Intermittent Fasting Window Calculator

Use this calculator to plan your intermittent fasting schedule. Choose from popular protocols like 16:8, 18:6, 20:4 (Warrior Diet), or OMAD, then set your preferred first meal time to see your complete eating and fasting windows. The calculator also estimates when you'll reach key metabolic states like fat burning and ketosis.

What Is Intermittent Fasting?

Intermittent fasting (IF) is an eating pattern that cycles between periods of eating and fasting. Unlike traditional diets that focus on what you eat, intermittent fasting focuses on when you eat. During fasting periods, you consume no calories (water, black coffee, and plain tea are typically allowed).

Research suggests intermittent fasting may offer benefits beyond simple calorie restriction, including improved insulin sensitivity, enhanced cellular repair processes, and favorable changes in hormones related to fat burning.

Fasting Protocols Explained

16:8 (Leangains)

The most popular and beginner-friendly protocol. You fast for 16 hours and eat within an 8-hour window each day. This typically means skipping breakfast and eating from noon to 8 PM, though you can adjust the timing to fit your schedule.

18:6

A moderate step up from 16:8, extending the fasting period to 18 hours with a 6-hour eating window. This protocol allows deeper entry into the fasted state while still permitting two substantial meals.

20:4 (Warrior Diet)

Based on Ori Hofmekler's Warrior Diet, this protocol involves a 20-hour fasting period with a 4-hour eating window. The original protocol allowed small amounts of raw vegetables and protein during the fasting period, though many practitioners follow a stricter water-only fast.

OMAD (One Meal a Day)

The most aggressive daily fasting protocol: 23 hours of fasting with a 1-hour eating window for a single meal. This requires careful attention to nutrition to ensure adequate intake of essential nutrients within the limited eating period.

5:2 (Fast Diet)

Unlike the time-restricted protocols above, the 5:2 method involves eating normally for five days per week while restricting calories to 500–600 on two non-consecutive days. This isn't strictly time-restricted fasting but rather periodic calorie restriction.

What Happens During Fasting: Metabolic Milestones

As you progress through a fast, your body undergoes several metabolic shifts. The timing of these transitions varies based on individual factors including metabolic rate, glycogen stores, activity level, and the composition of your last meal.

0–4 Hours: Fed State

Your body is actively digesting food, absorbing nutrients, and storing energy. Blood sugar rises, insulin is released, and excess glucose is stored as glycogen in the liver and muscles, or converted to fat for long-term storage.

4–8 Hours: Post-Absorptive State

Digestion is complete and blood sugar begins to normalize. Insulin levels drop, and your body starts accessing glycogen stores for energy. This transition period prepares your body for fat-burning mode.

12–14 Hours: Fat Burning Begins

With glycogen stores partially depleted, your body increasingly turns to fat oxidation for energy. Lipolysis (the breakdown of fat) accelerates, and free fatty acids are released into the bloodstream. This is when many people report increased mental clarity.

16–24 Hours: Ketosis Onset

As glucose becomes scarce, the liver begins producing ketone bodies from fatty acids. These ketones serve as an alternative fuel source, particularly for the brain. The degree of ketosis depends on individual metabolism, activity level, and glycogen depletion.

24+ Hours: Autophagy Activation

Autophagy ("self-eating") is a cellular cleanup process where cells break down and recycle damaged components. While autophagy occurs continuously at low levels, fasting significantly upregulates this process. Research in humans is limited, but animal studies suggest meaningful autophagy enhancement begins around 24–48 hours of fasting.

Tips for Successful Intermittent Fasting

Starting Out

If you're new to fasting, start with the 16:8 protocol. Many people find this as simple as skipping breakfast, since much of the fasting period occurs during sleep. Allow 2–4 weeks for your body to adapt before considering longer fasting windows.

Hydration

Stay well-hydrated during fasting periods. Water, black coffee, and plain tea are calorie-free and won't break your fast. Some practitioners add a pinch of salt to water to maintain electrolyte balance during longer fasts.

Breaking Your Fast

After longer fasts (18+ hours), consider breaking your fast with easily digestible foods rather than a large, heavy meal. Good options include bone broth, eggs, or small portions of protein with vegetables. This helps prevent digestive discomfort.

Nutrition Quality

Intermittent fasting isn't a free pass to eat anything during your eating window. Focus on whole foods, adequate protein (especially important with limited meals), healthy fats, and plenty of vegetables. Nutrient density becomes more important when you have fewer eating opportunities.

Exercise Timing

Many people exercise successfully in a fasted state, particularly for low-to-moderate intensity activities. For intense strength training, you may prefer to schedule workouts near your eating window to optimize performance and recovery. Experiment to find what works best for you.

Who Should Avoid Intermittent Fasting?

Intermittent fasting isn't appropriate for everyone. Consult a healthcare provider before starting if you're pregnant or breastfeeding, have a history of eating disorders, have diabetes or blood sugar regulation issues, take medications that require food, are underweight, or are under 18 years old.

The Science Behind Fasting

Research on intermittent fasting has grown substantially over the past two decades. Key findings include improved insulin sensitivity and reduced fasting insulin levels, favorable changes in lipid profiles (though results vary), upregulation of cellular repair mechanisms including autophagy, potential neuroprotective effects, and weight loss comparable to continuous calorie restriction.

However, much of the research has been conducted in animal models or short-term human trials. Long-term effects and optimal protocols for specific goals continue to be areas of active investigation.

References:

1. de Cabo R, Mattson MP. Effects of Intermittent Fasting on Health, Aging, and Disease. N Engl J Med. 2019;381(26):2541-2551. https://pubmed.ncbi.nlm.nih.gov/31881139/
2. Anton SD, Moehl K, Donahoo WT, et al. Flipping the Metabolic Switch: Understanding and Applying the Health Benefits of Fasting. Obesity. 2018;26(2):254-268. https://pubmed.ncbi.nlm.nih.gov/29086496/
3. Patterson RE, Sears DD. Metabolic Effects of Intermittent Fasting. Annu Rev Nutr. 2017;37:371-393. https://pubmed.ncbi.nlm.nih.gov/28715993/
4. Mattson MP, Longo VD, Harvie M. Impact of intermittent fasting on health and disease processes. Ageing Res Rev. 2017;39:46-58. https://pubmed.ncbi.nlm.nih.gov/27810402/
5. Harvie MN, Pegington M, Mattson MP, et al. The effects of intermittent or continuous energy restriction on weight loss and metabolic disease risk markers: a randomized trial in young overweight women. Int J Obes. 2011;35(5):714-727. https://pubmed.ncbi.nlm.nih.gov/20921964/
6. Alirezaei M, Kemball CC, Flynn CT, et al. Short-term fasting induces profound neuronal autophagy. Autophagy. 2010;6(6):702-710. https://pubmed.ncbi.nlm.nih.gov/20534972/
7. Cahill GF Jr. Fuel metabolism in starvation. Annu Rev Nutr. 2006;26:1-22. https://pubmed.ncbi.nlm.nih.gov/16848698/
8. Hofmekler O. The Warrior Diet: Switch on Your Biological Powerhouse for High Energy, Explosive Strength, and a Leaner, Harder Body. Blue Snake Books; 2007.
9. Berkhan M. The Leangains Method: The Art of Getting Ripped, Researched, Practiced, Perfected. 2018.