Intermittent Fasting Timer
A free intermittent fasting timer that tracks your fasting window live — pick 16:8, 18:6, 20:4 or OMAD, see elapsed time, your metabolic stage and projected end. Free, no signup.
How intermittent fasting timing works
Intermittent fasting doesn't change what you eat so much as when. During and shortly after a meal your body is in the fed state: blood sugar and insulin rise as food is digested and energy is stored. As the hours pass without incoming calories, insulin falls, your liver's glycogen stores draw down, and your metabolism shifts toward burning stored fat for fuel. Push further and the liver starts producing ketones, and cellular clean-up processes known as autophagy are thought to increase. A large review in the New England Journal of Medicine describes this "metabolic switch" from sugar-burning to fat- and ketone-burning as a central mechanism behind fasting's effects.
The stage timeline in this tool maps those phases onto rough hour marks — fat-burning building through the 8–12 hour range, ketosis ramping up toward 12–16 hours, and autophagy likely picking up around 16–18 hours. The important caveat: these hours are approximate. The exact timing depends on your last meal, how active you are, your muscle mass, and your individual metabolism, and much of the human evidence is indirect. Use the timeline as an educational guide to what's roughly happening, not a medical readout.
Why a visible timer helps you stick to your window
The hardest part of a fasting window is usually the last couple of hours, when hunger and boredom make "just eat now" feel reasonable. A running timer turns an abstract goal into something concrete: you can see exactly how far you've come, how little is left, and the projected time your window ends. That visible progress is a small commitment device — it's easier to hold on for 40 more minutes when a bar is nearly full than when the finish line is invisible. Pairing the timer with plain water or black coffee (see the best time to drink coffee) and having a plan for evening cravings (how to stop snacking) makes the window far easier to keep.
Where fasting really pays off, though, is repetition — one 16-hour fast does little, but a consistent window most days of the week is what adds up. The simplest way to make it stick is to treat "closed my eating window on time" as a daily yes/no habit. You can log that one-tap check-in in HabitBox, a free, ad-free habit tracker that runs fully on your device (no account needed) on iOS and Android — start the fast here, then tick off the day and watch the streak build. For more app options, compare intermittent fasting tracker apps.
Note: this timer and its stage timeline are educational estimates for generally healthy adults, not medical advice or a diagnostic tool. Intermittent fasting isn't suitable for everyone — if you're pregnant or breastfeeding, take medication, have a history of disordered eating, or have any health condition, talk to a clinician before fasting.
Frequently asked questions
What is a 16:8 fast?+
16:8 is the most popular intermittent fasting schedule: you fast for 16 hours and eat all your meals within an 8-hour window each day. For example, finishing dinner at 8 PM and not eating again until noon the next day gives you a clean 16-hour fast. Many people fold most of the fast into their overnight sleep, which makes it one of the easiest protocols to sustain.
When does autophagy start during a fast?+
Autophagy — your cells' natural recycling process — is generally thought to ramp up somewhere around 16 to 18 hours into a fast and increase further beyond 24 hours. The exact timing is highly individual and depends on your last meal, activity, and metabolism, and most human evidence is indirect. Treat the 16–18 hour marker on this timer as a rough guide, not a precise switch.
Can I drink coffee or water while intermittent fasting?+
Plain water, black coffee, and unsweetened tea contain essentially no calories and are widely considered fine during a fast — they can even help with hunger and focus. What breaks a fast is calories, so adding sugar, milk, cream, or sweeteners that spike insulin can end the fasted state. Stick to water, black coffee, and plain tea if you want to keep the timer honest.
Is intermittent fasting safe?+
For many healthy adults, time-restricted eating like 16:8 is generally well tolerated, but it isn't right for everyone. People who are pregnant or breastfeeding, have a history of disordered eating, are underweight, take medication such as insulin, or have a medical condition should be cautious. This is general information, not medical advice — talk to a clinician before starting, especially for longer fasts.
How do I start intermittent fasting?+
Start gently: pick an easy window like 12:12 or 14:10 for the first week or two, then extend toward 16:8 as it gets comfortable. Anchor the fast around your sleep so most of the hours pass overnight, stay hydrated, and eat normal, balanced meals in your eating window rather than restricting hard. Use this timer to see your progress in real time, and treat consistency over weeks as the goal rather than any single perfect fast.
What happens if I go past my goal fast length?+
Nothing breaks — the timer keeps counting up and simply shows that you've reached your target window. Some people naturally extend a 16-hour fast to 18 or 20 hours on days it feels easy. Just don't force long or extended fasts (24 hours or more) without medical guidance, and break your fast if you feel unwell, dizzy, or shaky.
Turn your fasting window into a daily streak
One fast barely moves the needle — a consistent window most days is what counts. HabitBox makes closing your eating window a one-tap daily check-in you can watch build into a streak. Free, on-device, no account, on iOS and Android.
Free to start · No account required · No ads