After digging into the source code and POSIX standard, I would say the answer by @antje-m and @Gilles are mostly correct.
It's worth to quote the comment from POSIX.1-2008, as a summary:
The use of 512-byte units is historical practice and maintains compatibility with ls and other utilities in this volume of POSIX.1-2008. This does not mandate that the file system itself be based on 512-byte blocks. The -k option was added as a compromise measure. It was agreed by the standard developers that 512 bytes was the best default unit because of its complete historical consistency on System V (versus the mixed 512/1024-byte usage on BSD systems), and that a -k option to switch to 1024-byte units was a good compromise. Users who prefer the more logical 1024-byte quantity can easily alias df to df -k without breaking many historical scripts relying on the 512-byte units.
For the block size in
The POSIX says that the default block size is implementation-defined, unless
-k option is given.
The default block size implemented in
ls is defined in
/* The default block size used for output. This number may change in
the future as disks get larger. */
# define DEFAULT_BLOCK_SIZE 1024
which comes from an old commit:
Author: Jim Meyering <email@example.com>
Date: Mon Jun 29 15:23:04 1998 +0000
The commit message itself didn't say anything about the number 1024.
And note that the block size used in
df is also 1024,
ls just chose to consist with them. Although for
df it is a confliction to the POSIX standard (so here the environment variable
POSIXLY_CORRECT comes). This is seems a decision of the GNU team, see the wikipedia page POSIX about this controversy.
For the command
It is not a part of POSIX standard, but the
stat system call is.
However the unit for block size is not standardized (sys_stat.h):
The unit for the st_blocks member of the stat structure is not defined within POSIX.1-2008.
stat command simply displays the information provided by
stat system call, and using 512 block size with few exception (they are non-Linux, e.g. HP-UX, IBM AIX etc. see the macros defined in
So the number 512 is more a historical choice and a Linux convention.
GNU coreutils (hence the
ls command) is not a part of Linux kernel (hence the
stat call), they are targeting different system aspect,
GNU coreutils is more for human (easier to read), and Linux kernel for hardware abstract (hence closer to hardware).
Edit: the 4096 block size is the "IO block" size, the real physical block size is likely still 512 Byte as explained in this question.