How to display free disk space of drives (hard disk)

The df utility displays statistics about the amount of free disk space on the specified file system or on the file system of which file is a part. Values are displayed in 512-byte per block counts. If neither a file or a file system operand is specified, statistics for all mounted file systems are displayed.

Use df -h to see the output "Human-readable" output. Uses unit suffixes: Byte, Kilobyte, Megabyte, Gigabyte, Terabyte and Petabyte.

 # df -h 

Example:
# df -h
Filesystem Size Used Avail Capacity Mounted on
/dev/ad4s1a 23G 2.8G 19G 13% /
devfs 1.0K 1.0K 0B 100% /dev