Storage Capacity

Storage capacity is measured at it's lowest in bytes. One byte can store a decimal value up to 255 or one ASCII code as shown in this chart. Combining more bytes increases capacity. Some key terms are:

  • Byte (8 bits)
  • Kilobyte (1024 bytes)
  • Megabyte (1024 Kilobytes, or 1048576 bytes)
  • Gigabyte (1024 Megabytes, or 1048576 Kilobytes, or 1073741824 bytes)
  • Terabyte (1024 Gigabytes)

The above values tend to hold true for main memory - RAM, Random Access Memory - within the computer. However the manufacturers of hard disks have a more metric approach:

  • Byte (8 bits)
  • Kilobyte (1000 bytes)
  • Megabyte (1000 Kilobytes)
  • Gigabyte (1000 Megabytes)
  • Terabyte (1000 Gigabytes)

This difference may seem minor, but over a larger hard drive may mean many millions of bytes less for storage. Each byte can store one ASCII code as shown in this chart.

Typical Drive Capacities

These are typical storage capacities for modern storage devices:

  • Floppy disk - 1.44MB (not modern, only used as a last resort; a 'legacy' device; removeable)
  • Zip Disk - 100-250MB (different drive models have different capacities; removeable)
  • CD-ROM - 650-800MB (anything above 650MB is a result of 'overburning'; removeable)
  • Hard disk - 20GB-250GB+ (capacities and speeds always increasing)
  • DVD-ROM - 4.7GB (Theoretical capacity much higher; removeable)
  • Backup Tapes - 12GB-100GB+ (always increasing; removeable)

Secondary Storage

All of the above are considered forms of Secondary Storage. They store programs and files until they are required by the computer. At that stage, the programs or files are called into main memory and processed. The larger the amount of secondary storage your computer has, the more data it can retain when switched off.

Note that removeable disks (see above) mean that the drive that operates them has no upper limit on storage (theoretically) as when one disk is filled, another can be inserted instead.

Understanding Capacity

To understand storage capacity slightly better, 1MB is about the size, on disk, of a modern novel. This file is about 4.5KB. But the method of storing a file can have a bearing on the storage size as well. An image can be stored in different forms. Below are three links to the same image file, but stored with different mechanisms:
  • BMP Windows Bitmap (2.25MB - 2,359,350 bytes)
  • JPG Joint Photography Experts Group (0.17MB - 180,671 bytes)
  • PNG Portable Network Graphic (72.08KB - 73,805 bytes)

Obviously the larger the file, the less files can be fitted onto the medium - in this case, the drive. If you click on each of the above from home and time how long they take to transfer across the network you will also get a sense of how file sizes can affect transfer times of webpages across the internet. This is known as bandwidth and is key to modern data communications (PDF, 3.89MB).