Alternatively referred to as disk space, disk capacity is the total number of bytes that a disk drive or disc is capable of holding. Typically, the storage device is the computer's hard drive, but it can also be a USB thumb drive, a CD or DVD, a memory stick/card, or a floppy disk. Disk space is usually measured in KB (kilobytes), MB (megabytes), GB (gigabytes), or TB (terabytes).
Disk capacity example
The image below shows the total disk capacity of a hard drive, as well as how much space is being used, and how much is available.