Page "POSIX" Paragraph 51

POSIX mandates 512-byte block sizes for the df and du utilities, reflecting the default size of blocks on disks.

When Richard Stallman and the GNU team were implementing POSIX for the GNU operating system, they objected to this on the grounds that most people think in terms of 1024 byte ( or 1 KiB ) blocks.

The environmental variable POSIXLY_CORRECT was introduced to force the standards-compliant behaviour.

The variable POSIX_ME_HARDER was also discussed and was implemented in a few packages before being obsoleted by POSIXLY_CORRECT.