Intermittent Fasting Calculator

The most popular methods are: 16:8 (16 hours fasting, 8 hours eating)—best for beginners, allows two to three meals daily. 18:6 (18 hours fasting, 6 hours eating)—intermediate level, typically two meals. 20:4 (20 hours fasting, 4 hours eating)—advanced, often one main meal plus snack. OMAD (One Meal A Day)—23:1 fasting, single large meal. Start with 16:8 and gradually extend fasting windows as your body adapts. Listen to your energy levels and hunger signals to find your optimal schedule.

Your body begins transitioning to fat burning 8-12 hours after your last meal when liver glycogen (stored glucose) depletes. True ketosis—where fat becomes the primary fuel—typically starts around 12-16 hours of fasting. By 18-24 hours, fat burning is fully engaged. This is why 16:8 fasting is effective: you spend 4-8 hours in fat-burning mode daily. Eating high-carb meals extends the time to enter ketosis, while low-carb eating before fasting accelerates the transition.

Autophagy is your body's cellular cleanup process—cells break down and recycle damaged components, proteins, and organelles. Research suggests autophagy begins around 16-18 hours of fasting and significantly increases by 24-48 hours. Deeper autophagy and cellular regeneration occur with extended fasts (36-72 hours), though these require medical supervision. Even 16:8 fasting triggers some autophagy, and regular intermittent fasting provides cumulative benefits for cellular health and longevity.

Zero-calorie beverages don't break your fast: Water (plain or sparkling), black coffee (no sugar/cream), plain tea (green, black, herbal), and electrolytes without sweeteners. These may actually enhance fasting benefits—coffee boosts ketosis and autophagy. Avoid: anything with calories, artificial sweeteners (may trigger insulin), cream, milk, or sugar. Some purists avoid flavored teas too. Bone broth (under 50 calories) is sometimes allowed for extended fasts but technically breaks a strict fast.

Align your eating window with your natural energy and social patterns: Morning eaters (6am-2pm): Good for early risers, aligns with circadian rhythm, may improve sleep. Midday window (11am-7pm): Most popular, allows lunch and dinner, works for social eating. Late window (2pm-10pm): Suits night owls, allows dinner with family, but may affect sleep quality. Consider: work schedule, workout timing (eat within 2 hours post-workout), social commitments, and hunger patterns. Consistency matters more than the specific hours—pick times you can maintain long-term.