A petabyte is 2 to the 50th power, or 1,125,899,906,842,624 bytes. However, petabytes are often estimated as 10 to the 15th power, or 1,000,000,000,000,000 bytes. To avoid ambiguity, the exact calculation is often referred to as a pebibyte instead of a petabyte, though both definitions are commonly accepted.
A petabyte is 1,024 terabytes and precedes the exabyte unit of measurement. Since even the largest hard drives are measured in terabytes, petabytes are only used to measure the storage space of multiple hard drives or other collections of data. And no, "petabyte" is not what you do to make a byte purr.
For a list of all the different units of measurements, view this Help Center article.
Updated: October 13, 2008