Extended Fasting Weight Loss Calculator
Free extended water fasting weight loss calculator is a physiology-based model that estimates fat loss, glycogen depletion, protein breakdown, and total weight change during multi-day fasting. Adjust your inputs to see how your projected weight curve changes in real time. For educational purposes only - estimates are based on simplified physiological assumptions and individual results may vary. Fasting carries potential risks, so consult a healthcare professional before attempting extended fasting.
📱 Now available — get the Fasting Center app
Your Starting Point
Enter your baseline parameters.
Physiological Parameters
If you don’t know your Body Fat or Bone Mass, leave them blank and we’ll auto‑estimate.
Daily Activity
Fasting Protocol
Model Parameters
What Happens in Your Body During a Fast
🍽️ Digestion (0–4h)
Your body is still living off the last meal. Glucose is the main fuel and insulin is elevated. The real shift is still ahead.
📉 Blood Sugar Falls (3–8h)
With the meal absorbed, blood glucose drifts down toward fasting levels. The dip is gentle and normal, and it triggers the hormone shift that comes next.
🔽 Insulin Falls (4–8h)
Insulin drops roughly 20–50% from its fed peak while glucagon rises, and your liver taps stored glycogen. This flip is the master switch of the whole fast.
🔥 Fat Oxidation Increases (8–16h)
Glycogen is spent faster than it is replaced, so your body leans more on stored fat. Often the hardest stretch: hunger waves and fatigue are common. The 2–5 lb drop here is mostly water, not fat.
🌱 Ketones Rising (16–24h)
Your liver starts turning fatty acids into ketones, climbing to around 0.1–0.5 mmol/L. Still low, but the first sign the metabolic switch is underway.
🪫 Glycogen Depletion (18–30h)
Liver glycogen is roughly 70–90% depleted, so fat and ketones carry more of the load. Sodium losses rise, which is why electrolytes start to matter here.
⚡ Moderate Ketosis (24–48h)
Ketones reach roughly 0.5–2.0 mmol/L and become real fuel for brain, heart and muscle. Many people notice a clear lift in mental clarity around here.
💪 HGH Surge (~24h)
Growth hormone surges around the one-day mark, rising several-fold in research. One of its jobs is protecting lean muscle while you run on fat.
♻️ Autophagy Increases (24–48h)
Cells step up autophagy, clearing and recycling worn or damaged parts. The strongest direct evidence comes from animal studies, so human timing is an informed estimate.
🟢 Deep Ketosis (48–72h)
Ketones often reach 2–5 mmol/L and fat loss accelerates. Hunger frequently drops off substantially and energy feels steady.
✨ Autophagy Peaks (48–72h)
Cellular cleanup is thought to be at its most active in this window. As with earlier autophagy, treat the timing as a best estimate.
📊 Insulin Bottoms (~54h)
Insulin reaches its lowest fasting level. That extended quiet is what drives improved insulin sensitivity, and the benefit outlasts the fast itself.
🛡️ Immune Renewal (72–96h)
Past three days, fasting is linked to signaling shifts that may prompt stem-cell and immune-cell activity. Electrolyte management becomes critical. Evidence is limited and largely from animal research.
🫧 Gut Content Cleared (~72h)
The last of your food has finished its journey. This is part of why early fasting weight loss looks so fast and so misleading: much of it was food and water, not fat.
🕯️ Steady Fat Burning (96h+, day 4+)
Glycogen is long gone, ketones are high and stable, and fat is your fuel around the clock. Daily loss is now mostly fat rather than water.
⚠️ Elevated Risk Zone (168h+, day 7+)
Past about a week, electrolyte losses add up and testosterone, T3 and IGF-1 sit suppressed. Fasts this long are not for everyone and are best done with medical supervision.
🍲 Refeeding (after your fast)
How you come out matters as much as the fast itself. Start with warm broth and a little steamed vegetable, ease back over several days, and keep electrolytes going.
Timing varies. Your last meal's carbohydrate content, activity level, and metabolic health can shift these ranges by hours. Autophagy and immune or stem-cell timings are drawn largely from animal research, so treat them as informed estimates.
Frequently Asked Questions
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Most people lose roughly 1 to 2 lb per day of total weight, but only about half of that is fat. The rest is glycogen, water, gut content, and a small amount of protein. This calculator separates those components so you can see the real fat loss instead of just the scale number. text goes here
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Your body can only oxidize a limited amount of fat per day, set largely by your fat mass and energy needs. Everything above that comes from glycogen and its bound water, gut content, and protein. Water and glycogen return within days of refeeding; fat loss is what stays.
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It is a simplified physiological model, not a measurement. It uses your RMR, activity level, body composition, and fast length to estimate the tissue breakdown. Individual results vary with metabolic health, hydration, activity, and how much you ate before the fast.
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The first two days shed glycogen, water, and gut content, which are finite and go quickly. After that, loss is mostly fat, which comes off at a slower and steadier rate. The curve flattening is normal and expected.
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Some protein loss occurs, but less than most people expect. Growth hormone rises sharply around 24 hours, which helps preserve lean tissue while your body runs on fat. The calculator estimates protein loss separately so you can see the scale of it.
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Fasts up to 72 hours are commonly done by healthy adults. Beyond about 7 days, electrolyte losses and hormonal suppression become more significant and medical supervision is advised. Fasting is not appropriate during pregnancy, with a history of eating disorders, when underweight, or alongside certain medications and conditions.
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Water, plain black coffee, and unsweetened tea. Electrolytes matter more the longer you fast: sodium, potassium, and magnesium losses accumulate from around 24 hours onward.
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Slowly. Start with warm broth and a small amount of steamed vegetable, chewed well, before anything heavy. A common guide is to refeed for at least half as long as you fasted, keeping electrolytes going throughout.
How This Calculator Compared to My Own 10-Day Fast
I ran a 10-day water fast and measured it with DEXA scans before and after, so I could check this model against real numbers.
The calculator predicted 12.2 lb of total loss, made up of 6.0 lb of fat and 6.2 lb of non-fat tissue. DEXA measured 14.0 lb of total loss, made up of 5.5 lb of fat and 8.3 lb of non-fat tissue.
The fat number, which is the one that matters, came within half a pound. The total was under-predicted, because I lost more water and glycogen than the model assumed. After a 12-day refeed the lean tissue came back and the fat loss stayed, which is exactly the pattern this calculator is built to show you before you start.
The day-by-day diary and data of this fast is here 👉 “10-Day Fast Results: Data, Experience, and Takeaways”.
One caveat: this is a single data point, and my inputs at the time were close to but not identical to the defaults shown on this page.
Interested in IF, OMAD, or calorie deficit weight loss? Check out my other calculator
Share Your Feedback and Fasting Results
Tried the calculator or completed an extended fast yourself? Share your results, observations, or questions. Real-world experiences help improve this model and help others understand what to expect during multi-day fasting.