Water Fast Weight Loss Calculator
This calculator projects how much weight you might lose during a water fast, broken down day by day. It separates water weight loss from actual fat loss based on your Basal Metabolic Rate, so you understand what to expect when the fast ends and you rehydrate.
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Maximum 30 days for projections
How It Works
During a water fast, Day 1 weight loss is primarily water and glycogen (typically 1-2 kg). Days 2-3 involve a mix of water, glycogen, and increasing fat burning (approximately 0.5 kg/day). From Day 4 onward, the body is in full ketosis and burns roughly 0.3-0.4 kg of fat per day, based on your BMR. Fat loss is calculated using the principle that burning 1 kg of body fat requires a deficit of approximately 7,700 kcal (32,200 kJ). Your personal BMR determines your daily energy expenditure and therefore your rate of fat loss.
Quick Reference
| Fast Duration | Total Loss (est.) | Fat Loss | Water/Glycogen |
|---|---|---|---|
| 1 day | 1.2 – 1.8 kg | ~0.1 kg | ~1.2 kg |
| 2 days | 2.0 – 2.8 kg | ~0.3 kg | ~1.8 kg |
| 3 days | 2.5 – 3.5 kg | ~0.5 kg | ~2.2 kg |
| 5 days | 3.5 – 4.5 kg | ~1.1 kg | ~2.5 kg |
| 7 days | 4.0 – 5.5 kg | ~1.7 kg | ~2.6 kg |
| 10 days | 5.0 – 7.0 kg | ~2.7 kg | ~2.8 kg |
| 14 days | 6.0 – 8.5 kg | ~3.9 kg | ~2.9 kg |
| 21 days | 8.0 – 11 kg | ~5.8 kg | ~3.0 kg |
Estimates based on average BMR of 1,600 kcal/day. Water weight returns when eating resumes. Extended fasts require medical supervision.
Frequently Asked Questions
- How much weight will I lose on a 3-day water fast?
- A typical 3-day water fast results in 2-4 kg of total weight loss. However, about 60-70% of this is water and glycogen that returns when you eat again. Actual fat loss over 3 days is roughly 0.5-1.0 kg depending on your metabolic rate.
- Is water fasting safe?
- Short water fasts (24-72 hours) are generally safe for healthy adults. Extended fasts beyond 3 days carry risks including electrolyte imbalances, muscle loss, and refeeding syndrome. Medical supervision is strongly recommended for fasts longer than 48 hours. Do not water fast if you are pregnant, underweight, or have a medical condition.
- Will I lose muscle during a water fast?
- Some muscle loss occurs during any fast, especially in the first 2-3 days before the body fully adapts to burning fat. Studies suggest approximately 25% of weight lost during extended fasting can be lean mass. Shorter fasts and adequate protein intake before and after minimise muscle loss.
- How much weight stays off after a water fast?
- After rehydrating and eating normally, most people regain 1-2 kg of water weight within 2-3 days. The fat loss component (roughly 30-40% of total weight lost) tends to remain off if you return to a balanced diet at maintenance calories.
How This Is Calculated
- BMR
- = Basal Metabolic Rate — calories your body burns at rest
- 7,700
- = approximate kcal stored in 1 kg of body fat
Day 1 weight loss is mostly water and glycogen stores. True fat loss begins around day 2-3 and stabilises from day 4. Most of the water weight returns within 2-3 days of eating. This calculator is for educational purposes — extended fasting should be supervised by a healthcare professional.
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Disclaimer
This calculator provides estimates for informational purposes only and should not be considered medical advice. Individual results may vary based on factors not captured by these formulas. Consult a healthcare professional before making changes to your diet or exercise routine, especially if you have any existing health conditions.