What Is a Terabyte and a Petabyte?

by Lou Martin

Data storage measurements are expressed in a variety of units, including bytes, gigabytes, terabytes and petabytes. Starting with bytes, each successive unit is equal to approximately 1,000 of the previous unit. For instance, a terabyte is equal to 1,000 gigabytes, while a petabyte is made up of 1,000 terabytes. Electronic data is stored on an assortment of memory devices, such as memory cards and computer hard drives.


The smallest unit of data measurement is the bit. Eight bits make up a byte, and 1,000 bytes equals a kilobyte. The units of measurement from this point include, in order, megabytes, gigabytes, terabytes, petabytes, exabytes, zettabytes, yottabytes, brontobytes and geopbytes. A terabyte contains approximately 8.9 trillion bits, while a petabyte contains about nine quadrillion bits. In relation to bytes, a terabyte is the same as about 1.1 trillion bytes, and a petabyte is equal to approximately 1.1 quadrillion bytes. As you come to understand what these numbers mean, you'll get a better idea of the massive amounts of data they represent.


A bit is the smallest unit of data measurement and is represented by two states of information, such as "yes" and "no." A byte's worth of data is represented by one character in this sentence, while a kilobyte has enough data to fill a short paragraph. As each unit increases, the amount of data represented multiplies exponentially. For instance, a terabyte can hold as much data as can be found in 1,000 encyclopedias. A petabyte can hold about 20 million four-drawer filing cabinets full of text, or, to illustrate further, about 500 billion pages of printed text. At the time of publication, a yottabyte, one of the larger units, could hold all the content found on the Internet.

Memory Cards

While computer hard drives and solid state drives are the most common memory devices used to store data, smaller devices are also used. For example, mobile phones and tablet computers support memory cards, such as SD cards, for expanding the machine's memory capacity. These small cards often provide as much, if not more, data storage space than their hard- and solid-state drive counterparts. SD cards are produced in SD, SDHC and SDXC formats and offer varying amounts of data storage space. SD cards, for instance, provide up to 2GB of memory and are available in a variety of capacities, including 256MB and 512MB. SDHC cards provide between 2GB and 32GB of memory and are also produced in an assortment of capacities. SDXC cards offer between 32GB and 2TB of memory. As of the date of publication, no memory card offers a capacity measured in petabytes.

Technological Advances

Computer hard-drive capacities were first measured in megabytes when computers were first introduced during the 1980s. By 1991, however, the first gigabyte hard drive was introduced, followed by the first terabyte hard drive in 2007. Hitachi was the first company to develop the 1TB hard drive, unveiling it at the Consumer Electronics Show in Las Vegas in 2007. At the time of publication, no stand-alone petabyte hard drives have been produced.

About the Author

Lou Martin has been writing professionally since 1992. His work has appeared in the "Los Angeles Times," the "Long Beach Press-Telegram" and the "Deseret Morning News." Martin holds a Bachelor of Science in history and communication.

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