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Free FastingCalculator

Intermittent Fasting Calculator

Enter your last meal time, choose a fasting duration, and get your complete schedule: when your fast ends, your full eating window, and when the next fast begins. A free fasting calculator that runs privately in your browser: no account, no app, no upload.

Fasting duration

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, thefasting timer, thefasting window calculator, and thefasting schedule calculator, so a 16:8 window computed here is identical everywhere else. One engine, one source of truth, no inconsistent answers between pages.

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. Thereverse 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 theschedule 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.

ProtocolFastingEatingExample window
12:1212h12h7:00 PM → 7:00 AM fast
14:1014h10h8:00 PM → 10:00 AM fast
16:816h8h8:00 PM → 12:00 PM fast
18:618h6h8:00 PM → 2:00 PM fast
20:420h4h8:00 PM → 4:00 PM fast
OMAD23h1h7:00 PM → 6:00 PM fast

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 won't invent one, and here is why. A 2026 network meta-analysis covering 167 trials and nearly 12,000 participants found that most intermittent fasting regimens produce weight loss comparable to continuous calorie restriction when the overall energy deficit is similar. Reviews repeatedly reach the same conclusion: the fasting window is a structure that helps some people eat less consistently, not a formula that converts hours into kilograms. 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, andeditorial standardsthat rule out invented statistics.

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, a refreshing difference from calculators that make you fill in a health questionnaire before answering a simple time question. Enter one time, pick one duration, get your schedule. If you want more, the timer, planner, and history are one tap away: free, private, and free of judgment.

Common questions

How does this intermittent fasting calculator work?

You enter the time of your last meal and choose a fasting duration: a preset like 16:8 or a custom number of hours. The fasting calculator adds the fasting hours to your last meal time to find when your fast ends, then shows your full eating window and when your next fast begins. Everything is computed in your browser; nothing is uploaded.

Is this fasting calculator really free?

Yes, completely. There is no premium tier, no account, no trial, and no locked features. The calculator, the fasting timer, the schedule planner, and your local history are all free, which is exactly what people searching for a "free intermittent fasting calculator" usually want and rarely get.

How do I calculate my fasting time?

Take your last meal time and add your chosen fasting hours: eating at 8 PM with a 16-hour fast means your fast ends at 12 PM the next day. The tricky parts are midnight crossings and date changes, which this calculator handles automatically. You can also work backwards from your first meal using the reverse calculator.

Which intermittent fasting schedule is best for weight loss?

Honest answer: research does not crown one schedule. Large reviews and meta-analyses comparing fasting regimens find that most produce similar weight-loss results when overall calorie intake is similar; adherence matters more than the exact window. The best schedule is the one that fits your sleep, work, and meals well enough that you actually keep it.

How much weight can you lose with intermittent fasting?

We deliberately do not predict a number, because no calculator honestly can. Studies show intermittent fasting can produce meaningful weight loss for some people, typically comparable to conventional calorie restriction, but individual results vary with intake, activity, and adherence. Any calculator that promises "X kg in Y weeks" from a fasting duration alone is guessing.

Does this calculator handle 36, 48, or 72-hour fasts?

Yes. The custom duration option accepts anything from 1 hour to 7 days, so it works as a 36 hour fast calculator, 48 hour fast calculator, or 72 hour fasting calculator. For fasts of 24 hours or longer there is no daily eating window, and the calculator says so plainly instead of inventing one. Extended fasting is also where the safety note matters most.

Is this a water fasting calculator?

It calculates the time windows for any fast, including water fasts of custom lengths. What it will not do is estimate weight loss from a water fast "by age" or otherwise. Those numbers cannot be honestly computed from a duration, and extended water fasting is exactly the situation where you should involve a healthcare professional first.

Do I need an account or app download?

No. This works in any modern browser on any device. Your fasting data is stored locally on your device, and you can export it as JSON or CSV, or delete it, at any time from the history page.

Can the calculator start a timer?

Yes. Every result includes a "Start fast" button that hands the exact schedule to the fasting timer, including a start time earlier than now if your last meal already happened. The timer survives closed tabs and lets you edit the start time if you forgot to press it.