Intermittent Fasting Calculator

What a fasting calculator actually does

A fasting calculator turns two numbers into a time. You give it the moment your last meal ended and how many hours you intend to fast; it gives you back the exact moment the fast ends, the eating window that follows, and the point where the next fast begins. That is the whole job, and it is the reason a fast calculator is worth using at 11 PM when you would rather not do arithmetic across midnight.

Notice what is absent from that list. A fasting calculator cannot tell you how much weight you will lose, what your metabolic rate is, or which hour a particular biological process begins. Those depend on things a web form cannot observe. Tools that ask for your age, sex and body weight before returning a fasting time are collecting information that does not change the answer, because none of it alters what eight o'clock plus sixteen hours equals.

The part worth getting right is the calendar. Most fasts cross midnight, so the answer lands tomorrow, and a fast begun after midnight ends the same day. Longer fasts land two or three days out. This calculator labels every result with the correct day and handles daylight saving changes and leap days, which is exactly where mental arithmetic and simpler tools go wrong.

Free, with nothing held back

People search for a free fasting calculator because so many of them are not. The pattern is familiar: the schedule is free, then the timer needs an account, the history needs a subscription, and a reminder costs money. What is being sold in each case is arithmetic that takes one line of code.

Everything here is free and there is no tier above it. The calculator, the fasting timer, the schedule planner, calendar export, CSV and JSON export, and your history all cost nothing and require no account. Your data stays in your own browser, so there is no account to create because there is no server to create it on.

Any number of fasting hours, not just the presets

The preset buttons cover the common protocols, but the custom field accepts any duration from one hour to seven days in half-hour steps. That makes this a fasting hours calculator rather than a protocol picker: enter 13.5 if that is what your evening allowed, or 36 if you are planning a longer fast.

Two behaviours are worth knowing. Fasts of 24 hours or longer have no daily eating window, and the calculator says so rather than inventing one. And the result always reports what you entered rather than rounding to the nearest tidy protocol, because a 13.5 hour fast is a 13.5 hour fast and your history should say so.

How the fasting calculation works

Intermittent fasting divides each day into two periods: a fasting window (no calories) and an eating window. The math is simple but easy to get wrong across midnight: your fast starts at your last meal, runs for your chosen number of hours, and your eating window fills the rest of the 24-hour cycle. A 16-hour fast that starts at 8:15 PM ends at 12:15 PMtomorrow, a date change most people compute wrongly at least once. This fasting time calculator does the arithmetic exactly, including midnight crossings, month boundaries, leap days, and daylight-saving transitions, and draws the result on a 24-hour timeline so you can see your whole day at a glance.

The same tested calculation engine powers every tool on this site: the protocol pages, the fasting timer, the fasting window calculator, and the fasting schedule calculator, so a 16:8 window computed here is identical everywhere else. The engine is covered by a unit test suite, which is what stops two pages quietly disagreeing about the same fast.

Three ways to use this fasting calculator

People arrive at a fasting hours calculator with three different questions, and this tool answers all of them:

• "When can I eat?" is the forward calculation above. Enter your last meal and duration; get your fast end time, your full eating window, and your next fast start. This is the everyday use: you finished dinner, you want tomorrow's answer.
• "When should I stop eating?" is the reverse calculation. You know when you want your first meal (lunch at noon, breakfast after the gym) and need the deadline for tonight. The reverse calculator works backwards from the meal you're protecting.
• "Which schedule actually fits my life?" is the planning question. Instead of choosing a protocol first, tell the schedule planner your wake, sleep, work, and dinner times and let it generate every feasible window, with conflicts flagged in plain language.

Compare the common fasting protocols

These are the schedules people most commonly search for. The table shows plain facts; we don't score protocols or claim a longer fast is automatically better. Pick the one whose eating window fits your routine, or use the planner to work backwards from your actual day.

Custom durations are first-class too: enter any number from 1 to 168 hours for alternate-day patterns, 36-hour fasts, or anything in between. Durations of 24 hours or more have no daily eating window, and the calculator reports that honestly rather than forcing one.

Fasting and weight loss: what the evidence actually says

Most people searching for a fasting weight loss calculator want a number: "how much will I lose?" We will not invent one, and the research explains why. A network meta-analysis published in The BMJ in June 2025 pooled 99 randomised trials covering 6,582 adults and found that intermittent fasting strategies produced weight loss broadly comparable to continuous calorie restriction.

The detail is worth reporting properly, because it is usually quoted badly. One strategy, alternate-day fasting, did beat continuous calorie restriction, by an average of 1.29 kg. The authors point out that this fell short of the 2 kg threshold they had set for a difference that matters clinically, and they rated the certainty of the evidence across most outcomes as low to moderate. So the honest summary is that the single statistically detectable winner did not win by enough to change anyone's plans, and the comparison rests on evidence the authors themselves describe as uncertain.

The fasting window is a structure that helps some people eat less consistently. It is not a formula that converts hours into kilograms, which is why two people on identical 16:8 schedules can have completely different outcomes depending on what and how much they eat inside the window.

What a fasting calculator can honestly do is remove the friction: give you exact boundaries, a timer that tolerates real life, and a history that shows your actual consistency, because consistency is the one variable the research keeps pointing at. That is the entire product philosophy here: real arithmetic, no fake precision, and editorial standards that rule out invented statistics.

The two questions everyone asks next

Once the calculator has given you a time, two questions follow almost every time, and both have short answers.

Does sleep count as fasting time? Yes. A fast is continuous clock time and being asleep does not pause it. This is the reason a 16-hour fast is workable at all: with eight hours in bed inside the window, you are awake and fasting for about eight hours, usually the morning. Push your eating window later and that arithmetic changes, since less of the fast overlaps your sleep. The schedule planner reports the overlap in hours for any window you give it, which is a more useful comparison than the protocol name.

Can I drink coffee while fasting? Black coffee, plain tea and water contain effectively no calories, so they leave the arithmetic above untouched. Milk, sweeteners and anything blended into the cup do contain calories, and how many depends on how much you add. Where we differ from most pages answering this: no trial establishes a calorie threshold at which a fast stops counting, so any specific number you have been given was chosen rather than measured. Our what breaks a fast reference lists the common items with their actual calorie counts and leaves the judgment to you.

What this tool does (and doesn't) claim

This calculator computes time windows. It does not predict weight loss, "autophagy hours", or metabolic stages. Current human research doesn't support assigning exact biological transitions to specific clock times, and outcomes vary substantially between individuals. It also doesn't ask your age, weight, height, or goals, because none of those change the arithmetic of a fasting window, which is why there is no health questionnaire in front of a simple time question. Enter one time and one duration to get your schedule. The timer, planner and history are one tap away if you want them, and they report what actually happened rather than grading it.

References

This page makes only two claims that need evidence behind them, so the list is short. Both sources are linked in full so you can check what they say rather than take our word for the summary.

• Semnani-Azad Z, et al. Intermittent fasting strategies and their effects on body weight and other cardiometabolic risk factors: systematic review and network meta-analysis of randomised clinical trials. The BMJ, June 2025, doi:10.1136/bmj-2024-082007. Read the full text. The source for the 99 trials, 6,582 participants, the 1.29 kg alternate-day fasting difference, and the authors' own rating of low to moderate certainty.
• Effects of time-restricted eating on weight loss and other metabolic parameters in women and men with overweight and obesity: the TREAT randomized clinical trial. JAMA Internal Medicine, 2020, doi:10.1001/jamainternmed.2020.4153. Read the full text. In 116 adults over 12 weeks, 16:8 time-restricted eating produced a 1.17% weight decrease against 0.75% in the control group, a difference that was not statistically significant.

Neither study supports predicting an individual's result from a fasting duration, which is why this calculator does not try. Our editorial standards set out what we will and will not publish, and the medical disclaimer lists tool by tool what each calculator never claims.