Fasting Weight Loss Calculator
Estimate your potential fat loss based on fasting duration and metabolic rate.
Calculation Results
Estimated fat loss: 0.00 lbs
Total Calorie Deficit: 0 kcal
Your Daily TDEE: 0 kcal/day
*Results assume a complete water fast with zero caloric intake. Actual results may vary based on metabolism, hydration levels, and electrolyte balance.
How the Fasting Weight Loss Calculator Works
This calculator estimates weight loss during a fast by calculating your Total Daily Energy Expenditure (TDEE) and applying it to the duration of your fast. When you abstain from food, your body must burn stored energy (mostly fat and glycogen) to maintain biological functions.
The Math Behind Fasting
The calculation uses the Mifflin-St Jeor Equation to determine your Basal Metabolic Rate (BMR), which is the energy required to keep your body functioning at rest. We then multiply this by your activity level to get your TDEE.
- For Men: BMR = (10 × weight in kg) + (6.25 × height in cm) – (5 × age in years) + 5
- For Women: BMR = (10 × weight in kg) + (6.25 × height in cm) – (5 × age in years) – 161
Because one kilogram of body fat contains approximately 7,700 calories (or 3,500 calories per pound), the formula for fasting weight loss is:
Weight Loss (kg) = (TDEE / 24 hours) × Total Fasting Hours ÷ 7,700
Realistic Expectations vs. Water Weight
It is important to note that the scale may show a much larger drop than this calculator suggests. This is due to water weight. During the first 24-48 hours of fasting, your body uses up its glycogen stores (stored carbohydrates). Each gram of glycogen is bound to roughly 3-4 grams of water. As glycogen is depleted, that water is released, leading to rapid initial weight loss that is not fat.
Example Calculation
If a 30-year-old male weighs 90kg, is 180cm tall, and has a sedentary lifestyle:
- BMR: (10 × 90) + (6.25 × 180) – (5 × 30) + 5 = 1,880 kcal
- TDEE: 1,880 × 1.2 = 2,256 kcal per day
- 72-Hour Fast: (2,256 / 24) × 72 = 6,768 total calorie deficit.
- Fat Loss: 6,768 ÷ 7,700 = 0.88 kg (approx. 1.9 lbs).
Safety Warning: Always consult with a medical professional before attempting extended fasts (over 24 hours), especially if you have underlying health conditions, are pregnant, or are taking medication.