how much water weight can you lose · water weight · glycogen
How Much Water Weight Can You Lose? Day vs. Week
By Simon Jounan · Founder of MyFightPrep · Kickboxer & Muay Thai fighter
Published · 7 min read
Contents
Most people can lose about 1–2% of their body weight in water over a few days, roughly 0.7–1.6 kg (1.5–3.5 lb) for a 70–80 kg adult, mainly by eating fewer carbohydrates and less salt (ISSN, 2025). On top of that, your weight normally swings 2–3 lb in either direction from one day to the next without you doing anything (Cleveland Clinic). Anything beyond that comes from sweating yourself dry. That is dehydration, it comes straight back when you eat and drink, and past about 2% of body weight it starts to cost you.
What is water weight?
Water weight is the fluid your body holds on top of fat, muscle and bone: in your blood, inside cells, bound to stored carbohydrate and in your gut. It rises and falls with what you eat, how much you drink, how much you sweat and your hormones.
Water retention is the most common cause of short-term weight changes, and a meal high in carbohydrates or sodium can make you hold more water for a while (Cleveland Clinic). That is why the scale can jump noticeably after a pizza night even though you could not physically have gained that much fat in one evening.
You control three main levers: glycogen (stored carbs), sodium (salt) and sweat. The first two change how much water your body wants to hold. The third simply pulls water out.
Glycogen: why cutting carbs drops weight so fast
Every gram of stored carbohydrate, called glycogen, is packed with about three grams of water. Empty your glycogen stores and that water goes with them, which is why the first days of a low-carb diet look so dramatic on the scale.
The exact ratio varies between studies. Glycogen is stored “in hydrated form (three to four parts water)” (Kreitzman et al., 1992), while the ISSN position stand on combat sports uses 1 g of glycogen to 2.7 g of water (ISSN, 2025).
How much glycogen do you have? Roughly 100 g in the liver and 400 g in muscle (Hearris et al., 2018). The rough maths:
| Estimate | |
|---|---|
| Glycogen stored (liver + muscle) | ~500 g |
| Water bound to it (2.7–4 g per g) | ~1.35–2 kg |
| Theoretical maximum if you emptied everything | ~1.9–2.5 kg (4–5.5 lb) |
You never empty your glycogen completely in real life, so treat that as a ceiling, not a target. In practice, cutting carbs and training depletes enough glycogen to lower body mass by 1–2% (ISSN, 2025).
A classic study shows the effect clearly. Six adults with obesity ate 800 kcal a day for 10 days on two different diets. On the low-carb (ketogenic) version, 61% of the weight they lost was water. On the same calories with normal carbohydrate, water made up 37% (Yang & Van Itallie, 1976). The low-carb diet looked faster on the scale mainly because it drained more water. The fat loss was not faster.
Sodium: the salt lever
Salt makes your body hold on to water, so cutting back from a high-salt diet sheds some fluid within days. The effect is real but smaller and less predictable than the glycogen effect.
Combat athletes reduce sodium in the last days before a weigh-in because, alongside water loading, it increases urine output and water loss (ISSN, 2025). For everyday life, just eating fewer heavily salted, processed foods usually takes the edge off bloating. Cutting salt to near zero is not the goal. You need sodium for your muscles and nerves to work, which is why fighters bring it back straight after the weigh-in (see electrolytes during a weight cut).
Drinking less water does not reduce water retention. It just dehydrates you.
Sweat: the fastest and riskiest lever
Hard training can make you sweat about 1 litre an hour, and each litre is about 1 kg on the scale. That is why sweating is the only way to lose several pounds in a single day, and also why it is the most dangerous way.
Across 1,303 athletes tested in the lab, average sweating rates ranged from about 0.8 L/h in baseball to 1.5 L/h in American football (Barnes et al., 2019). Individual sweat rates vary a lot, which is why the American College of Sports Medicine recommends measuring your own by weighing yourself before and after training (ACSM, 2007).
Sweat loss is not “losing water weight” in the sense of shedding retained fluid. It is emptying water your body needs, and every gram has to go back in.
Realistic ranges: in a day vs. in a week
In a single day you can only realistically lose normal fluctuation plus sweat. Over a week, the glycogen and salt effects add up. The table assumes a 70–80 kg (155–175 lb) adult:
| Timeframe | Where it comes from | Realistic range | Is it safe? |
|---|---|---|---|
| Any normal day | Food, drink, salt, bathroom | 2–3 lb either way (Cleveland Clinic) | Normal |
| One day, on purpose | Sweat | ~1 kg per hour of hard sweating (Barnes et al., 2019) | Dehydration; see limits below |
| 3–7 days of low carb and less salt | Glycogen water, some sodium-related water | 1–2% of body weight, ~0.7–1.6 kg (ISSN, 2025) | Fine for healthy adults |
| 10 days, very low-calorie low-carb diet | Mostly glycogen water | ~2.9 kg of water in adults with obesity (Yang & Van Itallie, 1976) | Temporary; comes back with carbs |
The ~2.9 kg figure is 61% of the 466.6 g a day those participants lost, over 10 days. Larger people with more muscle store more glycogen and tend to see bigger swings. Smaller, lean people see smaller ones.
How fighters use water weight
Fighters stack these levers in the final week before a weigh-in: lower carbs and fibre, reduce sodium, manipulate fluids, then sweat off a small final amount, and rehydrate after stepping off the scale. On average, combat athletes lose about 5% of body mass this way (ISSN, 2025).
That only makes sense because they have hours or a full day between the weigh-in and the fight to refill. How much is sensible depends on your days out and weigh-in timing. That is covered in detail in how much weight you can cut before a fight. For the fluid side, see the water loading guide. If the word “cutting” is new to you, start with what weight cutting is.
If you are not stepping on an official scale, there is no reason to dehydrate at all.
Why water weight always comes back
Water weight comes back because nothing structural changed. Once you eat carbohydrates and salt and drink normally, glycogen refills and pulls its water back with it.
Kreitzman and colleagues describe this as an “exaggerated regain” when people carb-load after dieting (Kreitzman et al., 1992). It explains two common experiences:
- The first-week “whoosh” on a diet. Much of it is water, so progress seems to slow in week two, even when fat loss continues.
- The post-weekend jump. A salty, carb-heavy meal can put a couple of pounds back overnight, well within the normal daily swing. That is water, not fat.
For fat loss, watch your weekly average rather than single weigh-ins. For fighters, this refill is the whole point: the weight you drop for the scale is meant to come back before the fight. That is what rehydrating after the weigh-in is about.
When losing water becomes dangerous
Once you lose more than about 2% of your body weight as water, performance measurably drops. Larger losses raise the risk of dizziness, fainting and heat illness.
| Water lost (% of body weight) | What the evidence says |
|---|---|
| ~1–1.5 kg (2–3 lb) either way, day to day | Normal fluctuation (Cleveland Clinic) |
| More than 2% | ACSM advises preventing losses beyond this during exercise to avoid compromised performance (ACSM, 2007) |
| ~2.5% | Can already impair performance (ISSN, 2025) |
| ~5% | The average rapid weight loss in combat sports, done with a planned rehydration window and ideally supervision (ISSN, 2025) |
Watch for the NHS warning signs of dehydration: thirst, headache, feeling dizzy or light-headed, tiredness, a dry mouth, and dark, strong-smelling urine or peeing less than usual. Dizziness on standing that does not go away, confusion, rapid breathing or unusual drowsiness need urgent medical help (NHS).
Never use diuretics (water pills), laxatives, enemas, diet pills or vomiting to lose water weight. Sports nutrition experts explicitly do not endorse these methods (ISSN, 2025).
MyFightPrep tracks your weight trend toward fight day and plans water and salt for fight week, so the water you drop is planned, small and comes back on time. Free.
Important
This guide is general information, not medical advice. If you have swelling or fluid retention you can’t explain, see a doctor. It can be a sign of a medical condition, and water pills should only ever be used on prescription. Dehydrating on purpose can be dangerous and, in extreme cases, life-threatening. Athletes under 18 should never dehydrate to make weight. If you feel any of the warning signs above, stop, drink, and get help.
Frequently asked questions
Can you lose 5 lbs of water weight in a day?
Only by sweating, and it would be dehydration rather than real weight loss. Five pounds is about 2.3 kg, which is more than 3% of body weight for a 70 kg adult, already past the roughly 2% level where performance starts to suffer. It comes back as soon as you eat and drink.
Can I lose 20 pounds of water weight?
Not safely, unless a doctor is treating fluid retention from a medical condition. Twenty pounds is about 9 kg, which is over 11% of body weight for an 80 kg (176 lb) person. That is far beyond the roughly 5% that combat athletes lose on average, and they do it under supervision with a rehydration window.
How much water weight do you lose before actual weight?
In the first days of a diet, a large share of the loss is water. In a small study of adults on an 800-calorie diet, water made up 61% of the weight lost over 10 days on a low-carb version and 37% on a version with normal carbohydrate. Once glycogen is emptied, that extra water loss stops.
Can 10 pounds be water weight?
In some people, yes, especially larger people switching to a very low-carb, low-calorie diet. In one study, obese adults lost roughly 2.9 kg (6.3 lb) of water in 10 days on a low-carb diet, and daily swings of 2–3 lb on top of that are normal. For a smaller, lean person, 10 pounds of pure water loss would mean serious dehydration.
How fast does water weight come back?
Within a day or two of eating carbohydrates and salt and drinking normally again. Glycogen refills and pulls its water back with it, which is why the scale often jumps after a high-carb meal even though you haven't gained fat.
Sources
This guide is based on the works below. Numbers in the text refer to them.
- Kreitzman SN, Coxon AY, Szaz KF (1992). Glycogen storage: illusions of easy weight loss, excessive weight regain, and distortions in estimates of body composition
- Ricci AA, Evans C, Stull C, et al. (2025). International society of sports nutrition position stand: nutrition and weight cut strategies for mixed martial arts and other combat sports
- Hearris MA, Hammond KM, Fell JM, Morton JP (2018). Regulation of Muscle Glycogen Metabolism during Exercise: Implications for Endurance Performance and Training Adaptations
- Yang MU, Van Itallie TB (1976). Composition of weight lost during short-term weight reduction. Metabolic responses of obese subjects to starvation and low-calorie ketogenic and nonketogenic diets
- Barnes KA, Anderson ML, Stofan JR, et al. (2019). Normative data for sweating rate, sweat sodium concentration, and sweat sodium loss in athletes: An update and analysis by sport
- American College of Sports Medicine; Sawka MN, Burke LM, Eichner ER, Maughan RJ, Montain SJ, Stachenfeld NS (2007). American College of Sports Medicine position stand. Exercise and fluid replacement
- Cleveland Clinic (2024). 7 Possible Reasons Why Your Weight Fluctuates
- NHS (2026). Dehydration