How much weight will I lose fasting?
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Medical Disclaimer: This tool provides general educational estimates. Always consult your prescribing physician or healthcare provider before making medication changes or interpreting results from population-based models.
Frequently Asked Questions
Most people lose 4-8 pounds (2-4 kg) in the first month. The first week includes 1-3 pounds of water weight from glycogen depletion. Actual fat loss averages 1-2 pounds per week depending on your deficit, activity level, and fasting protocol.
Yes. 16:8 (skipping breakfast) is the most sustainable protocol and typically produces 0.5-1 pound of fat loss per week. It works by naturally shortening your eating window, creating a caloric deficit without counting calories.
Minimal muscle loss occurs when you maintain adequate protein (1.6-2.2g per kg body weight) and do resistance training 2-3 times per week. Extended fasts beyond 48 hours carry higher muscle loss risk.
Yes. The first 1-3 pounds are primarily water from glycogen depletion. Each gram of glycogen binds 3-4 grams of water. This water weight returns when you eat carbs again. True fat loss begins 12-36 hours into a fast.
Longer fasts create larger daily deficits, but the best protocol is the one you can sustain. 16:8 produces reliable long-term results because it's easy to maintain. OMAD and 48-hour fasts lose more per session but have higher dropout rates.
Sources
- de Cabo R, Mattson MP. Effects of Intermittent Fasting on Health, Aging, and Disease. N Engl J Med. 2019;381(26):2541-2551. Link
- Varady KA, Cienfuegos S, Ezpeleta M, Gabel K. Clinical application of intermittent fasting for weight loss. Nat Rev Endocrinol. 2022;18(5):309-321. Link
- Mifflin MD, St Jeor ST, et al. A new predictive equation for resting energy expenditure in healthy individuals. Am J Clin Nutr. 1990;51(2):241-247. Link
- Anton SD, Moehl K, Donahoo WT, et al. Flipping the Metabolic Switch. Obesity. 2018;26(2):254-268. Link
- Tinsley GM, La Bounty PM. Effects of intermittent fasting on body composition and clinical health markers in humans. Nutr Rev. 2015;73(10):661-674. Link
Methodology
This calculator estimates weight loss during intermittent fasting using the Mifflin-St Jeor equation for BMR. Males: BMR = (10 × kg) + (6.25 × cm) - (5 × age) + 5. Females: BMR = (10 × kg) + (6.25 × cm) - (5 × age) - 161.
TDEE is BMR multiplied by an activity factor (1.2-1.9). For daily protocols (16:8, 20:4, OMAD), the caloric deficit assumes ~70% of normal hourly caloric intake during the eating window. Fat loss uses the standard 7,700 kcal per kg approximation. Water weight from glycogen depletion is estimated separately.
Intermittent fasting weight loss depends on your fasting protocol, body weight, and activity level. A 170-pound person doing 16:8 fasting can expect to lose approximately 1-2 pounds per week, or 4-8 pounds in the first month. The first week includes 1-3 pounds of water weight from glycogen depletion. The Mifflin-St Jeor equation is used to estimate BMR, multiplied by an activity factor for TDEE. Fat loss is calculated at 3,500 calories per pound of body fat.
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