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Why is the effective hard drive size lower than the actual size?

I have a 16 gb pen drive. when i formatted it with fat32..i got 14.8 gb to work with...while using ntfs i got 14.7 gb. Where did the 100 mb go?

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migrated from stackoverflow.com Dec 19 '12 at 10:04

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marked as duplicate by Oliver Salzburg Dec 19 '12 at 12:25

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NTFS is a lot more complicated file system. It needs additional space for keeping track of file permission (security tab) and other features that it is introducing. For pen drives you better format it with exfat - it is the next generation FAT file system without the limitation of 2 GB file size. It will allow you to store bigger files. It is Microsoft recommendation. –  mnmnc Dec 19 '12 at 10:11
    
Are those two sizes (14.8 GB, 14.7 GB) 'free space' or 'total space' ('capacity')? Also, you have dropped quite a few significant figures - it's likely not actually 100 MB, but a rounding error. –  Bob Dec 19 '12 at 12:47
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2 Answers

Filesystems store meta information (information about the files). Fat follows a legacy structure which was designed for small disks and low memory. Modern filesystems can have many additional features, backups of strucutres, and more info about the files themselves (permissions for example).

Cluster size (block size) is also a factor. This is the size of units which can be associated to a file. If it is 4k for example, then an average 2k per file is lost, because the FAT will not have the resolution to deal with details smaller than that. NTFS's default cluster size can be smaller than FAT32's, giving you a larger database about these allocation units which costs more space against the device's unallocated size.

A number of structures are allocated by default so it is understandable that there is a difference. If you start filling the drive with files the difference can grow even bigger - or NTFS can make up for this space originally lost, depending on the style you use the file system (few large files / many small ones).

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The reason for this is, that every filesystem needs a specific amount of memory (storage) to build the filesystem structure (List and position on the block device of directories, files, ... called ToC (Table of Contents), metadata, filesystem superblock, ...) on the storage device.

So you see FAT32 just needs 100MB less than NTFS.

See http://en.wikipedia.org/wiki/File_system for more informations.

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