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Fasting for Weight Loss Calculator

Calculate your expected weight loss through intermittent fasting based on your current weight, fasting schedule, and target weight.

Enter your current weight in pounds (lbs)
Enter your target weight in pounds (lbs)
Enter your height in inches
Enter your age in years

How to Use This Calculator

  1. Enter your current weight in pounds
  2. Enter your target weight in pounds
  3. Provide your height, age, and gender
  4. Select your typical activity level
  5. Choose your preferred fasting method
  6. Set your desired weekly weight loss goal
  7. Click Calculate to see your fasting weight loss timeline

Formula Used

BMR = 10 × weight(kg) + 6.25 × height(cm) - 5 × age + s
TDEE = BMR × Activity Factor
Caloric Deficit = TDEE × Fasting Reduction Factor
Weekly Weight Loss = (Caloric Deficit × 7) ÷ 3500

Where:

  • BMR = Basal Metabolic Rate (calories burned at rest)
  • TDEE = Total Daily Energy Expenditure
  • s = Gender factor (+5 for males, -161 for females)
  • Activity Factor = 1.2 (sedentary) to 1.9 (very active)
  • Fasting Reduction Factor = 0.15 (16:8) to 0.35 (OMAD)
  • 3500 = Approximate calories in 1 pound of fat

Example Calculation

Real-World Scenario:

Sarah, a 35-year-old female, wants to lose 20 pounds using the 16:8 intermittent fasting method.

Given:

  • Current Weight = 170 lbs (77.1 kg)
  • Target Weight = 150 lbs (68.0 kg)
  • Height = 65 inches (165 cm)
  • Age = 35 years
  • Gender = Female
  • Activity Level = Light (exercise 1-3 days/week)
  • Fasting Type = 16:8

Calculation:

BMR = 10 × 77.1 + 6.25 × 165 - 5 × 35 - 161 = 1,490 calories/day

TDEE = 1,490 × 1.375 = 2,049 calories/day

Caloric Deficit = 2,049 × 0.15 = 307 calories/day

Weekly Weight Loss = (307 × 7) ÷ 3,500 = 0.6 lbs/week

Result: It will take approximately 33 weeks (about 8 months) to reach the target weight of 150 lbs with 16:8 intermittent fasting.

Why This Calculation Matters

Practical Applications

  • Setting realistic weight loss expectations
  • Choosing the most effective fasting schedule
  • Tracking progress toward weight goals
  • Adjusting fasting approach for optimal results

Key Benefits

  • Personalized timeline based on your body
  • Helps maintain motivation with realistic goals
  • Provides a scientific approach to fasting
  • Reduces risk of unhealthy weight loss practices

Common Mistakes & Tips

Many people expect to lose 5-10 pounds per week, which is unsustainable and unhealthy. A safe rate of weight loss is typically 1-2 pounds per week. Intermittent fasting may lead to slightly faster initial weight loss due to water weight, but the long-term rate will slow to a more sustainable pace. Set realistic goals to maintain motivation and avoid disappointment.

Fasting doesn't give you permission to eat whatever you want during your eating window. Focus on nutrient-dense foods that provide adequate protein, fiber, vitamins, and minerals. Poor nutrition can lead to muscle loss, nutrient deficiencies, and other health issues, even if you're losing weight. The quality of your calories matters as much as the quantity.

As you lose weight, your caloric needs decrease. If you continue with the same fasting schedule and eating patterns, your weight loss will plateau over time. Periodically recalculate your needs using updated weight information, and consider adjusting your fasting approach or eating habits to continue making progress toward your goals.

Frequently Asked Questions

Intermittent fasting is generally safe for most healthy adults, but it's not recommended for certain populations. People who are pregnant or breastfeeding, under 18 years old, have a history of eating disorders, or have certain medical conditions like diabetes should consult with a healthcare professional before starting any fasting regimen. If you're taking medications that need to be taken with food, you'll also need to discuss timing with your doctor.

When done correctly, intermittent fasting can help preserve muscle mass while losing fat. To minimize muscle loss, ensure adequate protein intake during your eating window (typically 0.7-1.0 grams of protein per pound of body weight), incorporate resistance training into your routine, and avoid overly aggressive caloric deficits. Some research suggests that intermittent fasting may actually have muscle-sparing effects compared to continuous calorie restriction.

During fasting periods, you can generally consume zero-calorie beverages without breaking your fast. These include water, black coffee, unsweetened tea, and sparkling water. Be cautious with artificial sweeteners, as some research suggests they may trigger an insulin response in certain individuals. Anything that contains calories, including milk in coffee, sweetened beverages, or alcohol, will break your fast and should be consumed only during your designated eating window.

References & Disclaimer

Medical Disclaimer

This calculator provides estimates based on established formulas but should not replace professional medical advice. Weight loss results vary significantly between individuals. Consult with a healthcare provider before starting any new diet or exercise program, especially if you have pre-existing health conditions. This calculator is not suitable for pregnant or breastfeeding women, individuals under 18, or those with a history of eating disorders.

References

Accuracy Notice

This calculator uses standard formulas to estimate weight loss through intermittent fasting. Individual results may vary based on genetics, metabolism, adherence to the fasting schedule, dietary choices during eating windows, and other factors. The estimates provided are for informational purposes only and should not be considered medical advice. Weight loss of more than 2 pounds per week may not be sustainable or healthy for most individuals.

About the Author

Kumaravel Madhavan

Web developer and data researcher creating accurate, easy-to-use calculators across health, finance, education, and construction and more. Works with subject-matter experts to ensure formulas meet trusted standards like WHO, NIH, and ISO.

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health body-composition-weight-management fasting weight loss medical body