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⏱️ Intermittent Fasting Calculator

Plan your intermittent fasting schedule. Choose a protocol (16:8, 18:6, 20:4 or OMAD), set your eating window start time and see your full daily fasting and eating plan.

Fasting Protocol

Eating Window Start Time

What is this tool?

Intermittent fasting (IF) is an eating pattern that cycles between periods of voluntary food restriction and periods of unrestricted eating. Unlike most diets, it focuses primarily on *when* you eat rather than *what* you eat. The most popular protocols include 16:8 (16 hours fasting, 8 hours eating), 18:6 (18 hours fasting, 6 hours eating), 20:4 (also called the Warrior Diet), and OMAD (One Meal A Day, effectively 23:1). Research published by the National Institutes of Health and reviewed by Johns Hopkins Medicine suggests that time-restricted feeding can improve insulin sensitivity, lower fasting glucose, support modest weight loss, and trigger a cellular repair process called autophagy, in which cells clear out damaged proteins and organelles. During the fasting window the body depletes stored glycogen and shifts to burning fatty acids and ketones for fuel, which is thought to underlie many of the metabolic benefits. This intermittent fasting calculator helps you translate an abstract protocol like "18:6" into a concrete daily clock: you choose your protocol and the time you want your eating window to begin, and the tool computes the exact start and end of both the fasting and eating windows, then lays out a full seven-day schedule so you can see how the pattern repeats. It is designed for healthy adults exploring time-restricted eating for weight management or metabolic health, and it explicitly flags groups who should avoid unsupervised fasting. To round out your health tracking, pair this tool with the BMI calculator to monitor body composition, the BMR calculator to understand your baseline calorie burn, the calorie calculator to set intake targets, the water intake calculator to stay hydrated during fasting hours, and the body fat calculator for a finer composition picture.

How it works

You start by selecting a fasting protocol from the dropdown. Each protocol defines the length of the fasting window and the eating window in hours. You then enter the clock time at which your eating window should begin (for example 12:00 PM for a typical 16:8 schedule). The calculator computes the eating window end time by adding the protocol's eating hours, then derives the fasting window start and end from the remaining hours of the day, producing a clear four-line daily schedule. It also generates a seven-day text grid so you can verify that the pattern is stable across the week. | Protocol | Fasting (hr) | Eating (hr) | Typical Difficulty | |---|---|---|---| | 16:8 | 16 | 8 | Beginner-friendly | | 18:6 | 18 | 6 | Moderate | | 20:4 (Warrior) | 20 | 4 | Challenging | | OMAD (23:1) | 23 | 1 | Very challenging | | 5:2 (non-consecutive) | 24 (2 days/wk) | Normal (5 days) | Moderate | | Alternate-day | 24 (every other day) | 24 | Challenging | The table above summarises the six most common IF protocols. The 16:8 method is the most widely adopted because it usually requires nothing more than skipping breakfast, while OMAD and alternate-day fasting demand more discipline and should be approached gradually.

How to use

Choose your preferred fasting protocol (16:8, 18:6, 20:4, OMAD, 5:2 or alternate-day).
Enter the time at which you want your eating window to begin.
Optionally switch the clock format between 12-hour and 24-hour display.
Review the generated daily fasting and eating window schedule.
Use the seven-day grid to plan your week and verify the pattern is sustainable.

Frequently Asked Questions

Will drinking coffee break my fast?

Black coffee (without milk, sugar or cream) contains fewer than 5 calories per cup and is generally considered compatible with a fast. It may even enhance fat oxidation and suppress appetite. Adding milk, sugar, MCT oil or butter breaks the fast because it provides meaningful calories and triggers an insulin response.

How long does it take to see results from 16:8 fasting?

Most people notice reduced bloating and steadier energy within the first one to two weeks. Measurable weight loss typically appears after three to four weeks, provided the eating window does not compensate with excess calories. Improvements in fasting glucose and insulin sensitivity may take six to twelve weeks to show up in bloodwork.

What is the difference between 18:6 and OMAD?

18:6 allows a 6-hour eating window each day (for example noon to 6 PM), which usually accommodates two meals and a snack. OMAD (One Meal A Day) compresses all eating into roughly one hour, making it far more restrictive and harder to sustain. OMAD also makes it more difficult to meet daily protein, fibre and micronutrient needs.

Can I exercise while fasting?

Yes. Low to moderate-intensity cardio (walking, easy cycling, light jogging) is generally well tolerated in a fasted state and may increase fat oxidation. Heavy resistance training or high-intensity intervals are best performed toward the end of the fast so you can refuel during the eating window immediately afterward.

Who should avoid intermittent fasting?

Pregnant or breastfeeding women, individuals with a history of eating disorders, people with Type 1 diabetes, those taking insulin or sulfonylureas, children and adolescents, and anyone who is underweight or malnourished should not fast without medical supervision. Always consult a healthcare professional before starting a fasting regimen if you have any chronic condition.

What happens to autophagy during a fast?

Autophagy is a cellular cleanup process in which cells degrade and recycle damaged proteins and organelles. Animal studies suggest it increases after roughly 16–24 hours of fasting and peaks around 48–72 hours. Human data are still limited, but the findings are a major reason researchers study fasting for longevity and metabolic health.

Is intermittent fasting safe long-term?

Disclaimer: This intermittent fasting calculator is provided for general informational purposes only and does not constitute medical advice. Individual results vary. If you are pregnant, breastfeeding, taking medication, or have a medical condition (especially diabetes or an eating disorder), consult a qualified healthcare professional before changing your eating pattern.

Tips & Advice

If you are new to intermittent fasting, start with a 12-hour overnight fast and gradually extend the window by 30–60 minutes each week until you reach your target protocol — jumping straight into 20:4 or OMAD often leads to hunger, headaches and early abandonment. During the fasting window, water, black coffee and unsweetened tea are generally considered acceptable because they contain negligible calories and can help suppress appetite; however, anything with more than a handful of calories (bone broth, milk, juice) will break the fasted state and halt the metabolic switch to fat oxidation. Pay special attention to hydration, as fasting reduces the water normally obtained from food — use the water intake calculator to estimate your daily target. When you break your fast, prioritise protein, fibre and healthy fats to stabilise blood sugar, and avoid a large carbohydrate-only meal which can cause rebound hunger a few hours later. Regular resistance training during the eating window helps preserve lean muscle mass, which is otherwise at risk during prolonged calorie deficits. Certain groups should not undertake unsupervised intermittent fasting: pregnant or breastfeeding women, people with a history of eating disorders, individuals with Type 1 diabetes or those on insulin or sulfonylureas, children and teenagers, and anyone who is underweight. If you take prescription medications, consult your doctor before changing your meal timing because some drugs must be taken with food. Finally, intermittent fasting is a tool for energy balance, not a magic cure — if you overeat during the eating window you will still gain weight, so track intake with the calorie calculator alongside your fasting schedule.

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