Weight classes exist to make fights competitive, and in mixed martial arts they also shape who can realistically fight whom across a career.
The men's divisions
Flyweight tops out at 125 pounds. Bantamweight at 135. Featherweight at 145. Lightweight at 155. Welterweight at 170. Middleweight at 185. Light heavyweight at 205. Heavyweight at 265, the only division with a floor as well as a ceiling.
The women's divisions
Strawweight at 115 pounds. Flyweight at 125. Bantamweight at 135. Featherweight at 145.
The one-pound allowance
For a non-title fight, a fighter may weigh in up to one pound over the divisional limit. A welterweight bout can therefore be contested at 171 pounds.
For a championship fight there is no allowance at all. The limit is exact, and a champion who misses it forfeits the belt on the scale. The fight can still go ahead, but only the opponent can win the title.
Catchweight
When a fighter misses weight badly, or when two fighters from different divisions want to meet in the middle, the bout can be agreed at a catchweight: an agreed figure that belongs to no division. Catchweight bouts do not carry titles.
Why weight cutting dominates the sport
Fighters do not live at their competition weight. They compete below their walking weight by dehydrating in the days before the weigh-in, then rehydrating in the roughly twenty-four to thirty-six hours between weighing in and fighting.
The advantage is size. The risk is real: severe dehydration impairs performance, slows recovery and has been linked to serious harm. Earlier weigh-ins, hydration testing in some organisations and pressure from athletic commissions have all been attempts to reduce it.
Round lengths
Standard bouts are three rounds of five minutes. Championship bouts and designated main events are five rounds of five minutes, with one minute between rounds. That extra distance is why title challengers train differently, because pacing across twenty-five minutes is a distinct skill.
Moving up and down
A fighter who struggles to make weight will eventually move up, usually one division. Moving down is rarer and harder, because there is a floor beneath which the cut becomes dangerous rather than merely unpleasant.
Champions who compete in two divisions are unusual precisely because the physical profile that dominates one weight rarely dominates the next.
The short version
Eight men's divisions from 125 to 265 pounds, four women's divisions from 115 to 145. One pound of grace for non-title fights, none for titles. Three rounds normally, five for a belt.