Intermittent Fasting Calculator
Intermittent fasting (IF) is a structured approach to eating that alternates between defined fasting and eating windows. Unlike traditional diets, IF focuses on when you eat rather than what you eat. The most popular protocol is 16:8, where you fast for 16 hours and eat within an 8-hour window. Other protocols include 18:6, 20:4, and OMAD (one meal a day). This calculator takes your preferred fasting protocol and the time you plan to start or end your fast, then calculates your exact eating window start and end times, fast duration, and eating window duration. This makes it easy to schedule your meals and align fasting with your sleep schedule for minimum disruption to daily life.
How the fasting window is calculated
Eating window opens = Fast start time + Fasting hours
Eating window closes = Eating window opens + Eating hours
Eating hours = 24 - Fasting hours
If your fast starts at 8:00 PM and you follow 16:8, your eating window opens at 12:00 PM the next day and closes again at 8:00 PM.
Intermittent fasting: frequently asked questions
What is intermittent fasting?
Intermittent fasting (IF) is an eating pattern that cycles between periods of fasting and eating. It does not restrict what you eat, only when. Common protocols include 16:8 (16 hours fasting, 8 hours eating), 18:6, and 5:2 (normal eating 5 days, restricted calories 2 days).
What are the health benefits of intermittent fasting?
Research suggests intermittent fasting can help with weight management, improved insulin sensitivity, reduced inflammation, and metabolic health. The NIH notes promising evidence but emphasises that most long-term effects in humans are still being studied. Always consult a doctor before starting.
When does fasting actually begin?
Your fasting period begins after your last meal or caloric intake. Coffee without sugar or milk, plain water, and plain tea are generally considered fasting-safe as they contain no or negligible calories. Your body begins glycogen depletion and transitions toward fat oxidation after roughly 12-18 hours.
Which intermittent fasting protocol is best?
The most effective protocol is the one you can sustain. 16:8 is the most widely practiced and studied. 5:2 may suit those who prefer not to restrict daily. OMAD (one meal a day) provides the longest fasting window but requires careful nutritional planning to meet daily needs.
Is intermittent fasting safe for everyone?
IF is not suitable for pregnant or breastfeeding individuals, children, people with a history of eating disorders, those with type 1 diabetes, or people on certain medications. Always consult a healthcare provider before beginning any fasting regimen.
Official sources
- National Institute on Aging (NIH): Calorie Restriction and Fasting Diets.
- National Institutes of Health - MedlinePlus: Intermittent Fasting.
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology.