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Intermittent Fasting (IF) Calculator

Calculate eating & fasting windows for 16:8, 18:6, 20:4, OMAD protocols. Track metabolic states, autophagy timing, and weight loss estimates.

HEALTH

Intermittent fasting (IF) calculator for designing an optimal eating and fasting window. Supports 16:8, 18:6, 20:4, OMAD, 5:2, and ADF protocols.

Four tabs: protocol picker (cards with pros and cons), eating-window builder (24-hour visual), metabolic-state timeline (digestion → fat burning → autophagy → ketosis), and progress tracker with weight-loss estimate.

Disclaimer: Consult a doctor before starting IF, especially if you have diabetes, an eating disorder, are pregnant or nursing, or are under 18.

Intermittent Fasting (IF) Calculator

Calculate eating window, metabolic phases, and progress estimates based on popular Intermittent Fasting protocols. Based on research from Dr. Jason Fung and Mark Mattson.

Choose an IF protocol that fits your lifestyle, schedule, and goals. Beginners are strongly advised to start with 16:8.

16:8 (LeanGains)

Beginner
Pros: Easy to follow, flexible schedule, fits skipping breakfast.
Cons: Slower results than stricter protocols.
Suitable for: Beginners, office workers, fitness enthusiasts.

18:6

Intermediate
Pros: Deeper deficit, begins to trigger mild autophagy.
Cons: Narrow eating window, requires calorie planning.
Suitable for: Those already comfortable with 16:8 for 2-4 weeks.

20:4 (Warrior Diet)

Advanced
Pros: Stronger autophagy, maximum fat burning.
Cons: Hard to meet nutrient needs within 4 hours.
Suitable for: Experienced IF practitioners who have adapted.

OMAD (One Meal a Day)

Advanced
Pros: Very time-efficient, deep autophagy, high mental focus.
Cons: Risk of nutrient/calorie deficiency, not for everyone.
Suitable for: Experienced IF practitioners, not for beginners.

5:2

Intermediate
Pros: 5 days normal eating, only 2 days restricted to 500-600 kcal.
Cons: Restricted days feel hard, may overeat on normal days.
Suitable for: Those who struggle with daily fasting, prefer 2 days per week.

Eat-Stop-Eat

Advanced
Pros: Significant calorie deficit, simple: 1-2x 24h fasts per week.
Cons: Hard for beginners, requires mental adaptation.
Suitable for: Those who have graduated from daily protocols.

Alternate Day Fasting (ADF)

Advanced
Pros: Fast weight loss, strong metabolic benefits.
Cons: Hard to sustain long-term, muscle loss risk without enough protein.
Suitable for: Aggressive weight loss goals under supervision.
Consult a doctor before starting Intermittent Fasting if you have diabetes, a history of eating disorders, are pregnant, breastfeeding, a child, or have certain medical conditions. This calculator is educational and does not replace professional medical advice.

Calculator information

How to use this calculator

  1. Pick an IF protocol that fits your level: 16:8 (beginner), 18:6 (intermediate), 20:4 or OMAD (advanced), 5:2 or ADF (alternate-day fasting).
  2. Set your eating-window start time; most studies favor an earlier (morning-to-afternoon) window to align with circadian rhythm.
  3. Enter weight, height, age, sex, and activity level to compute TDEE and calorie deficit.
  4. Review the metabolic-state timeline to see when your body enters fat burning (12 hours), autophagy (16-18 hours), and ketosis (>24 hours).
  5. Log daily weight and calorie intake in the tracker tab to estimate weekly weight loss.
  6. Drink water, plain coffee/tea, and electrolytes during the fast to prevent dehydration and headaches.
  7. Stop IF and consult your doctor if you develop severe dizziness, palpitations, or menstrual disruption.

Calorie Deficit and Weight-Loss Estimate

Weight_loss_per_week (lb) = (TDEE - daily_intake) * 7 / 3500
  • TDEE = BMR * activity factor (Mifflin-St Jeor)
  • BMR (male) = 10W + 6.25H - 5A + 5
  • BMR (female) = 10W + 6.25H - 5A - 161
  • W = weight (kg), H = height (cm), A = age (years)
  • 3500 kcal ~ 1 lb body fat (7700 kcal ~ 1 kg)

An NEJM 2020 study found 16:8 IF reduced body weight ~3% over 12 weeks, comparable to conventional calorie restriction. Added benefits: lower fasting insulin, improved insulin sensitivity, and autophagy that becomes meaningful after 16-18 hours fasting.

Worked example: 30-year-old female, 143 lb (65 kg), 5'3" (160 cm), moderately active, 16:8 protocol

Given:
  • BMR: 10(65) + 6.25(160) - 5(30) - 161 = 1,339 kcal
  • TDEE: 1,339 * 1.55 = 2,075 kcal
  • Target intake: 1,500 kcal (deficit 575)
  • Eating window: 12:00 PM - 8:00 PM
Steps:
  1. Daily deficit = 2,075 - 1,500 = 575 kcal.
  2. Weekly deficit = 575 * 7 = 4,025 kcal.
  3. Weight loss/week = 4,025 / 3,500 = 1.15 lb (0.52 kg).
  4. 12-week projection ~ 14 lb (6.3 kg), about 10% of starting weight.
  5. At hour 12 of fasting (midnight) the body enters fat burning; at hour 16 (4:00 AM) autophagy ramps up.

Result: With a 16:8 schedule and a 575 kcal/day deficit, expect ~1 lb/week (0.5 kg) or about 13 lb (6 kg) in 3 months. Most loss comes from fat, not muscle, if protein is >=1.6 g/kg body weight and strength training is performed 2-3x/week.

Frequently asked questions

Is IF safe for women?
Most healthy adult women can do IF, but hormonal effects differ from men. Aggressive protocols (20:4, OMAD) may disrupt the menstrual cycle, lower T3, and raise cortisol. Start at 14:10 or 16:8 with a morning-to-afternoon window. Avoid IF during pregnancy, breastfeeding, or with a history of eating disorders.
Can I drink coffee while fasting?
Yes. Black coffee, tea, and plain water don't break the fast since they're under 5 kcal. Caffeine actually raises fat oxidation 10-15%. Skip milk, creamer, and caloric sweeteners; non-caloric sweeteners (stevia, sucralose) are generally considered fast-safe, though some studies show a minor insulin response. Add electrolytes (salt, magnesium) when fasting beyond 18 hours.
When does the body enter ketosis during a fast?
Nutritional ketosis (BHB > 0.5 mmol/L) typically requires 24-72 hours of water fasting, depending on glycogen stores, activity, and prior diet. Daily 16:8 IF rarely induces full ketosis, but the body still uses fat as its primary fuel after the 12-hour mark. To reach ketosis faster, combine IF with a low-carb (<50 g/day) diet.
Does IF cause muscle loss?
Risk is minimal if protein intake is sufficient (>=1.6 g/kg body weight) and resistance training continues 2-3x/week. Tinsley et al. (2017) found 20:4 IF preserved muscle mass equal to isocaloric controls in trained subjects. Muscle loss occurs when daily deficits exceed ~1,000 kcal, protein falls below 1 g/kg, or strength training is absent.
What's the difference between autophagy and fat burning?
Fat burning (lipolysis) starts around hour 12 of fasting once liver glycogen is depleted and triglycerides are mobilized for energy. Autophagy is cellular recycling of damaged proteins and organelles, stimulated by fasts of >=16 hours (rodent data) or >=24 hours (human data). Autophagy is thought to underlie IF's anti-aging effects, though human evidence is still limited.

Last updated: May 11, 2026

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