Intermittent Fasting Calculator

Plan your intermittent fasting schedule with this calculator. Enter your fasting protocol (16:8, 18:6, 20:4, OMAD, or 5:2), first meal time, age, weight, height, activity level, and goal to get your complete eating window, fasting window, estimated daily calories, and protein targets. The calculator also shows when you'll hit key metabolic milestones like fat burning and ketosis.

Choose your preferred intermittent fasting protocol.

The time you plan to break your fast each day.

years

Results

Eating Window

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Stop Eating By

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Break Fast At

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Daily Fasting Hours

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Fat Burning Begins

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Mild Ketosis Likely

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Daily Calorie Target

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Daily Protein Target

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Fasting vs Eating Window

Frequently Asked Questions

What is intermittent fasting?

Intermittent fasting (IF) is an eating pattern that cycles between periods of fasting and eating. Unlike traditional diets, IF focuses on when you eat rather than what you eat. Common approaches include the 16:8 method, 5:2 diet, and OMAD (one meal a day).

What should I consume during the fasting period?

During your fasting window, you should consume zero calories. Water, black coffee, and plain herbal tea are generally considered acceptable as they contain negligible to no calories and do not meaningfully break a fast. Avoid anything with sugar, cream, or milk.

How many hours of fasting is considered intermittent fasting?

Most experts consider a minimum of 12 hours of fasting to qualify as intermittent fasting. The most popular protocol is 16:8, meaning 16 hours fasted and an 8-hour eating window. Shorter windows (18:6, 20:4, OMAD) are more advanced approaches.

Do overnight hours count toward my fasting window?

Yes, overnight hours absolutely count. This is one reason intermittent fasting is accessible for many people — if you stop eating at 8 PM and sleep for 8 hours, you're already 8 hours into your fast when you wake up, making it easy to reach a 16-hour window by noon.

Does intermittent fasting have any proven benefits?

Research suggests IF may improve insulin sensitivity, support fat loss, promote cellular repair (autophagy), and lead to favorable hormonal changes. However, benefits vary by individual and most studies are still ongoing. IF is not a magic solution and works best alongside a balanced diet.

When does fat burning start during a fast?

Your body begins shifting to fat as a primary fuel source roughly 12 hours into a fast, once liver glycogen stores are depleted. Mild ketosis — where blood ketone levels measurably rise — typically occurs between 16 and 24 hours of fasting.

Who should avoid intermittent fasting?

IF is not suitable for everyone. Pregnant or breastfeeding women, children, people with a history of eating disorders, those with diabetes on insulin or blood-sugar-lowering medications, and anyone with certain medical conditions should consult a doctor before starting any fasting regimen.

How long has intermittent fasting been practiced?

Fasting has been practiced for thousands of years across many cultures and religions. As a structured dietary strategy for health and weight management, IF gained mainstream scientific attention in the early 2000s, with research accelerating significantly over the past decade.

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