⏱️ Intermittent Fasting Tracker & Timer
Track your daily intermittent fast with live countdown timer, 16:8, 18:6, 20:4 protocol presets, and biological metabolic stages (Ketosis, Autophagy).
What Intermittent Fasting Tracker & Timer Does
This is a fasting clock. Enter the time your fast started, pick a protocol — 16:8, 18:6, 20:4 or 14:10 — and it counts elapsed time against the target, shows progress, and names the metabolic stage the elapsed hours correspond to. It runs in the browser with no account and nothing to install.
The elapsed time is derived from the start time you enter, so it survives a reload and reflects a real fast rather than a counter that began when the page loaded. A start time later than the current time is read as yesterday evening, which is how most overnight fasts are actually entered.
About the stage labels: they are the part of every fasting app most likely to be wrong, and they have been rewritten here to match what the evidence supports. The shift from glucose to fat metabolism is well documented and typically happens somewhere between 12 and 36 hours without food. The claim that autophagy switches on at a specific hour is an extrapolation from animal and cell-culture work — it cannot be observed in a person without a tissue sample, and the timing in humans has not been established. This page says so instead of printing a number.
How to Use Intermittent Fasting Tracker & Timer
- Choose your preferred fasting protocol (16:8, 18:6, 20:4, or 14:10)
- Start the timer when you finish your last meal
- Monitor your live progress bar and current metabolic phase (Glycogen, Ketosis, Autophagy)
Formula Used by Intermittent Fasting Tracker & Timer
Elapsed fast and eating window
elapsed = now − start (rolled back a day if start is later than now); eating window = 24 − fasting hours
- start
- The clock time your last calories were consumed
- fasting hours
- The first number in the protocol — 16 in 16:8
Worked example
A 16:8 fast begun at 20:00, checked at 12:00 the next day.
- 20:00 is later than 12:00, so the start belongs to yesterday
- From 20:00 to midnight: 4 hours
- From midnight to 12:00: 12 hours
Result: 16 hours elapsed — the fast is complete, and the eating window runs from 12:00 to 20:00. Note that roughly eight of those sixteen hours were spent asleep, which is why 16:8 is the protocol most people can actually keep.
What the protocols mean
fasting hours : eating hours, summing to 24
- 14:10
- The gentlest on offer here, and close to what a conventional dinner-to-breakfast gap already produces
- 20:4
- The most restrictive here; all calories in a four-hour window
Worked example
The same 20:00 finish, across all four protocols.
- 14:10 — eat from 10:00 to 20:00
- 16:8 — eat from 12:00 to 20:00
- 18:6 — eat from 14:00 to 20:00
- 20:4 — eat from 16:00 to 20:00
Result: Each step tightens the window by two hours at the front. Pushing the start later is the usual failure point, because it moves the first meal past the point where most people are hungry enough to overeat at it.
What Happens Across a Fast, and How Well It Is Established
The honesty column is the one other fasting timers do not print. Timings for the metabolic switch follow the NEJM review cited below; the autophagy row reflects that human timing has not been established.
| Elapsed | What is described | How well established in humans |
|---|---|---|
| 0–4 h | Digestion and absorption; insulin elevated | Well established |
| 4–12 h | Blood glucose and insulin fall; liver glycogen is drawn on | Well established |
| 12–36 h | Metabolic switch: glycogen depletes, fuel shifts toward fatty acids and ketones | Well established, but the hour varies with the last meal, activity and individual physiology |
| 14–18 h | Ketone levels rising | Measurable, and highly variable between people |
| 18 h+ | Autophagy and other cellular repair pathways upregulated | Demonstrated in animals and cell culture. The timing in fasting humans is NOT established and cannot be measured without a tissue biopsy |
Protocols and Their Eating Windows
Shown against a 20:00 finish, the most common anchor because it puts the fast across the night.
| Protocol | Fast | Eating window | Example window | Hours typically asleep during the fast |
|---|---|---|---|---|
| 14:10 | 14 h | 10 h | 10:00 – 20:00 | ~8 |
| 16:8 (Leangains) | 16 h | 8 h | 12:00 – 20:00 | ~8 |
| 18:6 | 18 h | 6 h | 14:00 – 20:00 | ~8 |
| 20:4 (Warrior) | 20 h | 4 h | 16:00 – 20:00 | ~8 |
What the Randomized Evidence Shows
A 12-month randomized trial in 139 patients with obesity, published in the New England Journal of Medicine in 2022, comparing calorie restriction with and without an 8-hour eating window.
| Question | Finding |
|---|---|
| Did time-restricted eating produce more weight loss than calorie restriction alone? | No significant difference between the groups |
| Were both groups restricted on calories? | Yes — both followed the same calorie target; only the window differed |
| What does that imply for the window itself? | The benefit observed in practice is most plausibly explained by eating fewer calories, which a shorter window tends to cause |
| Does that make fasting useless? | No. It makes the window a tool for controlling intake rather than a separate metabolic lever |
How to Read Your Result
The window is a way of eating less, not a metabolic trick
This is the finding that most changes how to read a fasting timer. When calories were matched between groups in a year-long randomized trial, adding an eight-hour eating window produced no significant extra weight loss. Restricting when you eat reliably restricts how much you eat, which is a genuinely useful behavioral effect — it is simply not a separate mechanism operating on top of calorie balance. A timer that fills up is evidence you kept to a schedule, not evidence that something metabolically special occurred.
Why the autophagy number on every app is not a measurement
Autophagy is a real and important cellular recycling process, and fasting does upregulate it — in animals, where researchers can examine tissue. In a living person it cannot be observed without a biopsy, there is no blood marker that reliably tracks it, and no trial has established a clock time at which it begins or peaks. The specific hour figures circulating in fasting apps are extrapolated from rodent studies at rodent metabolic rates. Treat them as a motivational graphic rather than a physiological readout.
What breaks a fast, in practice
Water, black coffee and plain tea contain effectively no calories and do not meaningfully interrupt a fast. Anything with sugar, milk, cream or sweeteners that provoke an insulin response does, and that includes most of what people add to coffee without thinking of it as eating. The common confusion is between "no calories" and "no metabolic effect" — for the purpose this tool serves, keeping the window clean of calories is the practical rule.
Who should not use a schedule like this
Fasting protocols are not appropriate for everyone and this tool applies no screening. That includes people who are pregnant or breastfeeding, anyone with a history of disordered eating, people with type 1 diabetes or on insulin or sulfonylureas where the hypoglycaemia risk is real, children and adolescents, and anyone underweight or on medication that must be taken with food. A clock cannot know any of that. Speak to a clinician before starting if any of it applies.
Limitations & Accuracy Notes
- A timer, not a tracker. It does not store history, log completed fasts, chart a streak, sync across devices, or send notifications — the paid fasting apps do all of that.
- Nothing is saved. Reloading keeps the fast because elapsed time is computed from the start time you entered, but closing the page loses the setting.
- One fast at a time, and four fixed protocols. There is no custom ratio, no 5:2 or alternate-day pattern, and no OMAD preset.
- No calorie, macro or food logging, so it cannot tell you whether the eating window actually reduced intake — which, on the trial evidence, is the part that matters.
- The metabolic stage labels are descriptive and approximate. Where a fast puts any individual on that timeline depends on the last meal's size and composition, activity, sleep and individual metabolism.
- No health screening of any kind, and no medical advice. It counts hours.
Frequently Asked Questions
What is the most popular intermittent fasting schedule?
When does Autophagy occur during a fast?
What do 16:8 and 5:2 actually mean?
Does fasting work better than just eating less?
Does coffee or tea break a fast?
Who should not try intermittent fasting?
Is the timer accurate if I close the tab?
Is this medical advice?
References & Further Reading
- Liu D et al. — Calorie Restriction with or without Time-Restricted Eating in Weight Loss, N Engl J Med 2022;386:1495-1504 — The 12-month randomized trial behind the evidence table: time-restricted eating added no significant weight loss over calorie restriction alone
- de Cabo R, Mattson MP — Effects of Intermittent Fasting on Health, Aging, and Disease, N Engl J Med 2019;381:2541-2551 — Review describing the metabolic switch from glucose to fatty-acid-derived ketones, typically 12 to 36 hours after food intake ceases
- Anton SD et al. — Flipping the Metabolic Switch, Obesity 2018;26:254-268 — Detail on glycogen depletion and the shift to fat oxidation that the 12–36 hour row in the table describes