*n*. 2 to the power of 80 (approximately 10 to the power of 24) bytes, or a million trillion megabytes.

**Example Citation:**

**yottabyte**, or 1 trillion terabytes."

—"IBM Unveils Storage Capacity on Demand for Growing e-businesses,"

*Business Wire*, October 27, 2000

**Notes:**

PREFIX POWER UNITS NUMBER OF 10 kilo- 3 thousands 1,000 mega- 6 millions 1,000,000 giga- 9 billions 1,000,000,000 tera- 12 trillions 1,000,000,000,000 peta- 15 quadrillions 1,000,000,000,000,000 exa- 18 quintillions 1,000,000,000,000,000,000 zetta- 21 sextillions 1,000,000,000,000,000,000,000 yotta- 24 septillions 1,000,000,000,000,000,000,000,000

All the numerical prefixes are defined by the International Standards Organization in a document called ISO 1000, "SI units and recommendations for the use of their multiples and of certain other units." (How's that for a mouthful of a title?) These prefixes are agreed upon by various committees, but there is some logic to their etymology. For example, tera- comes from tetra-, "four," because tera- represents 1,000 to the 4th power. Similarly, peta- is derived from penta-, "five," exa- comes from hexa-, "six", zetta- is a variation of the Latin septum, "seven," and yotta- is a variation of octo-, "eight."

Since a kilobyte is actually 1,024 bytes, a megabyte is 1,024 kilobytes, and so on, the above numbers don't accurately reflect the exact byte values represented by each unit. Here are the exact values:

UNIT POWER ACTUAL BYTES OF 2 kilobyte 10 1,024 megabyte 20 1,048,576 gigabyte 30 1,073,741,824 terabyte 40 1,099,511,627,776 petabyte 50 1,125,899,906,842,624 exabyte 60 1,152,921,504,606,846,976 zettabyte 70 1,180,591,620,717,411,303,424 yottabyte 80 1,208,925,819,614,629,174,706,176To put this in some perspective (or not), it would take approximately 86 trillion years to download a 1-yottabyte file, and the entire contents of the Library of Congress would consume a mere 10 terabytes.

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