Water Fasting Calculator.
Planning a multi-day water fast? Estimate how much weight you'll lose over 24, 48, or 120+ hours, and crucially how much of it is real fat versus water and glycogen that returns the moment you eat carbs again.
Frequently Asked Questions
How much weight will I lose water fasting?+
Most of what the scale shows in the first day or two is water and glycogen, not fat. Each gram of stored glycogen holds roughly three grams of water with it, so several pounds can disappear before meaningful fat loss begins. This calculator separates the two, projecting fat loss and water loss for 24, 36, 48, 72 and 120-hour fasts so you can see which is which.
How much of the weight comes back afterwards?+
The water portion, almost entirely, and quickly: usually within a day or two of eating normally, as glycogen refills and brings its water back. That rebound is not failure and it is not fat regain; it is the reason fasted weigh-ins overstate progress so dramatically.
Is a longer fast proportionally better?+
No. The rate of fat loss does not accelerate with duration, while the risks do. Beyond about 72 hours, electrolyte disturbance and loss of lean tissue become the limiting factors rather than anything to do with fat, which is why extended fasts in clinical settings are supervised.
Will I lose muscle?+
Some, and more the longer the fast runs. The body protects lean tissue reasonably well in early fasting by shifting to fat and ketones for fuel, but protein breakdown rises as the fast extends. Resistance training in the weeks around a fast, and adequate protein when you resume eating, both blunt it.
Who should not water fast?+
Anyone pregnant or breastfeeding, anyone with type 1 diabetes, anyone with a history of eating disorders, and anyone on medication that requires food or that adjusts blood glucose or blood pressure. If you take any prescription medication at all, this is a conversation to have with a clinician before starting rather than after.
What is the difference between water fasting and intermittent fasting?+
Duration and intent. Intermittent fasting means a daily eating window (16:8 and similar patterns) that you repeat indefinitely. A water fast is a single continuous stretch of 24 hours or more with nothing but water, and is not something to run as a routine.
Why the scale drops so fast, and why most of it isn't fat.
In the first day or two of a water fast, the scale can fall 1–2 kg. It feels dramatic, but very little of it is fat. Your body stores carbohydrate as glycogen in the liver and muscles, and every gram of glycogen is held with roughly 3 grams of water. As you burn through those glycogen stores in the first 24–48 hours, you shed the water bound to them. That weight returns almost as quickly once you eat carbohydrate again, which is why this calculator separates the two.
Real fat loss is slower and steadier. It is capped by how much energy your body actually burns while resting: roughly 1 kcal per kilogram of body weight per hour, a little higher with light activity. Divide the total energy burned over the fast by ~7,700 kcal (the energy in a kilogram of body fat) and you get the honest fat number. For most people that is a few hundred grams a day, meaningful over a long fast, but far below the total scale drop.
The stages of an extended fast.
Hours 12–18: liver glycogen runs low, insulin falls, and the glycogen water is released. Hours 18–24: with glycogen depleted, the liver ramps ketone production and the body shifts toward fat as fuel. Around 48–72 hours: autophagy (the cell's recycling of damaged components) increases meaningfully as insulin and the growth-signaling protein mTOR stay suppressed. Beyond 72 hours you are in a deep fasting state with steady fat oxidation.
These timings are approximate and shift with your last meal's size, your activity, and how metabolically fat-adapted you already are. If you want to actually measure where you are (ketones versus glucose), the GKI is the tool for that.
Fasting safely, and who should not.
Extended water fasts stress your electrolytes. Sodium, potassium, and magnesium fall as insulin drops and the kidneys excrete more, the cause of most "keto flu" and fasting fatigue. Beyond 48 hours especially, electrolyte replacement and careful refeeding matter, and refeeding syndrome is a real risk after long fasts.
This is not for everyone. If you are pregnant or breastfeeding, underweight, diabetic (especially on insulin or SGLT2 inhibitors), have a history of eating disorders, or take blood pressure or blood sugar medication, do not run a multi-day fast without medical supervision. This calculator is an educational estimate, not medical advice.
- Kreitzman SN, Coxon AY, Szaz KF. (1992). Glycogen storage: illusions of easy weight loss, excessive weight regain, and distortions in estimates of body composition. Am J Clin Nutr, 56(1 Suppl), 292S–293S.
- Anton SD, Moehl K, Donahoo WT, et al. (2018). Flipping the metabolic switch: understanding and applying the health benefits of fasting. Obesity, 26(2), 254–268.
For education, not medical advice. Results are estimates, not a diagnosis. Discuss any abnormal value or health concern with a qualified clinician.
50 High-Protein Snacks. Free.
Drop your email. We'll send the PDF straight to your downloads and add you to the weekly research drop. Unsubscribe anytime.