Intermittent Fasting Timer: 16:8, 18:6, 20:4 and OMAD

Use the timer above for today's fast, or plan the same eating window for every day and put it in your calendar. Below: what each schedule means, where to start, who should skip fasting and what the research shows.

16:8, 18:6, 20:4 and OMAD: fasting schedules compared

The first number is the hours you fast, the second the hours you eat, and together they make 24. So 16:8 intermittent fasting means 16 hours without food and an 8-hour eating window, for example from 12 PM to 8 PM. OMAD (one meal a day) is about 23:1: one meal inside roughly an hour.

Your night counts. If you stop eating at 8 PM and sleep 8 hours from 11 PM, you've already fasted 11 hours when you wake up (12 if you sleep from midnight). On 16:8, that leaves about 4 or 5 waking hours to go.

12:12 and 14:10 are softer steps on the way to 16:8. Other forms, like 5:2 (two low-calorie days a week) and alternate-day fasting, work by the week instead of the day, so they aren't in this timer.

Which fasting schedule should you start with?

Start with 12:12 or 14:10 and move to 16:8 only once that feels easy. A stricter schedule isn't required for results: the research below finds small differences between fasting styles. The schedule you can keep every day beats the strict one you drop after a week.

Our suggestion for a calm build-up (an example, not a medical plan):

1. Week 1: 12:12

   Stop eating after dinner and wait until breakfast: a normal day with no late snack.

2. Week 2: 14:10

   Push breakfast back two hours, or eat dinner a little earlier. Keep the same times every day.

3. Week 3 and on: 16:8

   Only if week 2 felt fine. If you have a health condition or take medication, talk to your doctor before this step.

Scale back, or stop, if you notice strong hunger, headaches, poor sleep, dizziness or thoughts that circle around food all day. Going back to 12:12 is not failing.

Who should not try intermittent fasting

Intermittent fasting isn't for children and teens, people who are pregnant or breastfeeding, or anyone with a history of an eating disorder. If you have diabetes, especially type 1, or take insulin, other diabetes drugs or blood pressure medication, fasting can drop your blood sugar or blood pressure too far. Only try it with your doctor.

Stop and eat if you feel dizzy or faint or your heart races, and stop if fasting turns into ever stricter rules or binges. Eating disorders can affect anyone, at any weight, says the NIMH. If food feels hard right now, the ANAD helpline (888-375-7767) is staffed by trained volunteers; it isn't a crisis line. In a crisis, call or text 988, the 988 Suicide and Crisis Lifeline, any time.

This tool is for general information and tracking only. It is not medical advice.

What time should you eat on 16:8? Sample schedules

Pick the 8 hours that fit your work, family dinners and training. Most people choose between an early window (8 AM to 4 PM), a middle one (10 AM to 6 PM) and a late one (12 PM to 8 PM). No single window is best for everyone.

The late window keeps dinner with family or friends. Not sure if you're a morning or evening person? The Chronotype Quiz is a quick way to find out. And if a late window ends close to bedtime, the sleep cycle calculator helps you plan the night around it.

Earlier eating may have a small edge for some people. In a 2022 trial in JAMA Internal Medicine, Humaira Jamshed and colleagues had 90 adults with obesity cut calories; half of them ate only from 7 AM to 3 PM. After 14 weeks the early group had lost more weight than the group that ate over 12 hours or more. That's one trial, it combined timing with eating less, and it didn't compare an early window with a late 8-hour one.

Picked a window? Set it in the planner and press Add to calendar for a daily reminder when it opens and closes.

What can you drink while fasting?

Water, black coffee and plain tea without sugar or milk are fine during a fast for most people. Milk, cream, sugar and drinks with calories break it. Zero-calorie sweeteners and diet soda sit in between: sources disagree and the evidence is thin, so it depends on why you fast.

• Fine while fasting. Water, sparkling water, black coffee, plain tea, herbal tea.
• Depends on your goal. Diet soda, zero-calorie sweeteners, a splash of milk in coffee.
• Breaks your fast. Milk, cream, sugar, juice, smoothies, soda, alcohol, protein shakes.

Go easy on coffee late in the day, when caffeine can keep you up. And drink enough, since part of your usual water comes with food. The water intake calculator gives you a daily target.

Does intermittent fasting work? What research says

Intermittent fasting can help you eat less and lose some weight, but in head-to-head trials it works about as well as ordinary calorie cutting, not better. Its edge is practical: some people find a clock easier to follow than a calorie count.

The biggest review so far, a 2025 network meta-analysis in The BMJ by Zhila Semnani-Azad and colleagues, pooled 99 randomized trials with 6,582 adults. All fasting styles and plain calorie restriction lowered weight compared with eating freely. Only alternate-day fasting beat steady calorie cutting, by about 1.3 kg (roughly 3 pounds) on average. The authors rated the certainty of most findings low to moderate.

The TREAT trial (Dylan Lowe and colleagues, 2020) tested 16:8 without other diet advice in 116 adults with overweight or obesity. After 12 weeks the 16:8 group had lost 0.94 kg (about 2 pounds) and the three-meals group 0.68 kg (about 1.5 pounds): no significant difference between the groups.

Adding a time window to a diet didn't do much more either. In a 2022 trial in the New England Journal of Medicine, Liu and colleagues had 139 adults with obesity in China cut calories for 12 months; half of them also ate only between 8 AM and 4 PM. The window group lost about 8.0 kg and the calories-only group 6.3 kg, a difference that wasn't significant, and body fat and metabolic risk factors didn't differ either.

Is 16:8 or 18:6 better? There's no strong direct comparison, and how fasting works out over years is still not well studied.

Why this timer doesn't show "autophagy at 16 hours"

Many fasting timers show a "stage" for each hour: fat burning at 12 hours, ketosis, autophagy at 16. Those hour marks are estimates. They differ a lot between sources and have mostly not been measured directly in people, so this timer only shows time: halfway, 12 hours and your goal.

What's broadly accepted is gentler: after some hours without food, your body leans more on stored energy, including fat. How fast depends on you, your last meal and how active you are.

Autophagy, the cell's cleanup process, is real. But much of the research on fasting and autophagy comes from animals and cells, and how long a person would need to fast for a meaningful effect isn't known.

Fasting timer website or fasting app: which do you need?

For a fixed daily schedule, a web timer plus calendar reminders is often enough. A fasting app is handier if you want push notifications, a long history or a link with other health data. Many fasting apps charge weekly or yearly, so check what stays free before you sign up.

If you want an app, check these first:

• The price and the kind of subscription, and what stays free.
• What happens to your health data, and whether you can delete it.
• Whether it works offline and its reminders arrive on time.
• Whether it makes hour-by-hour body claims it can't back up.

How this timer works

The timer saves the moment your fast began and works out the difference with the current time on every update, so it stays right when you close the tab, lock your phone or put your laptop to sleep. Your eating window opens at your start time plus your fasting hours and stays open for 24 minus your fasting hours. Start at 8:00 PM on 16:8 and you eat from 12:00 PM to 8:00 PM the next day. End early and it shows how long you fasted, with no judgment, and keeps the same closing time. On the night the clocks change, a 16-hour fast is still exactly 16 hours; only its end on the clock shifts. What it can't do: browsers slow timers in background tabs (Chrome can hold them to once a minute) and can't alert you once closed, so put your schedule in your calendar. Your fast lives only in this page's address. Nothing is sent to us.

Make your eating window a daily routine

Fasting works best with fixed times: the same last meal every evening, or the same first meal every morning. Consistency matters more than a longer fast, so pick a window you can keep on a busy Tuesday and on a Saturday night out, and hold it for a few weeks before you change anything.

In Greatr you can add your own habit, such as "Kitchen closes at 8 PM", as a step in your evening routine, or "First meal at 12" in your morning routine. Every day you keep it adds to your streak and earns XP. A missed day is just a missed day: tomorrow is a new one.

How long until it feels automatic? In a 2010 study of 96 volunteers, Phillippa Lally and colleagues found it took a median of 66 days for a new daily habit to feel automatic, with a range from 18 to 254 days. Tonight, pick your kitchen-closes time and set it in the planner. If you plan a walk after your first meal, the steps to miles converter shows how far it takes you.