Unable to extend an NTFS volume

Good afternoon.  I ran into an interesting issue this afternoon I wanted to share.  I had a customer that was receiving the following error:

“The volume cannot be extended because the number of clusters will exceed the maximum number of clusters supported by the filesystem.”

Unable to extend, cluster size

He encountered the error when trying to extend a volume.  The volume was 20TB (Terabytes) and he was trying to add another 19TB for a total of 39TB.  After some research I found, based on his setup, that the maximum volume size was 32.75TB.  We extended the volume to that size and we were done.

I figured someone might find it useful if I l go over the process of determining the maximum size for a volume.  There are a couple pieces of key information that are required.  The first is the maximum number of clusters in a NTFS volume.  This is 2^32 -1 clusters, or roughly 4 billion.  The second piece of information we need is the bytes per cluster.  To get this information, run the following command: fsutil fsinfo ntfsinfo x:
Replace the x: with the actual drive letter.  Below is output from the command.  In this example we see that we are using 4096 bytes per cluster or 4K for short.

bytes per cluster

Now that we have both pieces of information we just need to do some simple math to find the maximum volume size.  Multiply the maximum maximum number of clusters by the cluster size.  Taking the example above, that would be 4,294,967,295 * 4096 = 17,592,186,040,320 bytes.  To convert this to megabytes, divide the number by 1,048,576 (1024*1024).  In this example we get 16,777,215 Megabytes.

To make this even easier, here is a handy table:

Cluster size NTFS Max Size
512 bytes 2,199,023,255,040 (2TB)
1024 bytes 4,398,046,510,080 (4TB)
2048 bytes 8,796,093,020,160 (8TB)
4096 bytes 17,592,186,040,320 (16TB)
8192 bytes 35,184,372,080,640 (32TB)
16384 bytes 70,368,744,161,280 (64TB)
32768 bytes 140,737,488,322,560 (128TB)
65536 bytes 281,474,976,654,120 (256TB)

I hope you enjoyed this article.  If you have any suggestions or comments please leave them below.


14 thoughts on “Unable to extend an NTFS volume

  1. How to find the Cluster size of my disk, its NTFS and I am trying to extend volume from 13TB to 18 TB and get the message “exceed max number of cluster supported…”


    1. To get the cluster size run the following command:
      fsutil fsinfo ntfsinfo x: (where x is the drive letter of the volume)

      Bytes Per Cluster is what you are looking for. If you run into this issue it is likely because the cluster size is too small, for instance 4096 or 8192 bytes. A larger volume will require a larger cluster size, as only a finite number of clusters are supported for a volume.


  2. And the follow on question is “How can I use all my disk space”??
    The answer is, backup all your data, format the space using a larger cluster size with the /A:size option (see here: https://www.computerhope.com/formathl.htm), then restore your data.
    Haven’t tested, but conceivable could be a Windows bare metal backup (if you’ve got a lot of data, it could take a while…) and be able to restore your whole system back into the newly formatted space – then you could do the expansion in Disk Mgmt.


  3. Hello to all of you guys,
    I have got same issue but I can’t backup all my data (30TB) because I haven’t got space for IT. I’ve just added more space to my machine cause of Arcserve Backups which are stored there. I’ve added other 20TB and when I’m tryin to expand this space I’m getting the same problems.
    [[Windows Server 2012 R2]]
    My configuration:
    NTFS Volume Serial Number : 0xac6c028e6c02540c
    NTFS Version : 3.1
    LFS Version : 2.0
    Number Sectors : 0x00000001ce9de1ff
    Total Clusters : 0x00000000e74ef0ff
    Free Clusters : 0x0000000000c8c5d1
    Total Reserved : 0x0000000000000000
    Bytes Per Sector : 4096
    Bytes Per Physical Sector : 4096
    Bytes Per Cluster : 8192
    Bytes Per FileRecord Segment : 4096
    Clusters Per FileRecord Segment : 0
    Mft Valid Data Length : 0x000000001fa00000
    Mft Start Lcn : 0x0000000000060000
    Mft2 Start Lcn : 0x0000000000000001
    Mft Zone Start : 0x00000000a09f4e80
    Mft Zone End : 0x00000000a09f6b60
    Resource Manager Identifier : BAF2ADA0-C86C-11E7-80D1-005056B671A6

    How much more I can expand my disk space to get a time to think about it?

    I will appreciate all kind of help.


    1. You should be able to expand the disk to 35.184TB.

      I arrived at this number by taking the total number of clusters that are allowed (2^32)-1 and multiplying by the cluster size (8192) and then dividing the product by 1,000,000,000,000 to get terabytes (TB).


      1. Thanks for your answer Glenn.
        I’ve tried to do it but i’m getting same error.. System let me expand this space to 32TB.


  4. Is there any chance of modify cluster size without damaging the data or reformatting?
    DiskPart needs to reformat before increasing cluster size.

    Thank you


      1. Hi Glenn,

        Any 3rd party software that you can recommend were in reformatting is not required?
        or is it safe to use 3rd party software for modifying cluster?


  5. I do not know of any software that could do this. It would be a considerable undertaking as every cluster would have to be copied into RAM and then combined with other clusters for the same file. Then it would need to write the new cluster to a free spot in the disk. While this is theoretically possible, the volume would need to be offline while the process was going on. I recommend purchasing one or more USB hard drives and using Robocopy. Once all the data is copied, format with a larger cluster size. Finally, use Robocopy to put the data back.


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