Previous ratings for individual fights have been based on quarter-hour ratings, which measure the average viewership for an entire 15-minute period. Given that some fights last for well over 15 minutes and some fights last for well under 15 minutes, measuring a fight's average viewership based on quarter-hour ratings has always been an inexact science.
In the majority of cases, the average viewership for any given fight is dragged down by the lower viewership in the minutes immediately preceding and following the fight. It's the minutes containing the actual fights themselves that generally draw the highest viewership levels, so why not track the ratings on a minute-by-minute basis?
Using minute-by-minute Nielsen ratings data, the most accurate possible viewership information for any given fight can be determined. Viewership levels are based on live viewership, plus same-day DVR, rounded to the nearest 1,000 viewers, and the times listed are ET/PT. The indicated times begin at the opening bell of a fight and end at the minute in which the winner of the fight is known.
(In the case of a fight that ends in submission or KO/TKO, the ending time is obviously when the fight ends. In the case of a judges' decision, the ending time is the minute in which the judges' decision is announced. In the case of a doctors' stoppage, the ending time is the minute in which the fight is officially stopped by the doctor. The ending time is always the minute in which the winner if the fighter is known.)
Note that these are just the most-watched fights in U.S. MMA history, not the most-watched fights in worldwide MMA history. In Japan, a fight would need to draw over 30 million viewers to crack the top five. It also excludes pay-per-view, as accurate data is not made readily available.