 15 years ago. Now I’m a happy linux user (linuxmint) but there is Centos, Fedora, Ubuntu depending on your needs.

  13. EP said on September 6, 2023 at 11:55 am

    motherboard maker MSI has recently released new BIOS/firmware updates for their Intel 600 & 700 series motherboards to fix the “UNSUPPORTED_PROCESSOR” problem (Sept. 6):


  14. Raphael Benzo said on September 24, 2023 at 9:52 pm

    I try to disable the Diagnostics Tracking Service (Connected Devices Platform User Services) but it wont let me disable it, any help will be greatly appreciated.
    Tank you for your help

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