Intermittent Fasting Calculator: Your Eating Window and Fasting Schedule
Pick a fasting protocol (12:12, 14:10, 16:8, 18:6, 20:4, OMAD, 5:2 or custom) and your first or last meal time to see your exact eating window, when the fast starts and ends, a fasting timeline and 5:2 fast-day calories.
How it works
Eating hours = 24 − fasting hours (12:12 → 12, 14:10 → 10, 16:8 → 8, 18:6 → 6, 20:4 → 4, OMAD → 1) · Window end = first meal + eating hours (or window start = last meal − eating hours) · 5:2 fast-day calories = 25% × maintenance calories
Intermittent fasting splits time into eating and fasting periods. Daily time-restricted plans set how many hours you eat: 12:12 and 14:10 are gentle starting points, 16:8 gives an 8-hour window, 18:6 gives 6, 20:4 gives 4 and OMAD (one meal a day) about 1. Enter either the time of your first meal or the time your last meal must finish, and the calculator works out the other end of the window, when the fast starts and when it ends the next day, plus a timeline of the fast. The timeline is a rough guide: digestion of a meal takes about 4 hours, the body then draws on stored glycogen, and a fasted state with more fat burning and slowly rising ketones can begin somewhere between 12 and 36 hours, varying widely between people. The 5:2 plan works differently: you eat normally five days a week and about 500 to 600 kcal, roughly a quarter of your usual needs, on two non-consecutive days; the calculator sets that target from your maintenance calories and estimates the weekly deficit. Research suggests intermittent fasting gives similar weight loss to ordinary calorie restriction, so the best plan is the one you can keep. Fasting is not advised for children and teens, pregnant or breastfeeding women, people with or at risk of eating disorders, or people on insulin.
Worked example
Default: 16:8 with a first meal at 12:00. The 8-hour window runs 12:00 to 20:00, and the 16-hour fast runs from 20:00 until 12:00 the next day, 112 fasting hours a week. On the timeline the post-absorptive phase starts at 00:00 and the 12-hour mark is 08:00. Choosing "Last meal time" with 19:30 on 18:6 gives a window of 13:30 to 19:30. On 5:2 with 2,000 kcal maintenance, fast days are 25% × 2,000 = 500 kcal, the weekly deficit is 2 × 1,500 = 3,000 kcal, about 0.39 kg (0.86 lb) a week.
Sources: Wikipedia: Intermittent fasting (methods, evidence and cautions) · FatCalc: Intermittent fasting calculator
Frequently asked questions
When should I place my eating window?
Whatever you can keep consistent. Many people prefer a midday-to-evening window so they can eat with family, but an earlier window may suit morning appetite better. Consistency matters more than the exact clock time.
Which intermittent fasting schedule is best for beginners?
A 12:12 or 14:10 schedule is an easy start, because most of the fast happens overnight. After a week or two many people move to 16:8, for example eating from 12:00 to 20:00. Longer fasts such as 20:4 and OMAD are harder to keep and to meet protein needs on.
Can I drink anything during the fast?
Water, black coffee and plain tea are generally considered fine and do not meaningfully break a fast. Anything with calories, such as milk, sugar or juice, starts digestion and ends the fasting state.
How many calories should I eat on 5:2 fast days?
About 500 to 600 kcal, roughly 25% of your usual daily needs, on two non-consecutive days a week. On the other five days you eat your normal maintenance amount, not extra to make up for the fast days.
Does fasting alone cause weight loss?
Only if it puts you in a calorie deficit. Fasting can make that easier by shrinking the hours you eat, but it is still possible to overeat in a short window, so total intake is what counts. Studies show similar results to ordinary calorie restriction.
Who should not try intermittent fasting?
Children and teens, pregnant or breastfeeding women, people with or at risk of eating disorders, people who are underweight and people using insulin or other glucose-lowering drugs should not fast without medical advice.
Related calculators
Educational estimate, not medical advice. If you have diabetes, are pregnant or breastfeeding, take medication or have a history of eating disorders, talk to your doctor before fasting.