Fasting Window Calculator
Pick a schedule and the one meal time you actually know. The rest of the day works itself out.
Schedule
Eating window
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Green is when you eat. The rest of the day is the fast.
Fast starts
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Fast ends
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This is a calculator, not medical advice. It works only on the numbers you type in. What suits you personally depends on things this page knows nothing about — your health, your medication, your history — and a doctor or registered dietitian is the right source for that. Fasting is not appropriate for everyone — see the note on who should check first.
Knowing the window, and holding it
Food Cal AI has a fasting timer built in, with the body's fasting stages marked as the hours pass — alongside the calorie and macro tracking, so the window and what goes in it live in the same place.
Reading the notation
The two numbers are hours and they always add to twenty-four: the fast first, the eating window second. So 16:8 is sixteen hours without food and eight hours to eat in, and 20:4 narrows that to four. One meal a day is written 23:1, though in practice nobody times it to the hour.
What makes 16:8 the common starting point is not the biology, it is the sleeping. If you stop eating at 8pm and first eat at midday, most of that sixteen hours happened while you were unconscious, and the part you actually experience is skipping breakfast. Move to 20:4 and that stops being true, which is why the shorter windows are much harder to keep to for months rather than days.
| Schedule | Fast | Eating window | In practice |
|---|---|---|---|
| 12:12 | 12 h | 12 h | Roughly a normal day with no late snacking |
| 14:10 | 14 h | 10 h | A gentle first step |
| 16:8 | 16 h | 8 h | The common one — usually skipping breakfast |
| 18:6 | 18 h | 6 h | Two meals, close together |
| 20:4 | 20 h | 4 h | One meal plus something small |
| 23:1 | 23 h | 1 h | One meal a day |
What the evidence actually supports
This is a topic with real research and a great deal of overstatement around it, so it is worth separating the two. Trials that compare intermittent fasting against ordinary continuous calorie restriction generally find similar weight outcomes when the calories are matched. The straightforward reading is that most of the effect comes from eating less, and that a shorter window is a mechanism for eating less without counting.
That is a genuinely useful thing to be. Some people find a closed window far easier than portion control, and for them it works precisely because it removes decisions rather than because of anything happening at hour fourteen. The claims that go further — that fasting specifically burns fat that calorie restriction does not, or produces benefits independent of intake — are on much thinner ground than the popular coverage suggests.
Where it goes wrong
The most common failure is not hunger, it is compensation: a four-hour window that ends up holding a whole day's food plus some, which leaves the calorie total unchanged and the eating far less comfortable. The window is a constraint on timing, not on quantity, and it only produces a deficit if the amount also comes down.
The second is fitting it to a life it does not fit. A window that ends before your family eats dinner is one you will break several times a week, and the schedule you keep is worth more than the one that looked best on paper.
Who should ask first
Restricting when you eat is not a neutral change for everyone. Anyone who is pregnant or breastfeeding, has diabetes or takes medication that must be taken with food, is under eighteen, or has any history of disordered eating should talk to a doctor before starting — in the last case particularly, because a rule about not eating can reinforce patterns that are difficult to unwind. None of that is a reason nobody should try it. It is a reason this page is a clock and not a recommendation.
Questions
What does 16:8 mean?
Sixteen hours without eating and an eight-hour window in which you eat. The numbers always add to twenty-four, so 18:6 is an eighteen-hour fast and a six-hour window, and OMAD — one meal a day — is usually written 23:1. Most of the sixteen hours in a 16:8 schedule are spent asleep, which is the main reason it is the version most people find manageable.
Does fasting break if I drink something?
Water, black coffee and plain tea are generally treated as fine, since none of them carries meaningful energy. Anything with sugar, milk or sweeteners is where opinions divide, and the honest answer is that it depends what you are fasting for — a splash of milk is irrelevant to a calorie total and not irrelevant to every claimed mechanism. The research is not settled enough to give you a confident rule.
Is intermittent fasting better than just eating less?
Probably not, on the evidence available. Trials comparing intermittent fasting to plain continuous calorie restriction generally find similar weight results when total calories match, which suggests most of the effect comes from eating less rather than from the timing. That is not an argument against it: a shorter eating window is a genuinely easier way for some people to eat less without counting anything. It is an argument against the stronger claims made for it.
Who should not do this?
Fasting is not appropriate for everyone, and this is a case where the exceptions matter more than the average result. Anyone pregnant or breastfeeding, anyone with diabetes or taking medication that has to be paired with food, anyone under eighteen, and anyone with a history of disordered eating should talk to a doctor before changing when they eat — restricting eating windows can reinforce patterns that are hard to unwind. A calculator cannot make that assessment.
Which end of the day should the window sit?
There is some evidence favouring an earlier window over a late one, on the grounds that eating late runs against normal circadian rhythm. It is not a large or a settled finding. The much larger practical factor is whether the window fits your life — one that requires skipping dinner with your family is one you will abandon, and the schedule you keep to beats the theoretically optimal one.
