Why do boxers weigh in the day before a fight?

To give fighters time to rehydrate. The practice is standard in professional boxing and is also one of the sport's most criticised safety issues.

Boxing is divided into weight classes so that fighters face opponents of similar size. Historically, weigh-ins happened on the morning of the fight. Since the 1980s most professional bouts have weighed in roughly 24 to 36 hours beforehand, following deaths that were attributed in part to fighters competing while severely dehydrated.

The intent was safety. The consequence was an arms race in weight cutting. A fighter whose natural weight is well above the limit can shed several kilograms of water in the days before the weigh-in — through restricted fluid intake, sweating in saunas and hot baths, and reduced carbohydrate and salt — then make the limit, and spend the following day rehydrating. It is common for a boxer to enter the ring five to ten kilograms heavier than they weighed the previous day.

This creates the outcome the divisions were meant to prevent. Two fighters can weigh the same at the official weigh-in and be substantially different sizes when they actually fight, with the advantage going to whoever cut more.

Severe dehydration also reduces the cushioning fluid around the brain, which is why medical bodies regard aggressive cutting as a serious risk in a sport based on head impacts.

Some organisations have responded with same-day weight checks, rehydration limits capping how much a fighter may regain, or hydration testing. Adoption is inconsistent.

Amateur and Olympic boxing generally weighs in on the day of competition, which limits how much can be cut.