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California study suggests half of UFC fighters cut over 10% of body weight

The California State Athletic commission is voting on new weight cutting rules. Ahead of the vote, Iain Kidd examines the data they gathered about weight cutting at the elite level.

UFC 196: Weigh-ins Photo by Rey Del Rio/Getty Images

The California State Athletic Commission is voting on a 10 point plan to overhaul weight cutting in MMA. The UFC came out in support of the changes last week, which seem to be intelligent, reasonable changes that attempt to tackle an extremely difficult problem.

Back in March 2017, the California State Athletic Commission released sample results from a weight-cutting study they performed, comprised of 82 fighters. CSAC executive officer Andy Foster was kind enough to supply me with a larger data set, roughly twice the size of the previously released information, and I analyzed the data in detail to see what it could tell us about weight cutting.


There are a few ways to calculate how much weight the fighters in the study cut. The results contained three numbers - the weight a fighter was contracted to fight at, the weight the fighters weighed in on the study scale (which tended to be a few pounds heavier than their weigh-in weight, as they had generally already started drinking), and the weight the fighter was on fight day.

The study scale weight has the advantage of always taking place on the same scale, but has the disadvantage of not accurately representing the fighter’s lowest weight. While the official weights are lower, they have the disadvantage of not taking place on the same scale. What I chose to do instead was to count every fighter as weighing in at the limit for their class. Some actually came in slightly below or above this, but not enough to change any of the results in any meaningful way.

I also decided to focus solely on the UFC and Bellator fighters in the data set, to gain the best possible understanding of the weight cutting practices of elite fighters. As a result, the data set I used was comprised of 72 fighters.

It must be noted that weight gained after the weigh-in is an imperfect way of measuring the size of a weight cut. That’s because it’s possible for a fighter to have gained more, or less, weight than they cut. That being said, the weight gained should broadly correlate to the weight cut, and it’s the best measure we have just now.


The results using the methodology of the original study, using the weights from the “study scale,” showed around 30% of athletes put on 10% or more of their body weight between weigh ins and fight time, with an average gain of around 8%.

I decided to examine the study in a slightly different way. First, I decided to focus only on UFC and Bellator fighters. Second, I used the fighter’s contracted weigh-in weight and compared that to their fight day weight. What I discovered is that around half of UFC and Bellator fighters regain over 10% of their body weight between weigh-ins and the fight.

UFC fighters, on average (mean), regained 9.74% of their body weight. Bellator fighters, on average, regained 9.08% of their body weight. The largest amount of weight regained was 16.76% of body weight in the UFC and 17.03% in Bellator. 43% of UFC fighters regained more than 10% of their body weight, and 43% of Bellator fighters did the same.

I also re-ran the numbers allowing the one pound allowance for the class (e.g. fighters competing at welterweight are allowed to weigh in at 171 lbs). This did not significantly change the results. For the rest of the study I used the contracted weight (e.g. 170lbs for welterweight).

How did the cuts affect fights?

For the UFC bouts examined, the winner regained on average (mean) 2.2% of his/her own body weight less than their opponent. For example, the winner may have regained 10%, while the loser regained 12.2%. The athlete who regained the smaller amount of weight won around 80% of the time.

At one extreme, a winner regained 5.17% of body weight more than his/her opponent, while on the other, one winner regained 7.59% of body weight less than his opponent. The UFC results suggest that the athlete who cuts less weight is more likely to win, but the extremely small sample size means that 80% ratio should be taken with a grain of salt.

For the Bellator fights examined, the winner regained on average (mean) 1.09% of his/her own body weight more than their opponent. For example, the winner would have regained 11.09%, while the loser would have put on 10%. In Bellator’s case, the split between winners/losers in terms of who cut the most weight is almost exactly 50/50; the winner regained more weight 54% of the time.

In Bellator, one competitor won while regaining 10.16% more weight than his/her opponent. In contrast, another won while regaining 9.76% less than his/her opponent. Bellator’s results suggest cutting more weight has little to no effect on the chances of victory.

I decided to examine the fights where one fighter weighed significantly more than their opponent on fight day. I considered 4% of body weight or higher significantly more. That happened a total of 20 times. Of those 20 times, the heavier fighter won 11 times, while the lighter fighter won 9 times.


These results suggest that there’s no strong correlation between cutting more weight than your opponent, and winning. Even when a person comes in significantly heavier than their opponent, there’s basically no difference in win rate. It’s always a risk to draw firm conclusions from a limited data set, but the data we do have suggests fighters are cutting dangerous amounts of weight for little to no benefit to their chances of winning a fight.