Calculate your eating window, fasting progress, and when to eat based on your fasting protocol. Supports 16:8, 18:6, 20:4, and custom schedules.
16:8
Fast for 16 hours, eat within 8-hour window
Fast complete
Start your next fast when ready
Times based on your selected 16:8 protocol. Metabolic states are approximate and vary by individual.
Intermittent fasting (IF) is an eating pattern that cycles between periods of fasting and eating. Unlike traditional diets that focus on what you eat, intermittent fasting focuses on when you eat. During the fasting period, you consume zero or minimal calories, while during the eating window, you eat your meals for the day.
The concept isn't new—humans have fasted throughout history for religious, cultural, and practical reasons. What's different today is the scientific understanding of how fasting affects the body at a cellular and metabolic level. Research over the past two decades has revealed that time-restricted eating can trigger beneficial changes in hormone levels, cellular repair processes, and gene expression.
Intermittent fasting has gained popularity because it doesn't require counting calories or eliminating food groups. Instead, you simply limit when you eat, making it more sustainable for many people than restrictive diets.
When you eat, your body spends several hours processing that food and burning what it can from what you just consumed. Because the body has readily available energy in the bloodstream, it doesn't need to burn stored fat. This is especially true if you've consumed carbohydrates, which your body preferentially burns for fuel.
During the fasted state, your body doesn't have a recent meal to use as energy, so it's more likely to pull from fat stores. Your body enters the fasted state approximately 12 hours after your last meal—this is when interesting metabolic changes begin to occur.
Understanding what happens in your body during a fast helps explain why intermittent fasting is effective:
| Hours fasted | What's happening |
|---|---|
| 0-4 hours | Fed state—body digests and absorbs nutrients |
| 4-8 hours | Post-absorptive state—blood sugar and insulin levels drop |
| 8-12 hours | Glycogen stores begin depleting |
| 12-14 hours | Fat burning increases, early ketone production |
| 16-24 hours | Ketosis and autophagy (cellular cleanup) accelerate |
| 24+ hours | Deep ketosis, growth hormone increases |
For most intermittent fasting protocols, the goal is to reach that 12-16 hour mark regularly, where fat burning becomes significant and cellular repair processes activate.
The 16:8 protocol is the most popular form of intermittent fasting. You fast for 16 hours and eat within an 8-hour window. Most people achieve this by skipping breakfast and eating between noon and 8 PM, though you can adjust the window to fit your schedule.
Example schedule:
This method works well for beginners because it essentially extends your overnight fast by a few hours. Many people naturally aren't hungry first thing in the morning, making the 16:8 protocol feel effortless.
The 14:10 protocol is ideal for beginners or those who find 16 hours too challenging. You fast for 14 hours and eat within a 10-hour window. This gives you more flexibility while still providing metabolic benefits.
Example schedule:
Research suggests that even a 14-hour fast provides meaningful benefits, particularly for circadian rhythm alignment and metabolic health.
The 18:6 protocol extends the fasting window to 18 hours with a 6-hour eating window. This is more challenging but can accelerate results for experienced fasters.
Example schedule:
With only 6 hours to eat, most people fit in two meals. This protocol puts you deeper into the fat-burning and ketosis zone each day.
The Warrior Diet involves fasting for 20 hours and eating within a 4-hour window. During the fasting period, small amounts of raw fruits and vegetables are sometimes permitted.
Example schedule:
This protocol typically involves one main meal and perhaps a small snack. It's challenging but can be effective for those who prefer eating larger meals less frequently.
OMAD is exactly what it sounds like—you eat one meal per day within a 1-hour window. This is the most extreme daily fasting protocol and requires careful planning to ensure adequate nutrition.
Research has identified numerous potential benefits of intermittent fasting. While individual results vary, the following benefits have scientific support:
By reducing your eating window, you naturally tend to consume fewer calories. More importantly, fasting triggers hormonal changes that facilitate fat burning—insulin levels drop, growth hormone increases, and norepinephrine signals fat cells to break down body fat.
A 2020 review in the New England Journal of Medicine found that intermittent fasting can lead to weight loss of 3-8% over 3-24 weeks, with significant reductions in waist circumference indicating visceral fat loss.
Intermittent fasting has been shown to reduce insulin resistance, lowering blood sugar by 3-6% and fasting insulin levels by 20-31% in some studies. This may protect against type 2 diabetes.
During fasting, cells initiate autophagy, a process where they remove dysfunctional proteins and organelles. This cellular "cleanup" may help prevent diseases and slow aging. Autophagy becomes significant after approximately 16-24 hours of fasting.
Studies suggest intermittent fasting may improve several cardiovascular risk factors, including blood pressure, cholesterol levels, triglycerides, and inflammatory markers.
Fasting increases levels of brain-derived neurotrophic factor (BDNF), a protein that supports brain health and may protect against depression and cognitive decline.
Calculating your eating window is straightforward once you choose a protocol:
For example, if you finish eating at 8:00 PM and follow the 16:8 protocol:
Your eating window then closes after the designated eating hours:
For the 16:8 example:
Understanding what breaks a fast is crucial for success. Generally, anything with calories breaks a fast by triggering an insulin response. However, there's some flexibility depending on your goals.
If your primary goal is weight loss, small amounts of calories (<50) likely won't significantly impact results. If you're fasting for autophagy or maximum metabolic benefits, stricter adherence is recommended.
If you're new to fasting, don't jump straight into 16:8 or longer. Start with a 12-hour fast (which most people do overnight anyway), then gradually extend to 14 hours, then 16 hours over several weeks.
Drink plenty of water during your fasting window. Many hunger pangs are actually thirst signals. Aim for at least 8 glasses of water throughout the day, and consider adding electrolytes for longer fasts.
Boredom and habit often trigger eating, not actual hunger. Stay busy during your fasting window, especially in the morning if you're skipping breakfast. The first few days are the hardest—hunger typically decreases significantly after the first week.
Intermittent fasting isn't a license to eat junk food during your eating window. Focus on whole foods, adequate protein (at least 0.7-1g per pound of body weight), healthy fats, and plenty of vegetables. Poor food choices will undermine the benefits of fasting.
Try to maintain consistent eating and fasting windows each day. Your body's circadian rhythm adapts to regular patterns, making fasting easier and potentially enhancing benefits.
Some days will be harder than others. If you feel unwell, dizzy, or excessively weak, break your fast. Intermittent fasting should enhance your wellbeing, not diminish it.
Intermittent fasting isn't appropriate for everyone. The following groups should avoid or carefully approach fasting:
Always consult a healthcare provider before starting intermittent fasting, especially if you have any medical conditions or take medications.
Consuming excessive calories during your eating window negates the benefits of fasting. While you don't need to count calories obsessively, be mindful of portion sizes and avoid treating the eating window as a free-for-all.
Conversely, some people under-eat, which can slow metabolism and lead to muscle loss. Ensure you're consuming adequate calories and protein during your eating window.
Poor sleep increases hunger hormones and cravings, making fasting much harder. Prioritize 7-9 hours of quality sleep for best results.
Life happens—social events, travel, and special occasions may disrupt your schedule. It's okay to adjust occasionally. Consistency over time matters more than perfection on any single day.
| Approach | Focus | Flexibility | Sustainability |
|---|---|---|---|
| Intermittent fasting | When you eat | High | High |
| Calorie counting | How much you eat | Medium | Medium |
| Keto diet | What you eat (low carb) | Low | Medium |
| Paleo diet | What you eat (whole foods) | Medium | Medium |
Intermittent fasting can be combined with other dietary approaches. Many people find success pairing IF with low-carb or keto diets, as ketosis is achieved more quickly during fasting periods.
Tracking helps maintain accountability and motivation. Use this calculator to:
Many people find that starting with shorter fasts and gradually extending them leads to the most sustainable results. The best fasting protocol is one you can maintain consistently over time.