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Fasting Calculator

Plan your intermittent fasting schedule. See your eating window, track which metabolic phase you\u2019re in, and get goal-specific tips.

Disclaimer: This calculator is for informational purposes only. Consult a healthcare provider before starting any fasting regimen.

Fast 16h, eat 8h

When did you finish your last meal?

Select your protocol and last meal time, then click Calculate to see your fasting schedule.

How Intermittent Fasting Works

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. It’s not a diet in the conventional sense but rather a timed approach to meals that can improve metabolic health, support weight loss, and promote cellular repair.

Popular Fasting Protocols

  • 16:8 — Fast 16 hours, eat within an 8-hour window. The most popular and beginner-friendly protocol.
  • 18:6 — Fast 18 hours, eat within 6 hours. A step up from 16:8 for those comfortable with longer fasts.
  • 20:4 — Fast 20 hours, eat within 4 hours. Also known as the "Warrior Diet" approach.
  • OMAD (23:1) — One Meal A Day. Eat one large meal within a 1-hour window.
  • 5:2 — Eat normally 5 days per week, restrict calories to 500–600 on 2 non-consecutive days.

Metabolic Phases During Fasting

Your body goes through distinct metabolic phases during a fast:

  • Fed State (0–4h): Your body digests and absorbs nutrients from your last meal.
  • Early Fasting (4–8h): Blood sugar and insulin levels begin to drop.
  • Fat Burning (8–16h): The body starts relying on stored fat for energy as glycogen depletes.
  • Ketosis (16–24h): Ketone production increases as the body fully shifts to fat metabolism.
  • Deep Autophagy (24–48h): Cellular cleanup processes ramp up, recycling damaged proteins and organelles.

Understanding these phases helps you align your fasting protocol with your health goals. For weight loss, reaching the fat-burning phase (8–16h) regularly is key. For autophagy benefits, occasional extended fasts beyond 24 hours may be beneficial.

Frequently asked questions

The 16:8 protocol is widely recommended for beginners. You fast for 16 hours (including sleep) and eat within an 8-hour window. Most people skip breakfast and eat from noon to 8 PM. It’s sustainable, doesn’t require calorie counting, and most people adapt within a week.

Yes! Water, black coffee, plain tea (no sugar or milk), and sparkling water are all fine during your fasting window. Staying hydrated is essential. Avoid anything with calories, sweeteners, or cream, as these can trigger an insulin response and break your fast.

Most people begin entering ketosis after 12–16 hours of fasting, as the body depletes glycogen stores and shifts to burning fat for fuel. Full ketosis typically occurs around 18–24 hours. Regular fasting practice can help your body become more efficient at switching to fat-burning mode.

Yes, studies show 16:8 intermittent fasting can be effective for weight loss, primarily by naturally reducing calorie intake and improving metabolic markers. However, what you eat during your window still matters. Combining fasting with a balanced diet and regular exercise produces the best results.

Anything with calories breaks a fast — this includes food, sugary drinks, juice, milk, cream, and most supplements. Black coffee, plain tea, water, and electrolytes (without sugar) generally do not break a fast. Some purists also avoid artificial sweeteners, though research is mixed on their insulin impact.

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