Fasting Calculator

Free water fasting calculator for weight loss on a 3-day water fast and longer. This fasting calculator estimates fat vs water weight from 24 hours to 21 days. No signup.

A daily 16:8 eating window is the intermittent fasting calculator. This fasting calculator is for a water-only fast.

This water fasting calculator estimates fat and water loss during a zero-calorie water-only fast (not 16:8). Most people lose 0.25–0.5 kg of fat per day, plus 2–3 kg of water weight in the first 48 hours. Enter your stats below to get a personalized estimate.

Based on the Mifflin–St Jeor equation (BMR) and clinical fasting research. 1 kg fat = 7,700 kcal.

Water Fasting Calculator with Weight Loss & Hydration Estimates

How much weight will I lose on a 3 day water fast?

On a 3-day (72-hour) water fast most people lose 4–10 lbs (2–4.5 kg) on the scale. About 70–80% is water and glycogen, not fat. Typical fat loss is 0.5–1.5 lbs (0.2–0.7 kg). After you eat again, water comes back in 2–5 days — the fat is what stays if you return to maintenance calories.

The calculator above defaults to 72 hours. Enter your weight and hit Calculate for a number from your BMR, not a generic range.

3-day water fast weight loss by starting weight

Estimates assume a sedentary or light-activity fast and a typical TDEE. Heavier bodies burn more per day, so the scale moves more — most of that extra drop is still water.

Starting weight Scale (3 days) Fat Water / glycogen After refeed (net)
130 lb (59 kg) 3–5 lbs 0.5–0.8 lb 2.5–4.2 lbs ~0.6 lb
160 lb (73 kg) 5–7 lbs 0.8–1.1 lb 4–6 lbs ~0.9 lb
190 lb (86 kg) 6–8 lbs 1.0–1.4 lb 5–7 lbs ~1.2 lb
220 lb (100 kg) 7–10 lbs 1.2–1.7 lb 6–8.5 lbs ~1.4 lb

Not medical advice. Ranges, not promises. Illness, meds, last meal, and hormones move the number.

What happens each day of a 72-hour fast

Day Scale Mostly What it feels like
Day 1 (0–24 h) 2–4 lbs Glycogen water Biggest drop. Hunger and headaches are common. Little of this is fat.
Day 2 (24–48 h) 1–2 lbs Water + some fat Ketones rise. Hunger often peaks, then eases. Autophagy starts to ramp up.
Day 3 (48–72 h) 0.5–1.5 lbs More fat Slower drop. Autophagy near its 3-day peak. Electrolytes matter more than on day 1.

So if someone asks how much weight will I lose on a 3 day water fast, the honest answer is: a few pounds on the scale, about one pound of fat for an average adult, and a rebound when you drink and eat again. Use the water fasting calculator for your own TDEE instead of copying a before/after screenshot.

How much weight can you lose fasting for 3 days calculator?

On a 3-day water fast most people see about 4–10 lb on the scale. About 70–80% of that is water and glycogen. Fat loss is closer to 0.5–1.5 lb. Set the duration to 72 hours in this fasting calculator for a number from your own stats. The water comes back after you eat.

How much weight will I lose water fasting calculator?

This water fasting weight loss calculator uses your size and the length of the fast. Expect about 0.25–0.5 kg of fat per day, plus 2–3 kg of water in the first 48 hours. A heavier person with a higher burn loses more fat. The scale number is not the fat number.

What is a water fast calculator?

A water fast calculator estimates scale weight and fat loss on zero calories, from 24 hours to 21 days. This page is that tool. A 16:8 schedule is a different calculator.

What is a water fasting weight loss calculator?

A water fasting weight loss calculator splits the scale drop into fat and water. It uses your size, activity, and how many days you fast. One kilogram of fat is about 7,700 kcal, so a day of zero food is only a fraction of a kilogram of fat. The rest of the early drop is glycogen and water.

Is this water fasting calculator free?

Yes. This water fasting calculator is free. Enter your stats, set the length of the fast, and see fat versus water weight. No signup.

What do 3 days of fasting do to the body?

On day one glycogen drops and the scale falls mostly from water. By day two or three the body uses more fat and makes more ketones. Hunger often peaks, then eases. Dizziness and low sodium become more likely, so this is not a first fast to invent on your own. The table above is the 72-hour picture. Talk to a doctor before a 3-day water fast if you have not done a shorter one.

How this water fasting calculator works

Same core as other serious water fasting calculators (Mifflin–St Jeor), plus a water/glycogen adjustment so the first 48 hours are not treated as pure fat:

  1. BMR from age, sex, height, and weight (Mifflin–St Jeor).
  2. TDEE = BMR × activity during the fast (resting is lower than your gym TDEE).
  3. Zero calories in → daily deficit ≈ TDEE.
  4. Fat ≈ deficit ÷ 7,700 kcal per kg.
  5. Early scale loss adds glycogen-bound water (largest in the first 1–2 days). That water returns after refeed.
  6. After day 3 the result applies a small metabolic slowdown (BMR dips a little on longer fasts) and a risk level from duration, BMI, age, and experience. You can copy or export the plan.

We also show hydration and sodium / potassium / magnesium targets, plus a day-by-day timeline. The number is a planning range, not a guarantee.

How much weight can you lose water fasting?

Plan on 0.25–0.5 kg (0.5–1.1 lb) of fat per day once glycogen is gone, and a faster scale drop on days 1–2. A week is not 7× the 3-day number — water loss plateaus, fat continues, and metabolism slows a little. The duration table below is for a ~70 kg person; the calculator scales it to you.

What is water fasting?

Water fasting is a type of fast where you consume only water (and sometimes zero-calorie electrolytes) for a set period of time. Unlike intermittent fasting, which cycles between eating and fasting windows each day, water fasting typically lasts 24 hours to several weeks. During a water fast, your body shifts from using glucose to burning stored fat for energy, a process called ketosis. Extended fasts may also trigger autophagy, a cellular cleanup process.

What does this water fasting calculator do?

This water fasting calculator estimates your expected weight loss, daily water intake needs, electrolyte requirements, and calorie deficit during a water fast. It also provides a day-by-day timeline of what to expect physically and mentally, so you can prepare and fast safely. Whether you're planning a short 24-hour fast or an extended water fast, this tool gives you a personalized plan.

How much weight will I lose water fasting?

Weight loss during water fasting varies by individual, but a common estimate is 0.5–1.0 kg (1–2 lbs) per day. However, much of the initial weight loss is water weight (glycogen depletion), not pure fat. Actual fat loss depends on your TDEE (total daily energy expenditure). For example, if your TDEE is 2,000 kcal/day, you'd burn roughly 0.25 kg (~0.55 lbs) of fat per day. After refeeding, some water weight returns, so the net fat loss is typically less dramatic than the scale suggests.

What happens to your body during a water fast?

Your body goes through several phases during a water fast:

  • Hours 0–12: Your body uses up stored glucose (glycogen). Blood sugar drops gradually.
  • Hours 12–24: Glycogen stores deplete. The body begins producing ketones from fat. You may feel hungry, irritable, or have headaches.
  • Days 1–3: Full transition to ketosis. Hunger often peaks on day 1–2 and then decreases. Energy may dip before stabilizing.
  • Days 3–5: Deep ketosis. Many people report mental clarity and reduced hunger. Autophagy ramps up.
  • Days 5–7+: The body is fully fat-adapted. Weight loss continues at a steadier rate. Electrolyte management becomes critical.

How much water should you drink during a water fast?

A good baseline is 2–3 liters (8–12 cups) per day, adjusted for your body weight and activity. Larger individuals and those in warm climates may need more. Don't over-drink — excessive water without electrolytes can cause hyponatremia (dangerously low sodium). Sip consistently throughout the day rather than drinking large amounts at once. For a daily target from your weight and climate, use the water intake calculator.

Do you need electrolytes during a water fast?

Yes, especially for fasts longer than 24 hours. Without food, you lose electrolytes in urine and sweat. Symptoms of imbalance: headaches, dizziness, muscle cramps, heart palpitations, fatigue.

Electrolyte Typical daily target Common source
Sodium 1,000–2,000 mg Salt or salty water
Potassium 500–1,000 mg Potassium chloride (salt substitute)
Magnesium 200–400 mg Magnesium citrate or glycinate

The calculator result shows targets scaled to your plan. These ranges are planning aids, not a prescription.

Is water fasting safe?

Short water fasts (24–72 hours) are generally safe for healthy adults. Extended fasts (5+ days) carry more risk and ideally should be done under medical supervision. Water fasting is not recommended for people who are pregnant, breastfeeding, underweight, under 18, have diabetes (especially type 1), have eating disorders, take certain medications, or have heart/kidney conditions. Always consult a healthcare professional before attempting an extended fast.

How do you break a water fast safely?

The longer the fast, the slower the refeed:

Fast length Refeed window Start with
1 day Same day Light meal — soup, fruit, small portions
2–3 days 1–2 days Bone broth, diluted juice, soft foods
5+ days About half the fast Broth → juices → soft foods → normal meals. Skip large meals, sugar, and processed food at first.

Refeeding syndrome is a rare but serious risk with extended fasts. It occurs when the body gets a sudden influx of carbohydrates after prolonged fasting, causing dangerous electrolyte shifts. This is why gradual refeeding is critical.

Water fasting vs. intermittent fasting: what's the difference?

IF still includes daily meals. Water fasting is zero calories for 24 hours to several weeks. Many people start with IF first — set a 16:8 window on the intermittent fasting calculator.

Intermittent fasting Water fasting
Eating Daily, inside a window (16:8, 18:6, OMAD) None — water only
Typical length 12–20 hours a day, ongoing 24 hours to 21 days
Fat loss pace ~0.5–1 kg / week if in a deficit ~0.25–0.5 kg fat / day after glycogen is gone
Risk Lower — you still eat Higher after day 3; electrolytes and refeed matter

What are the benefits of water fasting?

Research and anecdotal evidence suggest several potential benefits:

  • Fat loss: Your body burns stored fat for energy when no food is available.
  • Autophagy: Cellular cleanup process that removes damaged cells and proteins (typically starts after 24–48 hours).
  • Insulin sensitivity: Fasting can improve how your body responds to insulin.
  • Mental clarity: Many fasters report improved focus and cognitive function after the initial adjustment period.
  • Reduced inflammation: Some studies show decreased inflammatory markers during fasting.
  • Digestive rest: Gives your digestive system a break, which may help with gut health.

What can you consume during a water fast?

A strict water fast allows only:

  • Water (still or sparkling)
  • Electrolytes (sodium, potassium, magnesium — zero-calorie supplements)
  • Some fasters also allow: black coffee, plain tea (purists avoid these)

Anything with calories — including juice, broth, or supplements with sugar — technically breaks a water fast. However, some people follow a "modified" water fast that includes bone broth or minimal calories.

Metabolic stages during a water fast

Your body goes through distinct metabolic shifts as a water fast progresses. The exact timing varies based on glycogen stores, metabolic rate, and your last meal's composition.

Time Metabolic State What Happens Fuel Source
0–4 h Fed State Digesting last meal, absorbing nutrients. Blood sugar rises, insulin secreted. Excess glucose stored as glycogen or fat. Glucose (dietary)
4–8 h Post-Absorptive Digestion complete. Blood sugar normalizes, insulin drops. Body begins accessing liver glycogen stores for energy. Glycogen (liver)
8–12 h Early Fasting Glycogen stores depleting. Glucagon rises, stimulating fat breakdown (lipolysis). Free fatty acids released into bloodstream. Glycogen + fatty acids
12–18 h Fat Burning Liver glycogen largely depleted. Fat oxidation accelerates. Growth hormone rises 300–500% to preserve muscle. Many report improved mental clarity. Free fatty acids
18–24 h Ketosis Onset Liver produces ketone bodies (BHB, acetoacetate) from fatty acids. Brain switches partially to ketone fuel. Blood ketones reach 0.1–0.5 mmol/L. Ketones + fatty acids
24–48 h Deep Ketosis Full ketosis established. Autophagy begins to ramp up — cells break down and recycle damaged proteins and organelles. Hunger often decreases paradoxically. Ketones (primary)
48–72 h Peak Autophagy Autophagy peaks. mTOR pathway suppressed. Immune system reset may begin — old white blood cells broken down, stem cell regeneration triggered. Ketones + recycled amino acids
72 h+ Extended Fast Body fully fat-adapted. Steady fat loss ~0.25–0.5 kg/day. Electrolyte management critical. Refeeding syndrome risk increases with duration. Medical supervision recommended. Ketones + fat stores

Note: Times are approximate. High-carb last meals delay ketosis; low-carb or keto-adapted individuals enter ketosis faster. Individual genetics, fitness level, and metabolic health affect timing. Sources: Cahill (2006), Anton et al. (2018), Alirezaei et al. (2010).

Water fasting weight loss by duration

This table shows typical weight loss ranges for different water fast durations based on a 70 kg (154 lb) person with a TDEE of ~1,800 kcal/day.

Duration Scale Loss Actual Fat Loss Water Weight Difficulty
24 hours 1–2 kg (2–4 lbs) 0.2 kg (0.5 lbs) ~1.2 kg Beginner
48 hours 2–4 kg (4–8 lbs) 0.5 kg (1.0 lbs) ~2.4 kg Beginner
72 hours (3 days) 3–5 kg (6–10 lbs) 0.7 kg (1.5 lbs) ~3 kg Moderate
5 days 4–7 kg (8–15 lbs) 1.2 kg (2.6 lbs) ~3 kg Intermediate
7 days 5–9 kg (10–20 lbs) 1.6 kg (3.5 lbs) ~3 kg Advanced
14 days 7–14 kg (15–30 lbs) 3.3 kg (7.3 lbs) ~3 kg Medical supervision
21 days 9–18 kg (20–40 lbs) 4.9 kg (10.8 lbs) ~3 kg Medical supervision

Water weight returns within 2–5 days of refeeding. Fat loss is permanent if post-fast calorie intake matches maintenance levels. Heavier individuals lose more due to higher BMR.

Important Safety Warning

This calculator provides estimates for educational purposes only — it is not medical advice. Extended water fasting (5+ days) should only be done under medical supervision. Stop fasting immediately if you experience severe dizziness, fainting, chest pain, or confusion. Always consult your doctor before starting a water fast.

Bottom line

Water fasting can be a powerful tool when done safely:

  • Start with shorter fasts and build up gradually
  • Stay hydrated and supplement electrolytes
  • Break your fast carefully and slowly
  • Listen to your body — stop if something feels wrong

This water fasting calculator helps you plan ahead so you know what to expect. Use it alongside professional guidance for the best results.

Key Takeaways

  • Short water fasts (24–48 hours) are generally safe for healthy adults; extended fasts need medical guidance.
  • A 3-day water fast is usually 4–10 lbs on the scale and about 0.5–1.5 lbs of fat; water returns after you eat.
  • Most initial weight loss is water from glycogen depletion — actual fat loss is ~0.25 kg per day.
  • Key electrolytes: sodium (1,000–2,000 mg/day), potassium (500–1,000 mg/day), magnesium (200–400 mg/day).
  • Autophagy — cellular cleanup — ramps up significantly after 24–48 hours of fasting.
  • Break fasts gradually: start with broth and soft foods, refeeding over half the fast duration.

References

  1. Cahill GF Jr. Fuel metabolism in starvation. Annu Rev Nutr. 2006;26:1-22. PubMed
  2. Anton SD, Moehl K, Donahoo WT, et al. Flipping the Metabolic Switch: Understanding and Applying the Health Benefits of Fasting. Obesity. 2018;26(2):254-268. PubMed
  3. de Cabo R, Mattson MP. Effects of Intermittent Fasting on Health, Aging, and Disease. N Engl J Med. 2019;381(26):2541-2551. PubMed
  4. Alirezaei M, Kemball CC, Flynn CT, et al. Short-term fasting induces profound neuronal autophagy. Autophagy. 2010;6(6):702-710. PubMed
  5. Longo VD, Mattson MP. Fasting: molecular mechanisms and clinical applications. Cell Metab. 2014;19(2):181-192. PubMed
  6. Kerndt PR, Naughton JL, Driscoll CE, Loxterkamp DA. Fasting: the history, pathophysiology and complications. West J Med. 1982;137(5):379-399. PubMed
  7. Mifflin MD, St Jeor ST, Hill LA, et al. A new predictive equation for resting energy expenditure in healthy individuals. Am J Clin Nutr. 1990;51(2):241-247. PubMed
  8. Finnell JS, Saul BC, Goldhamer AC, Myers TR. Is fasting safe? A chart review of adverse events during medically supervised, water-only fasting. BMC Complement Altern Med. 2018;18(1):67. PubMed
  9. Patterson RE, Sears DD. Metabolic Effects of Intermittent Fasting. Annu Rev Nutr. 2017;37:371-393. PubMed
  10. Goldhamer AC, Lisle DJ, Sultana P, et al. Medically supervised water-only fasting in the treatment of borderline hypertension. J Altern Complement Med. 2002;8(5):643-650. PubMed

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