GNU coreutils: block size related to df command

ABOUT df

df (abbreviation for disk free) is a standard Unix command used to display the amount of
available disk space for file systems on which the invoking user has appropriate read access.
df is typically implemented using the statfs or statvfs system calls.

[bash]
$df
Filesystem 1K-blocks Used Available Use% Mounted on
udev 1921644 0 1921644 0% /dev
tmpfs 388676 6508 382168 2% /run
/dev/sda2 475184136 6024592 444951776 2% /
tmpfs 1943372 1460 1941912 1% /dev/shm
tmpfs 5120 4 5116 1% /run/lock
tmpfs 1943372 0 1943372 0% /sys/fs/cgroup
/dev/sda1 523248 3660 519588 1% /boot/efi
tmpfs
[/bash] 388672 104 388568 1% /run/user/1000

[bash]
$df -h
Filesystem Size Used Avail Use% Mounted on
udev 1.9G 0 1.9G 0% /dev
tmpfs 380M 6.4M 374M 2% /run
/dev/sda2 454G 5.8G 425G 2% /
tmpfs 1.9G 1.5M 1.9G 1% /dev/shm
tmpfs 5.0M 4.0K 5.0M 1% /run/lock
tmpfs 1.9G 0 1.9G 0% /sys/fs/cgroup
/dev/sda1 511M 3.6M 508M 1% /boot/efi
tmpfs 380M 104K 380M 1% /run/user/1000
$export DF_BLOCK_SIZE=2k
[/bash]

[bash]
$df
Filesystem 2K-blocks Used Available Use% Mounted on
udev 960822 0 960822 0% /dev
tmpfs 194338 3254 191084 2% /run
/dev/sda2 237592068 3012288 222475896 2% /
tmpfs 971686 136 971550 1% /dev/shm
tmpfs 2560 2 2558 1% /run/lock
tmpfs 971686 0 971686 0% /sys/fs/cgroup
/dev/sda1 261624 1830 259794 1% /boot/efi
tmpfs 194336 50 194286 1% /run/user/1000
$export POSIXLY_CORRECT=1
[/bash]

[bash]
$df
Filesystem 2K-blocks Used Available Use% Mounted on
udev 960822 0 960822 0% /dev
tmpfs 194338 3252 191086 2% /run
/dev/sda2 237592068 3018782 222469402 2% /
tmpfs 971686 618 971068 1% /dev/shm
tmpfs 2560 2 2558 1% /run/lock
tmpfs 971686 0 971686 0% /sys/fs/cgroup
/dev/sda1 261624 1830 259794 1% /boot/efi
tmpfs 194336 46 194290 1% /run/user/1000
$set -o posix
[/bash]

[bash]
$df
Filesystem 2K-blocks Used Available Use% Mounted on
udev 960822 0 960822 0% /dev
tmpfs 194338 3252 191086 2% /run
/dev/sda2 237592068 3018774 222469410 2% /
tmpfs 971686 618 971068 1% /dev/shm
tmpfs 2560 2 2558 1% /run/lock
tmpfs 971686 0 971686 0% /sys/fs/cgroup
/dev/sda1 261624 1830 259794 1% /boot/efi
tmpfs 194336 46 194290 1% /run/user/1000
$
[/bash]

https://en.wikipedia.org/wiki/Df_(Unix)
https://www.gnu.org/software/coreutils/manual/coreutils.html#df-invocation