Mayweather pleaded guilty to a reduced domestic-violence misdemeanor charge and no contest to two harassment misdemeanor charges in the Clark County (Nev.) courthouse.
The jail sentence complicates, and perhaps scraps, plans for a late-spring fight between Mayweather (42-0) and Filipino superstar Manny Pacquiao.
"You don't need to be a genius to count the days," said a person in Pacquiao's camp, who was unauthorized to speak publicly.
With Mayweather scheduled to report to jail Jan. 6, his 90-day term would expire in early April. Clark County Court spokeswoman Mary Ann Price said, "He will likely spend approximately 65 days in custody," based on receiving 22 days of "good-time [behavior] credit," in addition to three other days of credit for time served.