A petabyte is 2 to the 50th power, or 1,125,899,906,842,624 bytes.
It can be estimated as 10 to the 15th power, or 1,000,000,000,000,000 bytes. 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.
Think of it this way. It takes:
One gigabyte (GB) to hold seven minutes of HD video. One gigabyte (GB) also holds 10 yards of book on a shelf. The size of standard DVD-R is 4.7 GB.
There are a million gigabytes in a petabyte.