Barton had requested a personal hearing and an independent regulatory commission found the 29-year-old guilty of two counts of violent conduct.
The controversial midfielder was shown a red card at the Etihad Stadium for elbowing Carlos Tevez and he then kicked Sergio Aguero before attempting to headbutt Vincent Kompany.
The FA announced Barton received an automatic four-game suspension, with the additional eight matches a consequence of the manner of his dismissal. He was also fined £75,000.
QPR will now begin their own internal inquiry into what transpired on a dramatic final day of an extraordinary Barclays Premier League campaign.
Club spokesman Ian Taylor said on Twitter: "QPR will be making no comment in regard to the Joey Barton case. Internal investigation will begin in due course."