A free intermittent fasting timer
Fasting is a free intermittent fasting timer that runs in your browser. Pick 16:8, 18:6, 20:4 or OMAD, tap Start, and a ring fills toward your goal while the page shows roughly what your body is doing. End the fast when you eat; it goes in your history and, if you hit the goal, your streak.
No account and no app to install. The start time is saved on your device, so the timer is still right after you close the tab or restart your phone.
How to use it
- Pick a plan. 16:8 is the most common place to start.
- Tap Start after your last meal. Forgot? Start now and move the start time back.
- Check back whenever you like. Turn on the goal alert to get a notification when you get there.
- Tap End fast when you eat. The page then shows your eating window and when your next fast should start.
16:8, 18:6, 20:4 and OMAD
| Plan | Fast | Eating window | Example |
|---|---|---|---|
| 16:8 | 16 hours | 8 hours | Eat noon to 8 PM |
| 18:6 | 18 hours | 6 hours | Eat 1 PM to 7 PM |
| 20:4 | 20 hours | 4 hours | Eat 3 PM to 7 PM |
| OMAD | About 23 hours | About 1 hour | One meal around 6 PM |
With Plus you can set a custom plan of any length from 1 to 72 hours, like 14:10 for an easier start. Fasts longer than a day are a different thing; talk to a doctor before you try one.
What happens while you fast, carefully
Most websites print an exact timeline. The honest version is fuzzier. After a meal your body runs on the sugar from it. Over the following hours insulin falls and your liver releases stored sugar (glycogen). Researchers describe a "metabolic switch", when the body leans more on fat and starts making ketones, that usually happens somewhere from about 12 to 36 hours after eating, depending on what and how much you ate and how active you are (Anton and colleagues, Obesity, 2018). The stages in the app are a rough guide to that, not a measurement.
On results: a review in the New England Journal of Medicine (de Cabo and Mattson, 2019) describes promising effects in animals and early human studies. In people, trials that compared time-restricted eating with ordinary calorie cutting found similar weight loss (Liu and colleagues, NEJM, 2022), and a trial of 16:8 without other changes found little effect (Lowe and colleagues, JAMA Internal Medicine, 2020). So the best plan is the one you can keep.
Who should talk to a doctor first
Talk to a doctor first if you are pregnant or breastfeeding, have diabetes (especially if you take insulin or other medicine that lowers blood sugar), have or have had an eating disorder, are under 18, or take medicine that has to be taken with food. Stop and get help if you feel faint, confused or unwell.
Free, and what Plus adds
| Feature | Free | Plus |
|---|---|---|
| Timer, ring, stages, goal alert | Yes | Yes |
| Plans | 16:8, 18:6, 20:4, OMAD | + custom, 1 to 72 hours |
| Streak | Yes | Yes |
| History | Last 7 days | Everything |
| Insights and CSV | No | Average fast, goal rate, longest, hours per week |
| Sync across devices | No (saved in this browser) | Yes |
Tensorust Plus is $2.99 a month or $14.99 a year, and the yearly plan starts with a 7-day free trial. One subscription covers Fasting and Focus, Habits, Sleep and Water. It renews until you cancel, and you can cancel anytime from the Plus page.
Questions
What is 16:8 fasting?
16:8 means you fast for 16 hours and eat within an 8-hour window each day, for example eating from noon to 8 PM. Most of the 16 hours are overnight. 18:6 and 20:4 shorten the eating window to 6 and 4 hours, and OMAD (one meal a day) is about 23 hours of fasting.
Is this fasting timer free?
Yes. The timer, the 16:8, 18:6, 20:4 and OMAD presets, the progress ring, the stages, the streak and the last 7 days of history are free with no sign-up. Tensorust Plus ($2.99 a month or $14.99 a year) adds your full history with insights, custom plans of any length, and sync.
Does the timer keep running if I close the tab?
Yes. The start time is saved on your device, so the timer is right whenever you come back, even after a restart. You can also move the start time if you forgot to tap Start.
Does intermittent fasting help you lose weight?
It can, mostly because a shorter eating window often means eating less. In trials that matched calories, time-restricted eating did not beat ordinary calorie cutting: a 2022 study in the New England Journal of Medicine found no extra weight loss, and a 2020 trial in JAMA Internal Medicine found 16:8 alone did little. Use the plan you can keep.
When does the body start burning fat or making ketones?
It varies. Researchers describe a "metabolic switch" from burning sugar to burning more fat and making ketones that usually happens somewhere from about 12 to 36 hours after your last meal, depending on what you ate and how active you are. The stages on this page are approximate, not a lab result.
What about autophagy?
Autophagy is the way cells break down and recycle old parts. Fasting triggers it in animal studies, and you will see claims online that it starts at a certain hour. That timing has not been measured reliably in people, so we do not show an autophagy hour.
Who should not fast?
Talk to a doctor before fasting if you are pregnant or breastfeeding, have diabetes (especially on insulin or other medicine that lowers blood sugar), have or have had an eating disorder, are under 18, or take medicine that has to be taken with food.
Can I drink water while fasting?
Yes. Water is fine and important while fasting. Black coffee and plain tea have almost no calories, and most plans allow them. Our free water tracker can keep count.
Sources
- Anton SD, Moehl K, Donahoo WT, et al. Flipping the metabolic switch: understanding and applying the health benefits of fasting. Obesity 2018;26(2):254-268.
- de Cabo R, Mattson MP. Effects of intermittent fasting on health, aging, and disease. New England Journal of Medicine 2019;381:2541-2551.
- Liu D, Huang Y, Huang C, et al. Calorie restriction with or without time-restricted eating in weight loss. New England Journal of Medicine 2022;386:1495-1504.
- Lowe DA, Wu N, Rohdin-Bibby L, et al. Effects of time-restricted eating on weight loss and other metabolic parameters: the TREAT randomized clinical trial. JAMA Internal Medicine 2020;180(11):1491-1499.
This page is general information, not medical advice.
Pair it with a habit
We do not have a Fasting iPhone app. To make fasting stick, add it to Habits as a daily habit, and use Focus to get through the afternoon slump.
Keep going
- FocusA focus timer with eyes that watch you work. Free.
- HabitsOne-tap habit tracker with streaks and a year quilt. Free.
- SleepSleep sounds you can mix, a fade-out timer and a sleep log. Free.
- WaterA water tracker that fills up, with gentle reminders. Free.
- Tensorust PlusFull history and insights, custom plans, sync.
- Free toolsCalculators and converters, no sign-up.