Calculate exactly how much weight you will lose fasting — broken down by fat loss, water weight, and glycogen. Enter your body weight, height, age, body fat percentage, and fast duration (1 to 14 days) to get a personalised day-by-day projection.
A 72-hour fast typically produces 1.5–3 kg (3–7 lbs) of total scale weight loss. However, a large portion — often 50–70% — is water and glycogen rather than fat. True fat loss over 3 days is approximately 0.4–0.8 kg for most people.
Day 1 of a fast depletes liver and muscle glycogen. Each gram of glycogen binds 3–4 grams of water, so glycogen depletion alone accounts for 1–2 kg of rapid early weight loss. This weight returns within 24–48 hours of refeeding. From Day 2 onward, as ketosis deepens, the proportion of true fat loss rises to 65–92% of daily energy expenditure.
Walking during fasting is highly effective because low-intensity aerobic exercise in a fasted state draws predominantly from fat stores. 60–120 minutes of brisk walking per day can increase total fat loss by 15–25% compared to rest. Weightlifting and calisthenics provide a different benefit: they activate mTOR signaling, which reduces muscle protein breakdown by approximately 15–25%, preserving lean mass while the fast progresses.
This calculator uses the Mifflin-St Jeor equation for basal metabolic rate, Cahill starvation-phase metabolism research (New England Journal of Medicine, 1970), the Ainsworth MET Compendium (2011) for exercise caloric expenditure, Biolo et al. (1995) for resistance training protein turnover, Laurens et al. (2021) for 10-day fasting body composition data, and Nature Communications (2025) for 7-day fasting lean mass preservation.