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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Enter your details
Maximum 30 days for projections
Weight Loss Projections
ExampleExample figures, not your own. Worked for a 35 year old woman of 85 kg and 170 cm over a 5 day fast. Enter your own details and press Calculate.
Total Weight Loss
2.7kg
Over 5 daysFat Loss
0.7kg
Stays off after fastWater/Glycogen
2.0kg
Returns when eatingYour BMR: 1577 kcal/day - this drives your daily fat burn rate of ~0.20 kg/day.
| Day | Water | Fat | Day Total | Cumulative |
|---|---|---|---|---|
| Day 1 | 1.20 kg | 0.06 kg | 1.26 kg | 1.3 kg |
| Day 2 | 0.40 kg | 0.13 kg | 0.53 kg | 1.8 kg |
| Day 3 | 0.30 kg | 0.15 kg | 0.45 kg | 2.3 kg |
| Day 4 | 0.05 kg | 0.19 kg | 0.24 kg | 2.5 kg |
| Day 5 | 0.05 kg | 0.19 kg | 0.24 kg | 2.7 kg |
Medical warning: Water fasting beyond 24-48 hours should be done under medical supervision. Extended fasts carry risks including electrolyte imbalances, muscle wasting, and refeeding syndrome. This calculator is for educational purposes only.
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.
How to use it
- 1Enter your current weight in kilograms
- 2Enter your height in centimetres
- 3Enter your age and select your gender
- 4Enter the planned duration of your water fast in days
- 5Click "Calculate" to see daily weight loss projections
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.
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.