When referring to hard drive capacity, one gigabyte, or GB, equals one billion bytes, and one terabyte, or TB, equals one trillion bytes. Your computer’s operating system may use a different standard of measurement and report a lower capacity. In addition, some of the listed capacity is used for formatting and other functions, and thus will not be available for data storage. Actual quantities will vary based on various factors, including: file size, file format, features and application software