Intermittent Fasting Timer for 16/8, 18/6 and OMAD

Tracking an intermittent fast and its metabolic stages

This timer measures the time elapsed since your fast began and compares it with the target duration of the protocol you selected. It shows progress on a dial, the time remaining, and the metabolic stage matching the hours already completed. The count is stored in your browser, so closing the tab or reloading the page does not reset it.

The five protocols offered

• 14/10 beginner: 14 h fasting, 10 h eating window. The gentlest way to start.
• 16/8 Leangains: the standard protocol, sustainable day to day.
• 18/6: advanced version, 18 h fasting for a 6 h eating window.
• 20/4 warrior: 20 h fasting for a 4 h eating window.
• 24 h (Eat-Stop-Eat): a 24-hour fast from one dinner to the next, once or twice a week.

The stages shown and their thresholds

The stage changes at fixed hours counted from the start of the fast: under 4 h digestion and glucose storage, 4 to 8 h insulin falls and stored energy starts to be released, 8 to 12 h gluconeogenesis, the liver produces glucose, 12 to 16 h possible switch to fat, depending on liver stores (12 to 36 h), beyond 16 h extended fast, ketone bodies rising. These thresholds are average teaching markers: the real pace depends on your last meal, its carbohydrate content, physical activity and individual insulin sensitivity. In humans, the switch from carbohydrate to fat and ketone bodies usually happens 12 to 36 hours after the last meal, depending on liver glycogen stores and physical activity during the fast, so at 16 hours it may not have started yet. The tool shows no hour for autophagy: the sources cited below give none for humans.

How to use it

Start the timer when you stop eating. If you forgot, enter the time of your last meal in "I stopped eating at", tick "Yesterday" if it was the day before, then start from that time: the time elapsed since that meal is counted straight away and replaces the current count. A time that has not come yet is refused. Switching protocol only changes the target duration: the time already elapsed is kept, and only the reset button clears the counter. While the timer is running, it also shows the expected end time. When the elapsed time reaches the target, a "Goal reached" panel shows the time spent beyond it and, while the timer is running, the time at which the goal was reached; the counter keeps going until you pause or reset it. Pausing keeps the time already accumulated rather than discarding it. The timer measures only the length of the fast, not what you eat during the feeding window.

Caution

Intermittent fasting is not suitable for everyone. It is not advised during pregnancy or breastfeeding, with a history of eating disorders, with diabetes or blood sugar problems (the risk of hypoglycaemia is higher with insulin or sulfonylureas), or under the age of 18.

Method and sources

The 12 to 36 hour window for the metabolic switch comes from a review published in Obesity: Anton et al., 2018 (PMID 29086496). On weight, a network meta-analysis of 99 randomised trials in 6,582 adults (Semnani-Azad et al., BMJ, 2025) found that every form of intermittent fasting led to weight loss compared with unrestricted eating, roughly as much as continuous calorie restriction: only alternate-day fasting did slightly better (1.29 kg more on average), and in trials lasting 24 weeks or more the only benefit seen was that of these diets over unrestricted eating. The list of people who should avoid intermittent fasting follows Johns Hopkins Medicine. This timer is a tracking tool: it does not replace advice from a doctor, which you should seek before starting, especially if you take any medication. References as of 22/10/2026.

Frequently Asked Questions

Does the timer keep running if I close the tab?

Yes. The start time and the time already accumulated are saved in your browser's local storage; when you reopen the page, the elapsed time is recalculated from the clock. The count is lost if you clear the site's data or switch browser or device.

Does the timer stop when the target is reached?

No. The dial caps at 100% and a "Goal reached" panel appears with the time spent beyond the target and, while the timer is running, the time at which the target was reached. The counter keeps going until you pause or reset it, and no notification is sent. Switching protocol changes the target without clearing the elapsed time.

I forgot to start the timer: can I start it afterwards?

Yes. Enter the time of your last meal in "I stopped eating at", tick "Yesterday" if it was the day before, then choose "Start from this time". The time elapsed since that meal is counted straight away and replaces the current count; it is saved like a normal start and survives a page reload. A time that has not come yet is refused.

After how many hours does the stage shown change?

The thresholds are fixed: under 4 h, 4 to 8 h, 8 to 12 h, 12 to 16 h, then beyond 16 h. They are the tool's teaching benchmarks, not measurements; the real pace depends on your last meal and on each person.

How long does a usual overnight fast last?

According to the opinion of ANSES (the French food safety agency) of 6 March 2024 on the timing of food intake, the overnight fast lasts 11 h 22 min on average among adults in France (adults aged 18 to 79, Inca 3 survey of 2014-2015; time estimated between the last food intake of the evening and the first of the next day). A 14/10 protocol therefore extends it by about 2 h 40, a 16/8 by about 4 h 40.

Do health authorities recommend intermittent fasting?

In France, ANSES has not assessed intermittent fasting: its 2024 opinion deals with the overnight fast, for which it considers the data insufficient to conclude on the link between its duration and the risk of chronic disease. As a precaution, it only advises reducing the share of energy eaten in the evening and having dinner about two hours before bedtime. Talk to a health professional before starting, especially if you are pregnant, have treated diabetes or an eating disorder.

Is this tool really free?

Yes, all tools on Kristools are 100% free and accessible without registration.

Is my data protected?

Yes. All processing is performed directly and securely inside your browser. No personal data is sent to our servers.