Weight Cut Calculator (MMA/Boxing)

A weight cut is the process fighters use to temporarily reduce body weight before an official weigh-in, so they can compete in a lower weight class. The cut typically involves reducing body fat through diet over several weeks, followed by acute water manipulation (dehydration) in the final days. After weigh-in, fighters rehydrate before the fight to regain size and strength.

Ideally, the majority of your cut should come from fat loss through a calorie deficit over weeks — this is the safest approach. Water cutting should be limited to 3–5% of your body weight at most. Cutting more than 5% of body weight via dehydration is associated with significantly impaired performance and serious health risks.

This calculator uses the U.S. Navy Body Fat Method, which uses your height, neck, and waist measurements (plus hips for females) to estimate body fat percentage. While not as precise as DEXA or hydrostatic weighing, it is reliable enough for planning purposes and is widely used in athletic settings.

Most sports science guidelines recommend keeping water cuts to under 5% of body weight for same-day weigh-ins, and up to 7–8% if you have 24+ hours to rehydrate before competition. Anything beyond that dramatically increases the risk of heat illness, kidney stress, impaired cognition, and reduced fight performance.

Prioritize electrolyte-rich fluids (sports drinks, oral rehydration salts) alongside water to replenish both fluid and sodium/potassium lost during dehydration. Pair rehydration with carbohydrate intake to restore glycogen. A general guideline is to consume 150% of the fluid lost — so if you cut 5 lbs of water, aim for about 120 oz of fluid over the recovery window.

Same-day weigh-ins occur just 2–4 hours before the fight, giving fighters almost no time to rehydrate. This makes large water cuts extremely risky and means you should enter weigh-in much closer to your natural weight. Next-day or 24-hour weigh-ins allow substantial rehydration, which is why fighters in those promotions often cut more aggressively.

You should compete in the lowest weight class where your natural walk-around weight is no more than 10–15% above the limit — and your water cut does not exceed 5% of body weight for same-day weigh-ins. Competing too far below your natural weight compromises health and performance. Use this calculator to find the class where your cut is both feasible and safe.

Water loading involves drinking very high amounts of water (e.g., 2–3 gallons/day) for several days before the cut, then abruptly reducing intake. This tricks the body into continuing to excrete water at a high rate even after intake drops, accelerating natural diuresis. It is a common strategy used by experienced combat sports athletes to enhance the water cut in the final 48–72 hours.