Uil..from the FAQ:

What is BKR and how is it calculated?
BKR means "BrainKing Rating" and is calculated by US Chess Federation formula for each kind of a game separately according to your game results. The player who finishes at least 4 rated games of one kind (everybody who defines a new game can specify whether the game will be "rated" - the result will be calculated for BKR - or not), earns "provisional BKR" displayed by italic font (e.g. 1870). After finishing 25 rated games of one kind the provisional BKR changes to "established BKR" and is displayed by normal font (e.g. 1853). If you complete more games, your BKR is more accurate because is based on more results.