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Find your eating window for intermittent fasting

Choose an intermittent fasting protocol and the time of your first or last meal. We show your window on a 24-hour clock, suggested meal times, a live timer and a plan for the whole week.

Before you start: this is general information, not medical advice.

If you're pregnant or breastfeeding, have diabetes treated with medication or insulin, have a history of eating disorders, are under 18, are underweight or take medications that depend on food, don't fast without medical supervision.

01 Choose your protocol

02 Your schedule

I want to calculate from
Time of your first meal
You can also drag the start of your window on the clock.
Meals in your window
Time format

03 Live tracking

Your fasting timer

It's saved on this device: you can close the page and come back later.

Not started yet

Press “Start now” when you finish your last meal.

What may happen in your body · hours since your last meal

    Illustrative, approximate timeline. Timing varies with what you ate, your physical activity, your metabolism and your health. Human evidence on ketosis and autophagy during intermittent fasting is limited.

    04 Your weekly plan

    Monday to Sunday

    Consistency matters more than perfection. If you shift your window by an hour on the weekend, that's fine.

    05 Before you start

    What you should know

    What breaks a fast

    Generally doesn't break it

    • Plain or sparkling water
    • Black coffee, no sugar
    • Unsweetened tea or herbal tea
    • Water with a pinch of salt or sugar-free electrolytes

    Breaks it

    • Milk, cream or plant milks in your coffee
    • Sugar, honey or syrups
    • Juice, smoothies and regular soda
    • Broth, collagen or protein drinks
    • Alcohol

    Zero-calorie sweeteners and sugar-free gum are a gray area: they provide almost no energy, but they trigger hunger in some people.

    Tips that actually work

    • Start with 12:12 and add an hour every few days or every week. Jumping straight to 20:4 usually ends in hunger and binges.
    • Stay hydrated all day. Water, black coffee or unsweetened tea help you manage hunger in the first few weeks.
    • Watch your electrolytes on fasts of 18 h or longer: a pinch of salt in your water can prevent headaches and dizziness.
    • Prioritize protein in your window: include a good portion at every meal to protect your muscle mass and go into your fast less hungry.
    • Train close to your eating window, ideally before your first meal or between meals, so you recover better.
    • Sleep well. Short sleep increases hunger and cravings; a good part of your fast happens while you sleep.

    Common mistakes

    • Making up for it with binges or ultra-processed food in your window: fasting doesn't cancel out the calories you eat.
    • Skipping water or drinking only coffee: it dehydrates you and makes headaches worse.
    • Starting with the toughest protocol or switching protocols every week.
    • Ignoring signs like severe dizziness, palpitations or weakness: if they show up, break your fast.
    • Fasting to “earn” your food or feeling guilty about breaking it early. If that happens often, talk to a professional.

    Who shouldn't do it without medical supervision

    • Pregnancy or breastfeeding
    • Diabetes treated with insulin or other glucose-lowering medication
    • History of eating disorders (anorexia, bulimia, binge eating)
    • Under 18 years old
    • Underweight or malnutrition
    • Medications that must be taken with food or that affect blood pressure or glucose
    • Chronic conditions (kidney, heart, liver) or recent surgery

    If you feel severe dizziness, confusion, palpitations or weakness, break your fast and seek medical care. This calculator is for information only and is not a substitute for advice from a doctor or dietitian.

    01How it works

    Three steps and done

    1. 01

      Choose your protocol

      If you've never fasted, start with 12:12 or 14:10. 16:8 is the most popular. 18:6, 20:4 and OMAD are for people with experience.

    2. 02

      Set your schedule

      Enter the time of your first or last meal. See your window on the clock, drag it if you want to move it and choose how many meals to have.

    3. 03

      Start and track

      Start the timer when you finish your last meal, check which stage you're in and add your plan to your calendar with alerts to open and close your window.

    02How it works

    Fasting isn't magic, it's timing

    Intermittent fasting is an eating schedule, not a specific diet: you alternate a period without food (the fast) with a window in which you have your meals. On 16:8, for example, you fast for 16 hours and eat within 8. A good part of the fast happens while you sleep.

    If you lose weight with fasting, it's mostly because you eat fewer calories: with fewer hours to eat, it's easier to eat less. Clinical trials comparing it with a diet of the same calories found similar weight results. That's why what you eat in your window matters as much as the schedule.

    During a fast your body switches fuel sources. In the first hours it uses what you ate; after that, your liver releases glucose from its glycogen stores. When those stores run low, usually after 12 hours or more, fat use increases and small amounts of ketone bodies are produced.

    Autophagy, the recycling of components inside your cells, is the most talked-about part and the least clear. Animal studies show it increases with prolonged fasting, but in humans no one knows precisely when it starts or how much it changes with a 16 to 24 hour fast. Be wary of anyone promising exact timelines.

    For many people the biggest advantage is practical: fewer decisions, less late-night snacking and a clear structure. It isn't better than other ways of eating for everyone. If it makes you anxious, leads to binges or makes you feel unwell, it's not for you, and that's okay.

    • Interactive 24-hour clock

      Your eating window and your fast on a circle. Drag the start of the window to move it.

    • Seven protocols

      12:12, 14:10, 16:8, 18:6, 20:4, OMAD and 5:2, each with its difficulty level so you can choose calmly.

    • Live timer

      A countdown with a progress ring that's saved even if you close the page, plus an optional alert when you're done.

    • Fasting stages

      An illustrative timeline of what may happen in your body, written carefully and without promises.

    • Weekly plan and calendar

      Your schedule from Monday to Sunday and an .ics file with alerts to open and close your window.

    • Meals, summary and link

      Two or three suggested meal times, a summary to copy and a link with your settings.

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    04FAQ

    What people ask us

    Which intermittent fasting protocol should I try if I've never done it?
    Start with 12:12, which for many people just means not eating late at night or very early in the morning. After one or two weeks move to 14:10, then to 16:8 if you feel good. The 18:6, 20:4 and OMAD protocols are more demanding and aren't necessary to get results.
    Does coffee break a fast?
    Black coffee without sugar generally doesn't break it, just like water and unsweetened tea. Milk, cream, sugar, syrups and drinks with calories do. If coffee on an empty stomach gives you heartburn or jitters, have it within your eating window instead.
    How much weight can you lose with intermittent fasting?
    It depends on how many calories you eat in your window, your physical activity and your starting point. Studies show intermittent fasting works about the same as a diet with the same calories, with no magic advantage. Be wary of promises of an exact number of pounds per week.
    Can I work out while fasting?
    Many people train well while fasted, especially with moderate exercise. For intense or strength workouts it's usually better to train close to your eating window so you can have protein afterward. If you get dizzy or lose strength, change your workout time.
    Will I lose muscle if I fast?
    The risk goes up if you eat little protein or don't do strength training. To protect your muscle mass, include a good portion of protein at every meal in your window and train with weights or bodyweight at least two or three times a week.
    What happens if I break my fast early?
    Nothing serious. Go back to your usual schedule the next day. If you feel severe dizziness, weakness or discomfort, break your fast without guilt. Consistency over weeks matters much more than one perfect day.
    Who shouldn't do intermittent fasting?
    People who are pregnant or breastfeeding, have diabetes treated with insulin or other glucose-lowering medication, have a history of eating disorders, are under 18, are underweight or take medications that must be taken with food shouldn't do it without medical supervision. This calculator is for information only and is not a substitute for medical advice.
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