About the Fasting Timer
This fasting timer tracks an intermittent fast in real time. Press start when you finish your last meal, pick a goal such as 16, 18 or 24 hours, and the timer shows how long you have fasted, how much is left, the clock time your fast ends and the approximate metabolic stage you are in. Your current fast and recent history are saved in your browser, so you can close the page and come back.
It suits anyone following time-restricted eating — 14:10, 16:8, 18:6, 20:4 or one meal a day — as well as people doing an occasional 24- to 36-hour fast. Below the live timer, a planner works out the end time and eating window for any start time and goal, and where you will be at a given number of hours.
Fasting stages are approximate guides: how quickly glycogen runs low and ketones rise depends on what you last ate, your activity and your metabolism. Fasting is not suitable for everyone — people who are pregnant or breastfeeding, under 18, have diabetes or take glucose-lowering medication, or have a history of eating disorders should speak to a doctor first.
How to use the fasting timer
- 1Choose a fasting goal, such as 16 hours for 16:8.
- 2Press Start when you finish your last meal (or adjust the start time if you began earlier).
- 3Watch the elapsed time, progress bar and current stage update live.
- 4Press End fast when you eat; the fast is saved to your history.
- 5Use the planner below to work out end times and eating windows in advance.
Formula and method
The end of your fast is simply your start time plus the goal length, rolling over to the next day (or later) when needed. For daily schedules shorter than 24 hours, the eating window is what is left of the day — a 16-hour fast leaves 8 hours, an 18-hour fast leaves 6. Progress is elapsed hours divided by the goal.
Stages use commonly cited approximate thresholds: roughly 0–4 hours fed, 4–12 hours early fasting, 12–18 hours increasing fat use as liver glycogen falls, 18–24 hours rising ketones, 24–48 hours extended fasting and beyond 48 hours prolonged fasting. Real timing varies from person to person, so treat them as a guide, not a measurement.
- start
- Date and time you finished your last meal
- goal
- Planned fast length in hours
- elapsed
- Hours fasted so far
Worked examples
16:8 fast starting at 8 pm
Finishing dinner at 20:00 on Monday 5 January and fasting 16 hours takes you to 12:00 on Tuesday, leaving an 8-hour eating window. Ten hours in, you are 62.5% of the way there with 6 hours left.
18:6 fast starting at 7:30 pm
An 18-hour fast that starts at 19:30 on 14 March ends at 13:30 the next day, giving a 6-hour eating window. At 15 hours you have 3 hours to go (83% done).
36-hour fast
Starting at 18:00 on Sunday 1 February, 36 hours later is 06:00 on Tuesday 3 February. Fasts of 24 hours or more have no daily eating window. At 30 hours you are in the extended-fast stage with 6 hours left.
Frequently asked questions
When does a fast start?+
Count your fast from the moment you finish your last meal or calorie-containing drink. Water, black coffee and plain tea are generally considered fine during a fast; anything with meaningful calories ends it.
What is the 16:8 fasting schedule?+
16:8 means fasting for 16 hours and eating within an 8-hour window each day — for example finishing dinner at 8 pm and eating again at noon the next day.
When does your body start burning fat during a fast?+
The body always burns some fat, but reliance on fat rises as liver glycogen falls, commonly after about 12 hours. Ketone levels typically start rising noticeably somewhere around 18–24 hours, depending on diet and activity.
Does the timer keep running if I close the page?+
Yes. The start time is saved in your browser, so the timer picks up where it left off when you return on the same device and browser. Clearing site data resets it.
Who should not fast?+
Pregnant or breastfeeding people, children and teens, people with a history of eating disorders, and anyone with diabetes or on glucose-lowering medication should not fast without medical advice.
This tool gives general estimates and is not medical advice. Talk to a doctor or qualified professional about your health, diet or training.