How-to · Updated October 2, 2026
When does ketosis start on a water fast?
Direct answer
The calculator calls 0–12 hours glycogen, 12–24 hours entering ketosis, and 24 hours or more likely in ketosis. A 24-hour fast is labeled Likely in ketosis. On a 3-day example the scale still moves mostly from water: 2.1 kg water and 0.6 kg fat.
The four windows
| Time since the last meal | What the model says |
|---|---|
| 0–12 hours | Glycogen stores still fueling brain/muscles |
| 12–24 hours | Entering ketosis — liver glycogen falling |
| 24+ hours | Likely in ketosis — fat oxidation rising |
| Day 3+ | Metabolic rate may drop ~10–15% (adaptive) |
A default 72-hour fast is in the “likely in ketosis” row. Mild fatigue, headache, or acetone breath can show up; those symptoms are not a measure of fat lost.
Ketosis and the 3-day scale
Default profile: 70 kg, 170 cm, man, age 30. Status: Likely in ketosis. Modeled loss: 2.7 kg (2.1 kg water, 0.6 kg fat) from about 4,610 kcal. The water piece is 3% of body weight for fasts of 7 days or less. Fat is daily fasting calories × days ÷ 7,700.
Limits
- Educational estimate, not medical advice and not a reason to extend a fast. Ketone strips and a lab are different from this label.
- Skip unsupervised fasting with diabetes on insulin or sulfonylureas, in pregnancy, with an eating disorder, with kidney disease, under 18, or on medicines that depend on food.
- Stop for chest pain, fainting, confusion, or persistent vomiting. Fasts of several days need a clinician because of electrolytes and refeeding.
Use the calculator
The scale table is in how much weight a water fast loses. The fluid budget is in how much water to drink. Run your dates in the water fast calculator.