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 September 2, 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 shows your fasting window, eating window, next meal time, and whether your eating window is currently open. The estimated caloric reduction is based on clinical research comparing time-restricted eating to ad-libitum eating patterns.
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 landmark review in the New England Journal of Medicine (de Cabo & Mattson, 2019) found that intermittent fasting triggers metabolic switching — shifting from glucose to fatty acids and ketones as fuel. This switch improves glucose regulation, increases stress resistance, and suppresses inflammation. A 2018 RCT (Sutton et al.) demonstrated improvements in insulin sensitivity and blood pressure in just 5 weeks of early 16:8 eating.
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 and caloric reduction estimates
The caloric reduction percentages displayed by this calculator are derived from published clinical trials comparing time-restricted eating (TRE) with ad-libitum eating. A meta-analysis by Varady et al. (Nat Rev Endocrinol, 2022) reported that 16:8 TRE reduces daily caloric intake by approximately 10-20% without conscious calorie counting. More restrictive windows (18:6, 20:4, OMAD) show progressively larger reductions of 15-30%, while the 5:2 protocol achieves roughly 25% weekly caloric reduction. These estimates are population averages and individual results vary based on meal composition, adherence, and baseline intake. The metabolic switching mechanism described by de Cabo and Mattson (NEJM, 2019) underpins the health benefits beyond simple caloric restriction.
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 29953426
- 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 34633860
- Harvie MN et al. — The 5:2 Diet. The Fast Diet. Atria Books (Mosley M, Spencer M, 2013). ISBN 978-1-4767-1659-8
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. A randomized controlled trial (Sutton et al., Cell Metabolism 2018) found that early time-restricted eating improves insulin sensitivity, blood pressure, and oxidative stress even without weight loss.
- 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?
- Research (de Cabo & Mattson, NEJM 2019; Longo & Panda, Cell Metabolism 2016) suggests IF benefits include weight and body fat reduction, improved insulin sensitivity, reduced fasting blood glucose and insulin levels, decreased LDL cholesterol and triglycerides, reduced inflammation markers (CRP, IL-6), improved cellular repair processes (autophagy), potential neuroprotective effects, and simplified meal planning. Individual results vary, and benefits typically become apparent after 4–8 weeks of consistent practice.
- 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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