Intermittent Fasting Calculator — Schedule
Plan your 16:8, 18:6, 20:4, OMAD or 5:2 fasting schedule. Enter last meal time for eating window, fasting window and next meal time. Free.
CalcVita. (2026). Intermittent Fasting Calculator — Schedule. CalcVita. Retrieved October 17, 2026, from https://calcvita.com/en/calculators/intermittent-fasting
Suggested article
Intermittent Fasting for Beginners: Protocols, Benefits, and How to Start
Intermittent fasting is one of the most researched dietary strategies of the past decade. Learn the most popular protocols, what the science says about metabolic and health benefits, and how to begin safely.
Read the full article →How does this calculator work?
Enter your fasting protocol, the time of your last meal (which starts your fasting clock) and your wake time. The calculator adds the protocol’s fasting hours to your last meal to give you your fasting window, your eating window, your next meal time and whether your eating window is currently open. For 5:2 it shows the fast-day guidance instead of a time window.
Choosing the right protocol
16:8 is the most studied and beginner-friendly protocol — most people skip breakfast and eat from noon to 8 PM. 18:6 and 20:4 are more aggressive and suitable for those with IF experience. OMAD (23:1) is the most extreme form and should only be attempted by experienced practitioners. The 5:2 protocol is calorie-restricted rather than time-restricted and works well for those who cannot commit to daily fasting windows.
Evidence-based benefits
A review in the New England Journal of Medicine (de Cabo & Mattson, 2019) describes how fasting triggers metabolic switching — shifting from glucose to fatty acids and ketones as fuel — which in studies improves glucose regulation, increases stress resistance and reduces inflammation. In a 5-week crossover trial (Sutton et al., 2018), men with prediabetes who ate within a 6-hour window ending before 3 PM improved their insulin sensitivity and blood pressure even without losing weight.
Tips for success
Stay hydrated during your fasting window — aim for 2-3 litres of water daily. Break your fast with a balanced meal including protein, healthy fats, and vegetables. Avoid high-sugar foods immediately after fasting as blood glucose sensitivity is elevated. Start with 12:12 or 14:10 if 16:8 feels too difficult, and gradually extend your fasting window over 2-4 weeks.
Methodology
The schedule is simple arithmetic: next meal = last meal + fasting hours (16, 18, 20 or 23), and the eating window lasts the remaining 8, 6, 4 or 1 hours of the day. The calculator does not estimate how many calories you will cut: that depends on what and how much you eat within the window. The most studied forms of intermittent fasting are alternate-day fasting, the 5:2 diet and time-restricted eating (Varady et al., Nat Rev Endocrinol 2022), and much of the evidence on mechanisms comes from animal studies and short human trials (de Cabo & Mattson, NEJM 2019).
Scientific Sources
- de Cabo R, Mattson MP (2019) — Effects of Intermittent Fasting on Health, Aging, and Disease. N Engl J Med. 381:2541-2551. PMID 31881139
- Sutton EF et al. (2018) — Early Time-Restricted Feeding Improves Insulin Sensitivity, Blood Pressure, and Oxidative Stress Even without Weight Loss in Men with Prediabetes. Cell Metabolism. 27(6):1212-1221. PMID 29754952
- Longo VD, Panda S (2016) — Fasting, Circadian Rhythms, and Time-Restricted Feeding in Healthy Lifespan. Cell Metabolism. 23(6):1048-1059. PMID 27304506
- Varady KA et al. (2022) — Clinical application of intermittent fasting for weight loss: progress and future directions. Nat Rev Endocrinol. 18(5):309-321. PMID 35194176
- Harvie MN et al. (2011) — The effects of intermittent or continuous energy restriction on weight loss and metabolic disease risk markers: a randomized trial in young overweight women. Int J Obes. 35(5):714-727. PMID 20921964
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 protocols include 16:8 (fast 16 hours, eat in 8-hour window), 18:6, 20:4 (Warrior Diet), OMAD (23:1, one meal a day), and 5:2 (five normal days + two low-calorie days). Research published in the New England Journal of Medicine (de Cabo & Mattson, 2019) shows IF can improve metabolic health, reduce inflammation, and support weight management.
- How does the 16:8 protocol work?
- The 16:8 protocol is the most popular form of intermittent fasting. You fast for 16 consecutive hours and eat all your meals within an 8-hour window. For example, if your last meal is at 8:00 PM, your fasting window runs from 8:00 PM to 12:00 PM the next day, and your eating window is 12:00 PM–8:00 PM. Most people skip breakfast and eat lunch and dinner. Shorter, earlier windows have been tested too: in a controlled trial, eating within a 6-hour window ending before 3 PM improved insulin sensitivity and blood pressure in men with prediabetes even without weight loss (Sutton et al., Cell Metabolism 2018).
- Is intermittent fasting safe?
- Intermittent fasting is generally safe for healthy adults. However, it is not recommended for pregnant or breastfeeding women, children and teenagers, people with a history of eating disorders, individuals with type 1 diabetes or those on insulin, or people who are underweight. Always consult your healthcare provider before starting any fasting protocol, especially if you have existing medical conditions or take medications.
- What can I consume during the fasting window?
- During the fasting window, you should consume only water, plain black coffee (no milk, sugar, or sweeteners), plain tea (no additives), and sparkling water. Even small amounts of calories can break your fast by triggering an insulin response. Some people include black coffee for its appetite-suppressing effects. Medications should be taken as prescribed, even during fasting — discuss timing with your doctor.
- What are the benefits of intermittent fasting?
- Reviews (de Cabo & Mattson, NEJM 2019; Longo & Panda, Cell Metabolism 2016) describe possible benefits such as weight and body-fat loss, better insulin sensitivity, lower fasting glucose and insulin, improved blood lipids, lower inflammation markers and activation of cellular repair processes (autophagy). Much of this evidence comes from animal studies and short human trials, and individual results vary.
- What is the difference between 5:2 and time-restricted eating?
- Time-restricted eating (TRE) protocols like 16:8, 18:6, 20:4, and OMAD restrict eating to a specific daily time window — you fast every day. The 5:2 diet (Mosley & Spencer, 2013) is different: you eat normally five days per week and restrict calories to approximately 500 kcal on two non-consecutive days. TRE is generally easier to maintain long-term because the pattern is consistent daily, while 5:2 requires planning for 'fast days' each week.
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