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Kalcufy

Intermittent Fasting Calculator

Plan your fasting schedule, calculate daily calories and macros, and see your estimated results.

Reference table

AgeDaily CaloriesBasal Metabolic RateTotal Daily Energy
202,005 kcal1,617 kcal2,506 kcal
251,974 kcal1,592 kcal2,468 kcal
301,943 kcal1,567 kcal2,429 kcal
401,881 kcal1,517 kcal2,351 kcal
501,819 kcal1,467 kcal2,274 kcal
601,757 kcal1,417 kcal2,196 kcal
701,695 kcal1,367 kcal2,119 kcal
751,664 kcal1,342 kcal2,080 kcal
801,633 kcal1,317 kcal2,041 kcal

Fasting Tips

  • Stay hydrated — drink water, black coffee, and plain tea during your fast. Zero-calorie beverages don't break your fast.
  • Start with a shorter fast like 12:12 or 14:10 and gradually increase. Your body needs 1-2 weeks to adapt.
  • Break your fast with protein and healthy fats first, then add carbs. This prevents blood sugar spikes.
  • Schedule workouts near the end of your fast or right after eating for best performance and recovery.

What Is Intermittent Fasting?

Intermittent fasting (IF) is an eating pattern that cycles between periods of fasting and eating. Unlike traditional diets that focus on what you eat, IF focuses on when you eat. The most popular method is 16:8, where you fast for 16 hours and eat within an 8-hour window. Research published in the New England Journal of Medicine (2019) shows that intermittent fasting can improve metabolic health, reduce inflammation, and promote cellular repair through a process called autophagy. During fasting, your body shifts from using glucose as its primary fuel to burning stored fat, typically entering a state of ketosis after 12-16 hours without food.

How This Calculator Works

This calculator uses the Mifflin-St Jeor equation to estimate your Basal Metabolic Rate (BMR) based on your gender, age, weight, and height. It then multiplies your BMR by an activity factor to determine your Total Daily Energy Expenditure (TDEE). Based on your goal — lose weight, maintain, or body recomposition — it adjusts your daily calorie target. For weight loss, it applies a 20% caloric deficit; for recomposition, a 10% deficit with higher protein. Finally, it distributes your daily macronutrients (protein, carbs, fat) across your chosen number of meals within your eating window, giving you a complete per-meal breakdown.

Science-Backed Benefits of IF

  • Fat burning increases significantly after 12+ hours of fasting as the body depletes glycogen stores and switches to fat oxidation (ketosis)
  • Autophagy — cellular cleanup and repair — activates during extended fasting periods, removing damaged proteins and organelles
  • Insulin sensitivity improves, reducing the risk of type 2 diabetes. Studies show fasting can lower fasting insulin by 20-31%
  • Human Growth Hormone (HGH) levels can increase up to 5x during fasting, supporting muscle preservation and fat loss
  • Inflammation markers (CRP, IL-6) decrease with regular fasting, potentially reducing risk of chronic diseases
  • Warning: IF is not recommended for pregnant or breastfeeding women, people with eating disorders, type 1 diabetes, or children under 18

Fasting Methods Explained

  • 12:12 — Equal split between fasting and eating. Ideal for beginners since overnight sleep covers most of the fast
  • 14:10 — Slightly longer fast. Skip late-night snacking and delay breakfast by 1-2 hours for easy compliance
  • 16:8 — The most researched and popular method. Skip breakfast, eat from noon to 8 PM. Sustainable long-term
  • 18:6 — Intermediate protocol. Two larger meals with better autophagy activation and deeper ketosis
  • 20:4 (Warrior) — Advanced method with a 4-hour eating window. Usually one main meal plus a small snack
  • 5:2 — Eat normally 5 days per week, restrict to 500-600 calories on 2 non-consecutive days. More flexible schedule

Calculation Examples

Step-by-step examples of how fasting calories are calculated

Male, 30, 180 lbs, 5'10", Moderate Activity, 16:8, Lose Weight

  1. BMR = 10 × 81.6kg + 6.25 × 177.8cm − 5 × 30 + 5 = 1,778 cal
  2. TDEE = 1,778 × 1.55 (moderate) = 2,756 cal
  3. Weight loss target = 2,756 × 0.80 = 2,205 cal/day
  4. Protein = 180g (1g per lb), Fat = 61g (25%), Carbs = 208g (remaining)
  5. 3 meals → 735 cal/meal (60g protein, 20g fat, 69g carbs each)

Eat 2,205 cal/day from 12 PM to 8 PM — lose ~1.1 lbs/week

Female, 28, 140 lbs, 5'5", Light Activity, 14:10, Maintain

  1. BMR = 10 × 63.5kg + 6.25 × 165.1cm − 5 × 28 − 161 = 1,368 cal
  2. TDEE = 1,368 × 1.375 (light) = 1,881 cal
  3. Maintenance target = 1,881 cal/day (no deficit)
  4. Protein = 112g (0.8g per lb), Fat = 63g (30%), Carbs = 218g
  5. 3 meals → 627 cal/meal in a 10-hour eating window

Eat 1,881 cal/day from 9 AM to 7 PM — maintain current weight

Frequently Asked Questions

Does coffee break my fast?

Black coffee, plain tea, and water do not break your fast. They contain virtually zero calories and may even enhance fasting benefits by boosting metabolism. However, adding cream, sugar, milk, or artificial sweeteners can trigger an insulin response and technically break your fast. If you need flavor, a small squeeze of lemon in water is generally acceptable.

Which fasting method is best for beginners?

The 16:8 method is the most popular starting point because it's simple and sustainable — you essentially skip breakfast and stop eating after dinner. If 16 hours feels too long, start with 12:12 or 14:10 for the first 1-2 weeks, then gradually extend your fasting window. The key is consistency, not intensity.

Will I lose muscle while fasting?

Not if you eat enough protein and do resistance training. Intermittent fasting actually increases Human Growth Hormone (HGH) levels, which helps preserve muscle. Aim for 0.8-1.2g of protein per pound of body weight, prioritize protein-rich foods when breaking your fast, and maintain your strength training routine. Most research shows IF preserves lean mass better than continuous calorie restriction.

Can women do intermittent fasting safely?

Yes, but women may need a gentler approach. Some research suggests that extended fasting (18+ hours) can affect female hormones, particularly if you're already lean or under stress. Women are often advised to start with 14:10 or 16:8 and avoid fasting on consecutive days initially. Pregnant or breastfeeding women should not practice IF. If you notice menstrual irregularities, reduce your fasting window.

How long does it take to see results from intermittent fasting?

Most people notice initial changes within 2-4 weeks, including reduced bloating and improved energy. Significant fat loss typically becomes visible at 4-8 weeks with consistent fasting and appropriate calorie intake. Weight loss averages 0.5-2 lbs per week depending on your caloric deficit. The metabolic benefits like improved insulin sensitivity can be measured within 2-3 weeks.

Should I exercise while fasting?

Yes, moderate exercise during fasting is safe and can enhance fat burning. Light cardio, yoga, and moderate strength training work well in a fasted state. For intense workouts, consider scheduling them near the end of your fast or within your eating window for better performance. Always listen to your body — if you feel dizzy or weak, eat something and adjust your schedule.

What should I eat to break my fast?

Break your fast with easily digestible, nutrient-dense foods. Start with protein (eggs, chicken, fish, Greek yogurt) and healthy fats (avocado, nuts, olive oil), then add complex carbohydrates. Avoid breaking your fast with sugary foods, processed carbs, or large portions — this can cause digestive discomfort and blood sugar spikes. A balanced meal with 30-40g of protein is ideal.

Is the 5:2 method effective for weight loss?

Yes, the 5:2 method is effective and may be easier for people who prefer not to fast daily. On restricted days, you eat 500-600 calories (typically as one or two small meals). Research from the University of Illinois shows similar weight loss results between 5:2 and daily calorie restriction. The advantage is psychological — knowing you can eat normally most days improves adherence.

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