How can I find out the size of a block device, such as /dev/sda? Running ls -l gives no useful information.


20 Answers 20


fdisk doesn't understand the partition layout used by my Mac running Linux, nor any other non-PC partition format. (Yes, there's mac-fdisk for old Mac partition tables, and gdisk for newer GPT partition table, but those aren't the only other partition layouts out there.)

Since the kernel already scanned the partition layouts when the block device came into service, why not ask it directly?

$ cat /proc/partitions
major minor  #blocks  name

   8       16  390711384 sdb
   8       17     514079 sdb1
   8       18  390194752 sdb2
   8       32  976762584 sdc
   8       33     514079 sdc1
   8       34  976245952 sdc2
   8        0  156290904 sda
   8        1     514079 sda1
   8        2  155774272 sda2
   8       48 1465138584 sdd
   8       49     514079 sdd1
   8       50 1464621952 sdd2
  • This doesn't work for things like CD drives – GS - Apologise to Monica Apr 22 '10 at 20:30
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    And also only reports "native" names. On a server with iSCSI and/or multipath and/or LVM you get lots and lots of sd* and dm-* devices but usually you need to lookup the symlink of the one you're actually interested in (e.g.. a /dev/mapper/* one) – Luke404 Jul 30 '13 at 8:32
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    The values reported here are multiples of 1KiB which may be one sector (512 byte) too small. – Lekensteyn Dec 26 '15 at 21:31
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    This was useful to me. I ended up doing awk '/sd[a-z]$/{printf "%s %8.2f GiB\n", $NF, $(NF-1) / 1024 / 1024}' /proc/partitions – suprjami Aug 5 '16 at 3:01

blockdev --getsize64 /dev/sda returns size in bytes.

blockdev --getsz /dev/sda returns size in 512-byte sectors.

Deprecated: blockdev --getsize /dev/sda returns size in sectors.

blockdev is part of util-linux.

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    unfortunately needs superuser power – heinrich5991 Oct 3 '12 at 18:51
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    @heinrich5991 you have no choice. A normal user cannot access a block device directly ("raw reading" it), which is good for security. – Totor May 23 '13 at 7:38
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    @heinrich5991 when working with filesystems you should ask statfs() and friends. More importantly, you should not assume any direct relationship between the block device size and total / usable / effective / free / whatever space on filesystem. – Luke404 Aug 1 '13 at 7:24
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    The manpage at linux.die.net/man/8/blockdev says --getsize is deprecated, and suggests --getsz for consistent 512-byte sectors. If --getsize must be used, I guess --getss should also be used to make sure the sector size is what you expect. – mwfearnley Jan 28 '18 at 17:49
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    It doesn't seem that blockdev --getsize* (and, probably, underlying BLKGETSIZE64 ioctl) requires superuser powers if the user has read permission for the disk device node: ``` # sudo -u user id uid=314(user) gid=4000(eng) groups=4000(eng) # ls -l hdw1 brw-rw---- 1 user disk 8, 177 Apr 10 17:17 hdw1 # sudo -u user blockdev --getsize64 hdw1 983040 # ls -l $(which blockdev) # no setuid bit -rwxr-xr-x 2 root root 26752 Dec 31 1999 /sbin/blockdev ``` – Kirill Spitsyn Apr 19 '18 at 17:16
cat /sys/class/block/sda/size

This gives you its size in 512-byte blocks.

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    @heinrich5991 How can you be sure the device block-size is always 512 bytes? – Totor May 26 '13 at 19:19
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    The sector size is always 512 bytes according to the source. For the future, I am trying to get this interface documented. – Lekensteyn Dec 26 '15 at 21:39
  • Thank you for making that clear. I noticed that there's also a /sys/class/block/sda/queue/logical_block_size. I wasn't sure if I should use that, or the constant 512. – Edward Falk Oct 4 '19 at 23:10

This simple code. Couldn't find any documentation, but does the trick nicely:

#include <linux/fs.h>
ioctl(file, BLKGETSIZE64, &file_size_in_bytes);
  • Correction: it actually gets you the size in SECTORS. Edit it and I'll upvote. – JCCyC Jun 22 '12 at 1:14
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    @JCCyC: BLKGETSIZE64 returns the size in bytes. See linux/fs.h, which notes "return device size in bytes". Note that BLKGETSIZE (no "64") returns it "/512". – Thanatos Mar 3 '13 at 23:26
  • I worked out a fuller sample partly based on this at stackoverflow.com/a/48490543/446106. To use the above snippet here, file needs to be a file descriptor (e.g. from open()), and file_size_in_bytes should be a size_t. – mwfearnley Jan 28 '18 at 19:54
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    (Actually, file_size_in_bytes should be a 64-bit type, so it should be unsigned long long.) – mwfearnley Jan 28 '18 at 20:17

How about:

fdisk -l

This will give you a list of all your disks with their respective capacity, usage, and more.

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    Try it with sudo, otherwise it just prints nothing. – Alfe Jun 20 '12 at 10:23

Interactive use

lsblk -no SIZE /dev/block-device

Note that, if there are partitions, you get a list with the total size of the device followed by the size of each partition or you can append the partition to the device name.


If you need to store the size in a variable, use:

size=$(lsblk -bno SIZE /dev/path | head -1)

Now, the result is always in bytes and, if path does not include a partition, it is the total disk size, otherwise it is the partition size.

  • +1, THE best, most accurate answer. And it doesn't even need sudo! \o/ – MestreLion Sep 6 '20 at 10:09
  • small improvement: it seems -bo SIZE implies -r. Also, since devices such as /dev/sda may contain partitions, it's best to consider only the first line: lsblk -bno SIZE /dev/block-device | head -n1 – MestreLion Sep 6 '20 at 10:17
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    @MestreLion: I updated the answer, given your ideas – antonio Sep 16 '20 at 18:41

blockdev --getsize /dev/sda

  • Use with sudo, otherwise you get Permission denied. – Wimateeka Aug 9 '18 at 18:00
echo "`cat /sys/class/block/sda2/size`*512" | bc

or if you use bash or any other POSIX-like shell whose arithmetic operators work with 64bit integers, you don't even need to call bc

echo "$((512*$(cat /sys/class/block/sda2/size)))"

gives the size in byte.

The call to cat and fork (except for bash) can be optimised away with bash, ksh93 and zsh with:

echo "$((512*$(</sys/class/block/sda2/size)))"

No need for ioctl in C. Just seek to the end of the file and get the size (in bytes) that way:

/* define this before any #includes when dealing with large files: */
#define _FILE_OFFSET_BITS 64
#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>

// ...
int fd = open("/dev/sda", O_RDONLY);
off_t size = lseek(fd, 0, SEEK_END);
// Now size is the size of the file, in bytes, or -1 on error.
// lseek(fd, 0, SEEK_SET) to get back to the start of the file.
  • If you're looking for the size of the raw disk drive, you'll need root privs to open the file to seek to the end. – Edward Falk Oct 4 '19 at 23:12

The blockdev(8) has a different answer? Options --getsz and deprecated --getsize are not the same.

  • BLKSSZGET (blockdev --getss) is for physical sector size and
  • BLKBSZGET (blockdev --getbsz) is for logical sector size.
echo $(($(blockdev --getsize64 /dev/sda)/$(blockdev --getss /dev/sda)))
  • Actually, I think you have those reversed. (Unless my system has 512-byte physical blocks and 4096-byte logical blocks.) – Edward Falk Jul 10 '12 at 19:24
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    Well, I realized that block/ioctl.c in Linux kernel source contains misleading comments. BLKPBSZGET (notice the P) gets physical sector size, BLKSSZGET gets logical sector size and BLKBSZGET (or BLKBSZGET_32 in block/compat_ioctl.c) gets file system allocation unit (cluster) size. Logical and physical sector size differ when Advanced Format 512e is used. (512e @ WP) – Palec Feb 22 '15 at 21:03
  • @Palec Most knowledgeful comment among all answers and comments to this question. – Irfan Latif Sep 5 '19 at 9:26

First, my thanks to all who contributed. I learned a few useful things.

Still, my experience is that most of these answers are somewhat incomplete, at least where CD and DVDs are concerned, notably regarding whether they are available to normal users rather than restricted to the superuser.
This is based on tests on my Linux Mageia 2.

Commands intended for the superuser can always be accessed by a normal user by prefixing them with /sbin/, or sometimes with /usr/sbin/. Now they may or may not work for a normal user.

Many may work, for a normal user, for a DVD on the DVD drive, even when it is not mounted, while they will not work for a hard disk (again when invoked as normal user).

For example /sbin/fdisk -l /dev/cdrom will work on my system and give the "geometry" of the DVD in the drive ... which is apparently mostly nonsense. But it does give the size of the DVD in bytes and in sectors, and a correct sector size (of 2048 bytes as is usual for DVD).

The same is true of /usr/sbin/gdisk -l /dev/cdrom, which give only the size in sectors of 2048 bytes.

other examples (as non-root, normal user)

$ /sbin/blockdev --getss /dev/cdrom   # DVD sector size
$ /sbin/blockdev --getsize64 /dev/cdrom  # DVD byte size

$ cat /proc/partitions  # see below
   8        8  416027241 sda8
  11        0    5325504 sr0
   8       16 1465138584 sdb

This works for the DVD drive, here called sr0, since the device for it is actually /dev/sr0, /dev/cdrom being only a symbolic link to it. The size is given in chunks of 1k.

Similarly, as normal user, the command

$ cat /sys/class/block/sr0/size

will give the size of a DVD on device /dev/sr0, in chunks of 512 bytes (as well as the size of other disk, even not mounted). However cat /sys/class/block/cdrom/size will not work be cause /dev/cdrom is only a symbolic link

The command df, suggested by some, gives the size of mounted partitions, not of whole disks. Furthermore, for a mounted CD or DVD it gives less than the actual size of the CD/DVD. More precisely, there are two distinct sizes one may be interested in :

  • the overall size of the device, including what is used for its internal organization. Typically that is the size of the file obtained if it is copied whole with the command dd;
  • the size of the space available (possibly only in read-only mode) to the user when it is mounted, which is always less. That is the size that is indicated by the command df.
  • uptick for $ /sbin/blockdev --getss /dev/cdrom # DVD sector size – peterretief Oct 5 '13 at 11:02
  • You say 512k but you mean just 512 byte sectors. – Tom Hale Nov 10 '16 at 8:06

A block is a sequence of bit or Bytes with a fixed length ie 512 bytes, 4kB, 8kB, 16kB, 32kB etc.

blockdev --getbsz partition


# blockdev --getbsz /dev/sda1 

So the block size of this file system is 4kB.

Analogy: number of bottles in crates.
enter image description here

  • This is the correct answer. – Karl Morrison Mar 17 '18 at 18:19

If you are using Node.js, you can use this native add on to get block device size, physical sector size and logical sector size (with support for FreeBSD, Linux, macOS and Windows). It also has a few other helpers for doing direct IO:



Is /sys/block/sda/size in block size? If so which one?

The ioctl BLKGETSIZE has the same problem as it is in units of 512 rather than BLKSSZGET. BLKGETSIZE64 solves this ambiguity. The real block count is BLKGETSIZE64/BLKSSZGET.

#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/ioctl.h>
#include <linux/fs.h>
#include <assert.h>

int main(int argc, char **argv)
        int fd;
        long blk=0L;
        long ssz=0L;
        long long oct=0LL;

        if((fd=open(argv[1],O_RDONLY))<0) { perror(argv[1]); exit(1); }
        if(ioctl(fd,BLKGETSIZE,&blk)<0) { perror("BLKGETSIZE"); exit(1); }
        if(ioctl(fd,BLKSSZGET,&ssz)<0) { perror("BLKSSZGET"); exit(1); }
        if(ioctl(fd,BLKGETSIZE64,&oct)<0) { perror("BLKGETSIZE64"); exit(1); }
        if(close(fd)<0) { perror("close"); exit(1); }
        printf("BLKGETSIZE=%ld BLKSSZGET=%ld BLKGETSIZE64=%lld BLKGETSIZE64/BLKSSZGET=%ld SIZEGB=%f #%f\240GiB\n\n",\
blk,ssz,oct,(long)(oct/(long long)ssz),(double)oct/1000000000.0,(double)oct/1073741824.0);

        fflush(stdout); /* before exec */

"for i in \
/sys/block/?d?/{size,alignment_offset,?d??/size,?d??/alignment_offset,queue/*block*,queue/*sector*}; \
do test -f \"$i\" && echo \"$i: $(<$i)\"; done"\

        return 127; /* not reached */

See http://lkml.indiana.edu/hypermail/linux/kernel/0105.2/0744.html

  • 1
    Abuse of assert: If you disable assertions, your necessary side-effects won't happen anymore. – Alfe Jun 20 '12 at 10:25

There is an EMC utility called inq which gives information about all types of block devices like local attached, SAN based, etc.

Try it out.


Here is a brief description of what it does: http://slashzeroconf.wordpress.com/2009/02/09/emc-inq-utility/

echo "`blockdev --getbsz /dev/sdc`/1024"|bc

will show output in KB

[root@veritas datadg2]# echo "`blockdev --getbsz /dev/sdc`/1024"|bc
[root@veritas datadg2]#
  • bash can do calculations inline: echo $(( $(blockdev ...)/1024 )). Also note, --getbsz gives the size of the block, not the size of the device. – mwfearnley Jan 28 '18 at 18:59

More simply:

sudo parted -l /dev/sda

Is for me the easiest to remember and type


If you want to find it programmatically in C, checkout code of util-linux package - https://github.com/karelzak/util-linux/blob/master/lib/blkdev.c#L81

OR this is also useful - http://www.linuxproblem.org/art_20.html (The code works on Linux Kernel version 5.0.0-32-generic.)

Both these codes use ioctl system call.

fdisk -l /dev/sda | grep -m1 ^Disk | awk '{print $3 " " $4}'

I find it very useful than all those standard tools or proc entries

df -k | grep /dev/sda

will give you the size in KB (first number) as well as the space used (second number) and space available (third number)

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    That's only if the filesystem is mounted. – lfaraone Jun 22 '09 at 12:54
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    This isn't about block device space, it will give you used and usable space of filesystems mounted that happen to be on /dev/sda. What about unpartitioned space? What if you have multiple partitions? – kbyrd Apr 25 '14 at 22:01

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