At Family Tree DNA, a Y-DNA haplogroup prediction is calculated using genetic distance. The calculation finds your most likely haplogroup. The prediction program compares your Y-DNA STR (short tandem repeat) profile to FTDNA's results database. The program uses the results of exact and near matches.
The process balances using a reliable number of Y-DNA STR markers and using a database of confirmed haplogroups that is sufficient in size. The program uses 14 Y-DNA STR markers for haplogroup predictions. These are the first 12 STR markers in all of FTDNA's Y-DNA tests and two additional markers that they test as check values.
It is important to understand that you must take an SNP test to establish your haplogroup for certain. It is also important to understand that haplogroups are not the same as family groups. Haplogroups are much larger than family groups and contain many different family groups and surnames.