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Intermittent Fasting Calculator

Choose a fasting protocol and your fast start time to see exactly when your fasting window ends and your eating window opens and closes.

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Intermittent Fasting Calculator
Calculate your eating window and fasting end time
Select protocol and start time

Medical disclaimer. This calculator is provided for educational and informational purposes only and does not constitute medical advice, diagnosis, or treatment. Always consult a qualified healthcare professional regarding your health, diet, exercise, or medication decisions. Read full disclaimer →

About Intermittent Fasting Calculator

Intermittent fasting (IF) cycles between eating windows and fasting periods. Common protocols: 16:8 (fast 16 hours, eat within 8-hour window - most popular); 18:6 (more aggressive, shorter eating window); 5:2 (eat normally 5 days, restrict to 500-600 calories on 2 non-consecutive days); OMAD (one meal a day). Our calculator identifies eating window, fasting start/end times, and tracks adherence.

Metabolic benefits of fasting begin after approximately 12 hours: glycogen stores deplete and fat oxidation increases. At 18-24 hours, autophagy (cellular repair) is upregulated. IF works primarily through calorie restriction and does not require changing food composition. Black coffee, tea, and water do not break a fast. People with diabetes, eating disorder history, or who are pregnant should consult a doctor before attempting IF.

Frequently Asked Questions

16:8 is the most popular intermittent fasting protocol: you fast for 16 hours and eat all your meals within an 8-hour window. This calculator supports 16:8 along with other common protocols like 18:6 (a shorter, more restrictive eating window) and shows your exact fasting-end and eating-window times based on when you start your fast.
No. Water, black coffee, and plain tea (without milk, sugar, or cream) do not break a fast and are generally fine to consume during fasting hours since they contain negligible or no calories.
Metabolic changes begin after roughly 12 hours of fasting, as glycogen stores deplete and fat oxidation increases. Around 18-24 hours, autophagy - a cellular repair and recycling process - is upregulated. Intermittent fasting works mainly by restricting the time window you eat in, which typically reduces total calorie intake, rather than by changing what you eat.
People with diabetes, a history of eating disorders, or who are pregnant or breastfeeding should consult a doctor before attempting any fasting protocol, since blood sugar regulation and nutritional needs differ significantly in these situations.
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