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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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NAME | SYNOPSIS | DESCRIPTION | OPTIONS | BUGS | SEE ALSO | HOMEPAGE | AUTHORS | COLOPHON |
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MKFS.FAT(8) System Manager's Manual MKFS.FAT(8)
mkfs.fat - create an MS-DOS FAT filesystem
mkfs.fat [OPTIONS] DEVICE [BLOCK-COUNT]
mkfs.fat is used to create a FAT filesystem on a device or in an
image file. DEVICE is the special file corresponding to the
device (e.g. /dev/sdXX) or the image file (which does not need to
exist when the option -C is given). BLOCK-COUNT is the number of
blocks on the device and size of one block is always 1024 bytes,
independently of the sector size or the cluster size. Therefore
BLOCK-COUNT specifies size of filesystem in KiB unit and not in
the number of sectors (like for all other mkfs.fat options). If
omitted, mkfs.fat automatically chooses a filesystem size to fill
the available space.
Two different variants of the FAT filesystem are supported.
Standard is the FAT12, FAT16 and FAT32 filesystems as defined by
Microsoft and widely used on hard disks and removable media like
USB sticks and SD cards. The other is the legacy Atari variant
used on Atari ST.
In Atari mode, if not directed otherwise by the user, mkfs.fat
will always use 2 sectors per cluster, since GEMDOS doesn't like
other values very much. It will also obey the maximum number of
sectors GEMDOS can handle. Larger filesystems are managed by
raising the logical sector size. An Atari-compatible serial
number for the filesystem is generated, and a 12 bit FAT is used
only for filesystems that have one of the usual floppy sizes
(720k, 1.2M, 1.44M, 2.88M), a 16 bit FAT otherwise. This can be
overridden with the -F option. Some PC-specific boot sector
fields aren't written, and a boot message (option -m) is ignored.
-a Normally, for any filesystem except very small ones, mkfs.fat
will align all the data structures to cluster size, to make
sure that as long as the partition is properly aligned, so
will all the data structures in the filesystem. This option
disables alignment; this may provide a handful of additional
clusters of storage at the expense of a significant
performance degradation on RAIDs, flash media or large-sector
hard disks.
-A Select using the Atari variation of the FAT filesystem if that
isn't active already, otherwise select standard FAT
filesystem. This is selected by default if mkfs.fat is run on
68k Atari Linux.
-b SECTOR-OF-BACKUP
Selects the location of the backup boot sector for FAT32.
Default depends on number of reserved sectors, but usually is
sector 6. If there is a free space available after the backup
boot sector then backup of the FAT32 info sector is put after
the backup boot sector, usually at sector 7. The backup must
be within the range of reserved sectors. Value 0 completely
disables creating of backup boot and info FAT32 sectors.
-c Check the device for bad blocks before creating the
filesystem.
-C Create the file given as DEVICE on the command line, and write
the to-be-created filesystem to it. This can be used to
create the new filesystem in a file instead of on a real
device, and to avoid using dd in advance to create a file of
appropriate size. With this option, the BLOCK-COUNT must be
given, because otherwise the intended size of the filesystem
wouldn't be known. The file created is a sparse file, which
actually only contains the meta-data areas (boot sector, FATs,
and root directory). The data portions won't be stored on the
disk, but the file nevertheless will have the correct size.
The resulting file can be copied later to a floppy disk or
other device, or mounted through a loop device.
-D DRIVE-NUMBER
Specify the BIOS drive number to be stored in the FAT boot
sector. For hard disks and removable media it is usually
0x80–0xFF (0x80 is first hard disk C:, 0x81 is second hard
disk D:, ...), for floppy devices or partitions to be used for
floppy emulation it is 0x00–0x7F (0x00 is first floppy A:,
0x01 is second floppy B:).
-f NUMBER-OF-FATS
Specify the number of file allocation tables in the
filesystem. The default is 2.
-F FAT-SIZE
Specifies the type of file allocation tables used (12, 16 or
32 bit). If nothing is specified, mkfs.fat will automatically
select between 12, 16 and 32 bit, whatever fits better for the
filesystem size.
-g HEADS/SECTORS-PER-TRACK
Specify HEADS and SECTORS-PER-TRACK numbers which represents
disk geometry of DEVICE. Both numbers are stored into the FAT
boot sector. Number SECTORS-PER-TRACK is used also for
aligning the total count of FAT sectors. By default disk
geometry is read from DEVICE itself. If it is not available
then LBA-Assist Translation and translation table from the SD
Card Part 2 File System Specification based on total number of
disk sectors is used.
-h NUMBER-OF-HIDDEN-SECTORS
Specify the number of so-called hidden sectors, as stored in
the FAT boot sector: this number represents the beginning
sector of the partition containing the file system. Normally
this is an offset (in sectors) relative to the start of the
disk, although for MBR logical volumes contained in an
extended partition of type 0x05 (a non-LBA extended
partition), a quirk in the MS-DOS implementation of FAT
requires it to be relative to the partition's immediate
containing Extended Boot Record. Boot code and other software
handling FAT volumes may also rely on this field being set up
correctly; most modern FAT implementations will ignore it. By
default, if the DEVICE is a partition block device, mkfs.fat
uses the partition offset relative to disk start. Otherwise,
mkfs.fat assumes zero. Use this option to override this
behaviour.
-i VOLUME-ID
Sets the volume ID of the newly created filesystem; VOLUME-ID
is a 32-bit hexadecimal number (for example, 2e24ec82). The
default is a number which depends on the filesystem creation
time.
-I Ignore and disable safety checks. By default mkfs.fat refuses
to create a filesystem on a device with partitions or virtual
mapping. mkfs.fat will complain and tell you that it refuses
to work. This is different when using MO disks. One doesn't
always need partitions on MO disks. The filesystem can go
directly to the whole disk. Under other OSes this is known as
the superfloppy format. This switch will force mkfs.fat to
work properly.
-l FILENAME
Read the bad blocks list from FILENAME.
-m MESSAGE-FILE
Sets the message the user receives on attempts to boot this
filesystem without having properly installed an operating
system. The message file must not exceed 418 bytes once line
feeds have been converted to carriage return-line feed
combinations, and tabs have been expanded. If the filename is
a hyphen (-), the text is taken from standard input.
-M FAT-MEDIA-TYPE
Specify the media type to be stored in the FAT boot sector.
This value is usually 0xF8 for hard disks and is 0xF0 or a
value from 0xF9 to 0xFF for floppies or partitions to be used
for floppy emulation.
--mbr[=y|yes|n|no|a|auto]
Fill (fake) MBR table with disk signature one partition which
starts at sector 0 (includes MBR itself) and spans whole disk
device. It is needed only for non-removable disks used on
Microsoft Windows systems and only when formatting whole
unpartitioned disk. Location of the disk signature and
partition table overlaps with the end of the first FAT sector
(boot code location), therefore there is no additional space
usage. Default is auto mode in which mkfs.fat put MBR table
only for non-removable disks when formatting whole
unpartitioned disk.
-n VOLUME-NAME
Sets the volume name (label) of the filesystem. The volume
name can be up to 11 characters long. Supplying an empty
string, a string consisting only of white space or the string
"NO NAME" as VOLUME-NAME has the same effect as not giving the
-n option. The default is no label.
--codepage=PAGE
Use DOS codepage PAGE to encode label. By default codepage
850 is used.
-r ROOT-DIR-ENTRIES
Select the minimal number of entries available in the root
directory. The default is 112 or 224 for floppies and 512 for
hard disks. Note that this is minimal number and it may be
increased by mkfs.fat due to alignment of structures. See
also mkfs.fat option -a.
-R NUMBER-OF-RESERVED-SECTORS
Select the minimal number of reserved sectors. With FAT32
format at least 2 reserved sectors are needed, the default is
32. Otherwise the default is 1 (only the boot sector). Note
that this is minimal number and it may be increased by
mkfs.fat due to alignment of structures. See also mkfs.fat
option -a.
-s SECTORS-PER-CLUSTER
Specify the number of disk sectors per cluster. Must be a
power of 2, i.e. 1, 2, 4, 8, ... 128.
-S SECTOR-SIZE
Specify the number of bytes per filesystem sector. Must be a
power of 2 and greater than or equal to 512, i.e. 512, 1024,
2048, 4096, 8192, 16384, or 32768. Values larger than 4096
are not conforming to the FAT filesystem specification and may
not work everywhere. Some operating systems have requirement
that filesystem sector size must match logical disk device
sector size. So default value is logical disk sector size.
-v Verbose execution.
--offset SECTOR
Write the filesystem at a specific sector into the device
file. This is useful for creating a filesystem in a
partitioned disk image without having to set up a loop device.
--variant TYPE
Create a filesystem of variant TYPE. Acceptable values are
standard and atari (in any combination of upper/lower case).
See above under DESCRIPTION for the differences.
--help
Display option summary and exit.
--invariant
Use constants for normally randomly generated or time based
data such as volume ID and creation time. Multiple runs of
mkfs.fat on the same device create identical results with this
option. Its main purpose is testing mkfs.fat.
mkfs.fat can not create boot-able filesystems. This isn't as easy
as you might think at first glance for various reasons and has
been discussed a lot already. mkfs.fat simply will not support it
;)
fatlabel(8), fsck.fat(8)
The home for the dosfstools project is its GitHub project page
⟨https://github.com/dosfstools/dosfstools⟩.
dosfstools were written by Werner Almesberger ⟨werner.almesberger@
lrc.di.epfl.ch⟩, Roman Hodek ⟨Roman.Hodek@informatik.uni-
erlangen.de⟩, and others. Current maintainers are Andreas Bombe
⟨aeb@debian.org⟩ and Pali Rohár ⟨pali.rohar@gmail.com⟩.
This page is part of the dosfstools (Tools for making and checking
MS-DOS FAT filesystems) project. Information about the project
can be found at ⟨https://github.com/dosfstools/dosfstools⟩. If
you have a bug report for this manual page, see
⟨https://github.com/dosfstools/dosfstools/issues⟩. This page was
obtained from the project's upstream Git repository
⟨https://github.com/dosfstools/dosfstools.git⟩ on 2025-08-11. (At
that time, the date of the most recent commit that was found in
the repository was 2023-10-10.) If you discover any rendering
problems in this HTML version of the page, or you believe there is
a better or more up-to-date source for the page, or you have
corrections or improvements to the information in this COLOPHON
(which is not part of the original manual page), send a mail to
man-pages@man7.org
dosfstools 4.2+git 2021-01-31 MKFS.FAT(8)
Pages that refer to this page: dosfsck(8), dosfslabel(8), fatlabel(8), fsck.fat(8), fsck.msdos(8), fsck.vfat(8), systemd-makefs@.service(8)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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