Fasting Timer Countdown
Free live fasting timer that counts down to your eating window for 16:8, 18:6, 20:4, OMAD, or custom fasts, with metabolic phase milestones.
Fasting Timer Countdown
Fasting protocol
Min 1, max 72. Extended fasts over 24 hours hide the eating-window close time.
When did you start fasting?
If the time you pick is later than right now, we assume the fast started yesterday so the countdown stays positive.
Current metabolic phase
Eating window
Fasting phases by hour
Metabolic phase timings (fat burning, ketosis, autophagy) are approximate and vary with your diet, activity, and individual metabolism. This tool is general wellness information, not medical advice. Talk to a clinician before fasting if you are pregnant, nursing, under 18, managing diabetes or an eating disorder, or on medication.
How to use the fasting timer countdown
This is a live intermittent fasting timer. It counts down to your eating window in real time and shows where you are in the metabolic phase timeline. No signup, and everything runs in your browser.
- Pick a protocol: 16:8, 18:6, 20:4, OMAD, or Custom (Custom reveals an hours field).
- Set the time you started fasting. If you tap "Start now" it uses the current time; if you enter a time later than now, the tool assumes you started yesterday.
- Read the live countdown, your eating-window open and close times, the progress bar, and the metabolic phase you are in.
Fasting stages by hour: what happens during your fast
As the hours add up, your body changes how it fuels itself. The phases below match the in-tool timeline. The ranges are approximate and depend on your last meal, activity, and metabolism, so treat them as rough markers rather than promises.
- 0 to 4 hours (fed / anabolic): your body is digesting and using food for energy. Blood sugar and insulin rise.
- 4 to 12 hours (blood-sugar drop): insulin falls and glycogen stores begin depleting.
- Around 12 hours (fat burning begins): glycogen is mostly spent and the body turns to stored fat, the shift often called the metabolic switch.
- Around 16 hours (autophagy ramps up): cellular cleanup and recycling signaling increases.
- Around 18 hours (early ketosis): detectable ketones (about 0.5 mmol/L) start to appear.
- Around 24 hours (deeper ketosis / autophagy): ketones fuel most cells and autophagy is meaningfully elevated.
- 48 to 72 hours (peak autophagy): maximum cellular repair, with growth hormone elevated, on extended fasts.
If you want the underlying research, Harvard Health, Johns Hopkins Medicine, and Michigan Medicine all publish balanced overviews. None of this is medical advice.
Choosing a protocol: 16:8 vs 18:6 vs 20:4 vs OMAD
16:8 is the most popular and the easiest to start with: 16 hours fasting, an 8 hour eating window, and enough room for two or three meals. 18:6 and 20:4 tighten the eating window to 6 and 4 hours, which can deepen the fasted state but takes more planning. OMAD (one meal a day) fasts roughly 23 hours and leaves a single 1 hour window, which works for people who prefer one big meal. The longer the fast, the further into the phase timeline you get, but the harder it is to hit your nutrition targets in such a short window.
Build the fasting habit into a 75-day challenge
Fasting works best as a daily streak, not a one-off. Folding it into a 75-day challenge alongside workouts, water, and reading gives the habit a structure that tends to stick. Reset75 lets you log a daily fasting checkbox next to the rest of your routine, with progress photos and stats and no account required. Pair this timer with the water intake calculator for hydration during the fast, the macro calculator to plan the eating window, and the calorie deficit calculator if weight loss is the goal.
Frequently Asked Questions
Common questions about fasting timer countdown
How does the fasting timer countdown work?
Pick a protocol (16:8, 18:6, 20:4, OMAD, or custom), enter the time you started fasting, and the tool counts down in real time to the moment your eating window opens. It recalculates every second from your start time, so the HH:MM:SS countdown, progress bar, and metabolic phase all stay accurate even if you leave the tab and come back.
When does fat burning start during a fast?
Fat burning tends to pick up around 12 hours into a fast, once your body has mostly used up its glycogen stores and starts pulling from stored fat (this shift is sometimes called the metabolic switch). It is a rough average and changes with your last meal, activity, and your own metabolism. General information only, not medical advice. Johns Hopkins Medicine has a helpful primer on how intermittent fasting works.
How long until I reach ketosis when fasting?
Measurable ketones (around 0.5 mmol/L) often show up near 18 hours, and many people hit deeper ketosis around 24 hours. The timing leans heavily on your diet (carbs especially), how active you are, and your metabolism, so treat these hours as rough markers. General information only, not medical advice.
When does autophagy begin?
Autophagy is your body's cellular cleanup process, and it is thought to pick up around 16 hours of fasting and keep climbing through about 24 hours. Peak activity often gets cited in the 48 to 72 hour range on extended fasts. Most of the human evidence is indirect, so these windows are approximate. General information only, not medical advice. Harvard Health covers the research in plain language.
What is the difference between 16:8, 18:6, 20:4, and OMAD?
The numbers split each 24 hour day into fasting hours and eating hours. 16:8 means 16 hours fasting and an 8 hour eating window. 18:6 is 18 hours fasting with 6 hours to eat. 20:4 is 20 hours fasting with a 4 hour window. OMAD (one meal a day) is roughly 23 hours fasting with a single 1 hour window. The longer the fast, the less time you have to eat, so it takes more planning.
What time should I start my fast for a 16:8 schedule?
A common 16:8 setup is to finish dinner around 8 PM and break the fast around noon the next day, which puts the eating window from noon to 8 PM. Aim for a start time that lands your eating window during your most social or active hours. Try a few different start times in the timer and see exactly when your window opens and closes.
Can I drink water, coffee, or tea during the fast?
Plain water, black coffee, and unsweetened tea are generally fine during a fast and will not break it for most people, since they have little to no calories. Adding sugar, cream, or milk brings in calories that can interrupt the fasted state. General information only, not medical advice.
Is intermittent fasting safe, and how long should I fast?
Plenty of healthy adults handle intermittent fasting well, and protocols like 16:8 are a common place to start. It is not right for everyone, though: people who are pregnant or nursing, under 18, managing diabetes or an eating disorder, or on certain medications should check with a clinician first. General information only, not medical advice. Michigan Medicine and Harvard Health both have balanced overviews.