Use case · Educational model · Updated August 4, 2026
How much weight will I lose on a water fast?
Direct answer
On The Calcs model, a 70 kg male, age 30, 3-day water fast loses about 2.7 kg on the scale (2.1 kg water/glycogen + 0.6 kg fat from the calorie deficit). Day 1–2 is mostly water; fat loss accrues from BMR-based burn. Not medical advice.
Citeable estimate table
| Duration | Profile | Total | Water | Fat (model) |
|---|---|---|---|---|
| 3 days | 70 kg male, 30 | 2.7 kg | 2.1 kg | 0.6 kg |
| 3 days | 65 kg female, 35 | 2.45 kg | 1.95 kg | 0.5 kg |
| 3 days | 154 lb male, 30 | 11.67 lb | 10.19 lb | 1.48 lb |
| 5 days | 75 kg male, 35 | 3.19 kg | 2.25 kg | 0.94 kg |
| 7 days | 70 kg male, 30 | 3.35 kg | 2.1 kg | 1.25 kg |
Model notes: Mifflin-St Jeor BMR; fasting multiplier 0.95 (≤3 days) or 0.85 (>3 days); water component ≈ 3% of body weight for ≤7-day runs; fat from kcal ÷ 7700. Individual results vary.
Safety limits (read this)
- Not for pregnancy, eating disorders, type 1 diabetes, or without clinician clearance if medicated.
- Electrolytes matter after ~48 hours — plain water alone is not always enough.
- 5–7+ day fasts commonly need medical oversight; refeeding syndrome is a real risk.
- Scale rebound when you eat again is normal (glycogen + water).
Use the free calculator
Enter your weight, height, age, sex, and dates: Water fast calculator. Related: intermittent fasting window.