How to Determine Total Unused Cluster Space on a Drive

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This article describes how to determine to total amount of unused space on a drive. This is the total amount of space on a drive that is unused when clusters are occupied by files that are smaller than the clusters. For example, if a 1-kilobyte (KB) file is stored in a 4-KB cluster on a drive, there is 3 KB of unused space in this cluster.


By design, the FAT file system allocates files to the drive on a per- cluster basis. Therefore, two files cannot be stored in the same cluster at the same time. To determine the total amount of unused space on a drive, use the following steps:

  1. Click Start, point to Programs, and then click MS-DOS Prompt.
  2. Type the following commands, pressing ENTER after each command:
    dir /a /s /v
  3. The following items are displayed:
    - Bytes
    - Bytes Allocated
    - Bytes Free
    - Bytes Total Disk Space

    Note the value for Bytes and Bytes Allocated.
  4. Subtract the number of bytes from the number of bytes allocated, and the resulting number is the total amount of unused space on the drive in kilobytes. If you want to convert this number to megabytes (MB), divide this number by 1024, and then divide the resulting number by 1024 again.
For related information about clusters and the FAT file system, see the following articles in the Microsoft Knowledge Base:
TITLE : FAT Type and Cluster Size Depends on Logical Drive Size

TITLE : MS-Dos Partitioning Summary

ARTICLE-ID: 192322
TITLE : Description of Default Cluster Sizes for FAT32 File System


Article ID: 195441 - Last Review: January 9, 2015 - Revision: 1.2
  • Microsoft Windows 98 Standard Edition
  • Microsoft Windows 95
kbnosurvey kbarchive kbinfo KB195441

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