A real countdown that survives a closed tab, because the start time lives in your browser rather than on our server. Six phases are marked as you pass them: fed for the first 4 hours, catabolic to 8, fat burning to 12, ketosis to 16, deep ketosis to 24, extended fasting beyond. No app, no account, no sign-up.
There is no arithmetic to check here beyond a subtraction. You set a start time, the timer counts the seconds since it, and the elapsed figure is the difference between that moment and your device clock. The start time is kept in your browser's own storage, which is why closing the tab does not lose the fast; nothing is sent anywhere and there is no account behind it.
The phase labels are the site's fasting-zone model, and they are the same boundaries every other tool here uses so that two pages can never hand you different answers: fed for the first 4 hours, catabolic from 4 to 8, fat burning from 8 to 12, ketosis from 12 to 16, deep ketosis from 16 to 24, and extended fasting past 24. Those hours are population estimates read off the fasting literature, not measurements of you. How quickly you reach any of them depends on what your last meal was, how much of it there was, how active you have been and how often you fast — a low-carbohydrate last meal moves the whole set several hours earlier. Treat the labels as signposts on a road, not as a reading off an instrument.
Nothing on this site is medically reviewed — it cites its sources so you can check them. Read our editorial policy. Method last checked 21 August 2026.
Intermittent fasting is one of the simplest and most evidence-backed approaches to improving metabolic health, but knowing where you are in your fast makes a real difference. This free browser-based fasting timer shows your elapsed time, remaining time, and current metabolic phase so you can fast with confidence.
Your body transitions through five distinct phases during a fast: Fed State, Catabolic, Fat Burning, Ketosis, and Deep Ketosis. Each phase comes with different metabolic benefits. By seeing exactly when you cross each threshold, you can make informed decisions about when to break your fast and maximize the benefits you are after.
This timer runs entirely in your browser. Your data is saved in localStorage so it persists across page reloads. No account, no download, no tracking. For the full experience with streaks, history, and push notifications, try the Fasted app.
A detailed hour-by-hour timeline of what happens in your body during a fast -- from 0 to 72 hours. Understand the science behind every stage of fasting.
getting started · 8 min readA practical step-by-step guide to starting intermittent fasting. Choose your schedule, set your window, and build the habit.
fasting methods · 7 min readLearn how 16:8 intermittent fasting works, what the science says, and how to build a sustainable 16-hour fasting routine. Complete guide with meal timing tips.
Interactive fasting zones chart showing what happens to your body at every hour. See when fat burning, ketosis, and autophagy begin.
Search any food or drink and get a straight answer for three goals: strict fasting, fat loss, and autophagy. Calories, the reason, and the honest gray zones.
Calculate your ideal fasting and eating windows based on your schedule, sleep time, and lifestyle. Get a personalised daily timeline.
The Fasted app saves every fast automatically, builds your streak calendar, and shows detailed stats so you can see your consistency improve over time.
Free to use — no account required
