A zettabyte is 1021 or 1,000,000,000,000,000,000,000 bytes.
One zettabyte (abbreviated "ZB") is equal to 1,000 exabytes and precedes the yottabyte unit of measurement. Zettabytes are slightly smaller than zebibytes, which contain 1,180,591,620,717,411,303,424 (270) bytes.
A single zettabyte contains one sextillion bytes, or one billion terabytes. That means it would take one billion one terabyte hard drives to store one zettabyte of data. Because the zettabyte unit of measurement is so large, it is only used to measure large aggregate amounts of data. Even all the data in the world is estimated to be only a few zettabytes.
NOTE: View a list of all the units of measurement used for measuring data storage.