dired-free-space-program is a variable defined in `dired.el'.
Its value is

Program to get the amount of free space on a file system.
We assume the output has the format of `df'.
The value of this variable must be just a command name or file name;
if you want to specify options, use `directory-free-space-args'.

A value of nil disables this feature.

If the function `file-system-info' is defined, it is always used in
preference to the program given by this variable.

You can customize this variable.