There is exactly one gigabyte in a gigabyte.
4.2 gigabytes
1073741824 http://www.whatsabyte.com/P1/byteconverter.htm
.733
There are 1024 megabytes in a gigabyte, or about 1000 MB. Therefore, 13000 MB = 12.6953125 GB, or about 13 GB.