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Intermittent fasting calculator: when your eating window opens

Worked example: last meal at 20:00 on 16:8worked example

12:00 to 20:00

Window opens
12:00 (next day)16 hours after the last meal
Window closes
20:00 (next day)8 hours long

This is clock arithmetic and nothing more. It says when a 16:8 window falls if the last meal was at 20:00 — not whether that schedule is a good idea, which depends on things a calculator cannot see.

24-hour or 12-hour, whichever you think in.

Schedule

Sixteen hours between the last meal and the first, leaving an eight-hour window.

How much of the fast am I asleep for?

Asked because it is the one genuinely useful thing this arithmetic can tell you: most of a long fast is spent asleep, which is why "sixteen hours" and "skipping breakfast" can describe the same day. For sleep timing itself, that is theballastprinciple.com.

What this page does not tell you

Which schedule to pick, whether to use one at all, or what following one would do for you. None of the schedules above is marked as recommended or as a starting point, because that would be a suggestion wearing a calculator's clothes. The arithmetic of 16:8 is addition; everything people actually want to know about it is not.

Time-restricted eating is not suitable for everyone. Among others, that includes people who are pregnant or breastfeeding, anyone with a history of disordered eating, people taking medication for diabetes — where changing meal timing can change how that medication behaves — and children and adolescents. If any of those describes you, this is a conversation with a clinician, not with a clock.

  • Time-restricted eating window (clock arithmetic)convention — window opens = last meal + fasting hours; window closes = opens + eating hours. Verified against de Cabo & Mattson, New England Journal of Medicine 2019 (There is no uncertainty in the arithmetic - it is addition on a clock. The uncertainty is entirely in what a schedule does for any individual, which this calculator does not estimate and does not claim.)

If what you are really after is how much you eat rather than when, the macro calculator and the protein calculator cite the reference intakes they use.

This works out when a time-restricted eating window opens and closes from the time of your last meal, for 16:8, 18:6, 20:4 and one meal a day. It also shows how much of the fast falls while you are asleep. It is clock arithmetic, and it does not recommend a schedule or predict a result.

  • Time-restricted eating window (clock arithmetic)convention — window opens = last meal + fasting hours; window closes = opens + eating hours. Verified against de Cabo & Mattson, New England Journal of Medicine 2019 (There is no uncertainty in the arithmetic - it is addition on a clock. The uncertainty is entirely in what a schedule does for any individual, which this calculator does not estimate and does not claim.)

Everything you type here is computed in your browser. Nothing you enter is stored on a server, sent anywhere, or shared — there is no account and no health data to leak.

Frequently asked questions

If I stop eating at 8pm, when does a 16:8 window open?
At noon the next day, and it closes again at 8pm. The fast is measured from the end of the last meal, so sixteen hours from 20:00 is 12:00, and the eight-hour window runs from there back round to where it started.
What do the numbers in 16:8 mean?
The first is the hours spent not eating, the second the hours in which eating happens, and together they make twenty-four. So 18:6 is an eighteen-hour fast with a six-hour window, and one meal a day is usually written 23:1.
How much of a 16-hour fast am I actually awake for?
Often about half. If you sleep from 23:00 to 07:00, eight of those sixteen hours are spent asleep, which is why the same day can be described as a sixteen-hour fast or as skipping breakfast. Enter your usual sleep times above and the tool works out the overlap.
Which fasting schedule is best?
This calculator will not answer that, and no calculator can. It does not know your medical history, your medication, your work pattern or your relationship with food, and each of those matters more than the arithmetic. None of the schedules here is marked as recommended for that reason.
Who should not use a fasting schedule?
Time-restricted eating is not suitable for everyone. That includes people who are pregnant or breastfeeding, anyone with a history of disordered eating, people taking medication for diabetes - where meal timing can change how the medication behaves - and children and adolescents. Those are conversations for a clinician.
Does the clock change if I have a coffee?
That depends on a definition rather than on arithmetic, and different protocols draw the line in different places. This tool computes the window you specify; what counts as breaking it is not something it can decide for you.

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The next number

No body metric means much on its own. Here is what this one feeds into.

Maintenance calorie calculatorWhen you eat is a clock question; how much is the one with a published equation behind it.Protein calculatorA shorter eating window makes hitting a protein target harder, so it is worth knowing what the target is.

Related calculators

Macro calculatorNutrition targetsProtein calculatorNutrition targetsMaintenance calorie calculatorMetabolism

Every formula on this page is listed with its source on the methodology page, and changes to any reference value are logged in the data changelog. Canonical URL: https://myhealthmywellness.com/intermittent-fasting-calculator

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