Weight is the most consequential number in the sport and the least understood. A fighter's listed division tells you what they weighed for about an hour on a Friday, not what they weigh when the cage door closes.
The divisions
The men's divisions under the unified rules run from strawweight at 115lb through flyweight (125), bantamweight (135), featherweight (145), lightweight (155), welterweight (170), middleweight (185), light heavyweight (205) and heavyweight (265). The women's divisions in common use are strawweight, flyweight, bantamweight and featherweight at the same limits.
Catchweight is any agreed limit between divisions, usually arranged when a fighter misses weight or when a bout is made on short notice.
The one-pound allowance
Non-title fights carry a one-pound allowance: a lightweight bout can be contested at up to 156lb. Title fights carry no allowance, and a champion who misses weight is generally stripped, with the challenger able to win the belt if they win the fight. This is why the same scale reading can be a non-event in one bout and a disaster in another.
What a cut actually is
Two different processes get called "cutting weight" and conflating them causes most of the confusion.
The first is ordinary fat loss over a training camp — weeks of controlled deficit, gradual, physiologically unremarkable.
The second is acute dehydration in the final days, and it is the part that carries risk. Fighters manipulate water and sodium intake to shed several kilos of body water, then rehydrate after the scale. The weight comes off fast, comes back fast, and none of it is tissue. A fighter can plausibly weigh ten or more pounds above their division by the time they compete.
Why early weigh-ins changed things
Weigh-ins moved to the morning of the day before the fight in most jurisdictions, giving roughly thirty hours to rehydrate rather than a few. The intent was safety, and it does give more recovery time. The side effect is that it made larger cuts more survivable, so some fighters simply cut more. Whether the rule reduced harm or shifted it is still argued about, and the honest answer is that it did some of both.
Reading a missed weight
Missing by half a pound and missing by four pounds are different events. A small miss is usually a timing error in the last hours. A large miss usually means the cut failed days earlier, and it often shows in the fight: a fighter who could not make the weight frequently could not rehydrate properly either. A fighter who misses badly and then looks flat in round one is a common pattern, not a coincidence.
Fighters who miss weight typically forfeit a percentage of their purse to their opponent, and the opponent retains the right to refuse the bout.