How Windows Calculates Fragmentation Percentage

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When running defrag -a -v C: on Windows 2008 R2, I got the following output:

Windows Disk Defragmenter
Copyright (c) 2006 Microsoft Corp.

Analysis report for volume C:

    Volume size                         = 40.00 GB
    Cluster size                        = 4 KB
    Used space                          = 13.94 GB
    Free space                          = 26.06 GB
    Percent free space                  = 65 %

File fragmentation
    Percent file fragmentation          = 19 %
    Total movable files                 = 73,181
    Average file size                   = 233 KB
    Total fragmented files              = 1,574
    Total excess fragments              = 5,611
    Average fragments per file          = 1.09
    Total unmovable files               = 21

The Percent file fragmentation value seems very strange to me. From the file counts, 1574 fragmented files is only 2% of the total number of files.

How does Windows calculate the Percent file fragmentation value?

Best Answer

It seems that:

Percent file fragmentation = Percent free space (after defragmentation) - Percent free space (before defragmentation)

In other words, the percent free space added by performing full defragmentation.

As you can see, 7 % = 31 % - 24 %:

C:\Windows\system32>defrag c: -w -v
Windows Disk Defragmenter
Copyright (c) 2006 Microsoft Corp.`

Defragmentation report for volume C:

    Volume size                         = 74.53 GB
    Cluster size                        = 4 KB
    Used space                          = 55.96 GB
    Free space                          = 18.57 GB
    Percent free space                  = 24 %

File fragmentation
    Percent file fragmentation          = 7 %
    Total movable files                 = 203,178
    Average file size                   = 342 KB
    Total fragmented files              = 3,641
    Total excess fragments              = 32,280
    Average fragments per file          = 1.19
    Total unmovable files               = 26

Free space fragmentation
    Free space                          = 18.57 GB
    Total free space extent             = 22,689
    Average free space per extent       = 858 KB
    Largest free space extent           = 4.09 GB

Folder fragmentation
    Total folders                       = 40,150
    Fragmented folders                  = 155
    Excess folder fragments             = 567

Master File Table (MFT) fragmentation
    Total MFT size                      = 203 MB
    MFT record count                    = 205,234
    Percent MFT in use                  = 98
    Total MFT fragments                 = 3

    Note: On NTFS volumes, file fragments larger than 64MB are not included in the fragmentation statistics

Defragmentation report for volume C:

    Volume size                         = 74.53 GB
    Cluster size                        = 4 KB
    Used space                          = 50.90 GB
    Free space                          = 23.62 GB
    Percent free space                  = 31 %

File fragmentation
    Percent file fragmentation          = 0 %
    Total movable files                 = 203,178
    Average file size                   = 342 KB
    Total fragmented files              = 3
    Total excess fragments              = 1,152
    Average fragments per file          = 1.00
    Total unmovable files               = 26

Free space fragmentation
    Free space                          = 23.62 GB
    Total free space extent             = 10,109
    Average free space per extent       = 2 MB
    Largest free space extent           = 6.99 GB

Folder fragmentation
    Total folders                       = 40,150
    Fragmented folders                  = 1
    Excess folder fragments             = 0

Master File Table (MFT) fragmentation
    Total MFT size                      = 203 MB
    MFT record count                    = 205,234
    Percent MFT in use                  = 98
    Total MFT fragments                 = 3

    Note: On NTFS volumes, file fragments larger than 64MB are not included in the fragmentation statistics

C:\Windows\system32>

On the other hand, when, after defragmenting a drive, the Percent free space does not change, nor the Percent file fragmentation does not change, even if the Total fragmented files number does change: 

C:\Windows\system32>defrag d: -w -v -f
Windows Disk Defragmenter
Copyright (c) 2006 Microsoft Corp.

Defragmentation report for volume D:

    Volume size                         = 932 GB
    Cluster size                        = 4 KB
    Used space                          = 841 GB
    Free space                          = 90.89 GB
    Percent free space                  = 9 %

File fragmentation
    Percent file fragmentation          = 2 %
    Total movable files                 = 400,108
    Average file size                   = 2 MB
    Total fragmented files              = 7,671
    Total excess fragments              = 1,420,753
    Average fragments per file          = 4.70
    Total unmovable files               = 7

Free space fragmentation
    Free space                          = 90.89 GB
    Total free space extent             = 213,633
    Average free space per extent       = 446 KB
    Largest free space extent           = 27.46 GB

Folder fragmentation
    Total folders                       = 16,189
    Fragmented folders                  = 526
    Excess folder fragments             = 2,304

Master File Table (MFT) fragmentation
    Total MFT size                      = 421 MB
    MFT record count                    = 405,678
    Percent MFT in use                  = 94
    Total MFT fragments                 = 3

    Note: On NTFS volumes, file fragments larger than 64MB are not included in the fragmentation statistics

Defragmentation report for volume D:

    Volume size                         = 932 GB
    Cluster size                        = 4 KB
    Used space                          = 841 GB
    Free space                          = 90.89 GB
    Percent free space                  = 9 %

File fragmentation
    Percent file fragmentation          = 2 %
    Total movable files                 = 400,108
    Average file size                   = 2 MB
    Total fragmented files              = 353
    Total excess fragments              = 1,226,342
    Average fragments per file          = 4.19
    Total unmovable files               = 7

Free space fragmentation
    Free space                          = 90.89 GB
    Total free space extent             = 260,094
    Average free space per extent       = 366 KB
    Largest free space extent           = 27.46 GB

Folder fragmentation
    Total folders                       = 16,189
    Fragmented folders                  = 1
    Excess folder fragments             = 0

Master File Table (MFT) fragmentation
    Total MFT size                      = 421 MB
    MFT record count                    = 405,678
    Percent MFT in use                  = 94
    Total MFT fragments                 = 3

    Note: On NTFS volumes, file fragments larger than 64MB are not included in the fragmentation statistics

C:\Windows\system32>
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