Water Fasting Calculator.

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.

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.

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.