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

What is the Intermittent Fasting Timer?

Intermittent fasting is not a diet that tells you what to eat, it is an eating pattern that tells you when to eat. You split each day into a fasting window, when you take in only water, black coffee or plain tea, and an eating window, when all of your meals happen. The method is written as two numbers: the first is the fasting hours and the second is the eating hours, and they always add up to 24. So 16:8 means 16 hours fasting and an 8 hour eating window, while OMAD (one meal a day) is effectively 23:1. An intermittent fasting timer like this one exists because most people find it much easier to follow a schedule of clock times than to keep recalculating hours in their head every day.

The maths behind any intermittent fasting timer is simple. Take the time your last meal finishes, that is when the clock starts. Add the fasting hours to get the moment you can break the fast and your eating window opens. Add the eating hours to that, and you are back at the start of the next fast. Because the cycle is 24 hours, the eating window the next day lines up with the same clock times, which makes the routine easy to repeat. The calculator above does this arithmetic and rolls past midnight correctly, so a fast started at 8 PM under 16:8 ends at 12 PM (noon) the next day.

What actually happens inside your body while the timer counts down is where most of the interest in intermittent fasting comes from. In the first few hours after eating, your body burns the glucose from your last meal for energy. By roughly 8 to 12 hours in, glycogen stores in the liver start to run low and the body shifts toward burning stored fat for fuel. Research on longer fasts suggests that autophagy, the process where cells clear out and recycle damaged components, increases somewhere in the 14 to 18 hour range and continues to build the longer the fast runs, with growth hormone also rising well into a fast. These are general patterns reported in fasting research, not a guarantee of what happens at an exact hour for every person, since factors like your last meal size, activity level and metabolism all shift the timing.

Most beginners start with 16:8 because it is the gentlest: you simply skip breakfast, stop eating after dinner, and let sleep cover a big chunk of the fast. Longer fasts (18:6, 20:4, OMAD) shrink the eating window further, which some people find makes a calorie deficit easier to hit, but they are also harder to sustain and leave less room to meet your nutrition needs. There is also a milder end of the spectrum, 12:12 and 14:10, that some clinicians recommend as a starting point for women or anyone with a history of hormone sensitivity to calorie restriction, since aggressive fasting can disrupt menstrual cycles or spike cortisol in some women. Men are not exempt either: pairing very long fasts with a large calorie deficit for extended periods has been linked to lower testosterone in some studies.

Exercise timing is another question this intermittent fasting timer helps answer indirectly, once you can see your eating window on the clock. Light to moderate activity, such as walking or an easy cardio session, is generally well tolerated in a fasted state. Heavier strength training or high intensity work is often better scheduled close to or inside the eating window, so you have fuel available for the session and protein available afterward for recovery. The best schedule, in the end, is the one you can keep up consistently and that fits your work, sleep, training and social life, and that is exactly what setting a fixed start time in the calculator above helps you plan around.

When to use it

• Planning a 16:8 routine around your work day so the eating window covers lunch and dinner.
• Working out what time you can eat tomorrow when you finished dinner late tonight.
• Comparing how 16:8, 18:6, 20:4 and OMAD change the length and timing of your eating window.
• Setting a consistent last-meal cut-off time so your fasting window lines up the same way every day.
• Checking whether your usual bedtime and wake-up routine will land your eating window at a sociable time.
• Using a gentler 14:10 or 12:12 schedule to ease into intermittent fasting before trying longer windows.

How to use the Intermittent Fasting Timer

1. Pick your fasting method: 16:8, 18:6, 20:4 or OMAD.
2. Enter the time you start fasting, which is when your last meal ends.
3. Read off when the fast ends and your eating window opens.
4. Check the schedule table for the eating window close and the next fast start.
5. Save or note your times so the same intermittent fasting timer schedule repeats every day.

Formula & method

Worked examples

Common intermittent fasting methods and their windows

Example 16:8 eating windows by last-meal time

What research suggests happens by hour into a fast (approximate)

Common mistakes to avoid

• Counting the start time as when you sit down, not when you finish. The fasting clock starts when your last bite or sip of anything caloric is done, not when the meal begins. Starting the count too early makes you break the fast sooner than planned.
• Breaking the fast with calorie drinks. Coffee with milk, juice, soda or a sports drink contains calories and ends the fast. During the fasting window stick to water, plain black coffee or unsweetened tea.
• Jumping straight to long fasts. Starting with 20:4 or OMAD on day one often leads to intense hunger, low energy and giving up. Most people do better easing in with 12:12 or 16:8 and extending the window gradually.
• Eating anything you like in the window. Fasting controls timing, not quality. If the eating window is filled with ultra-processed food and you overeat, the benefits people are after may not show up.
• Ignoring hydration and electrolytes on longer fasts. The longer the fasting window, the more it matters to keep drinking water and, on 20:4 or OMAD schedules, to consider sugar-free electrolytes, since headaches and fatigue are often a hydration issue rather than a hunger one.
• Using the same aggressive schedule regardless of sex or life stage. Women, especially those with irregular cycles or hormone sensitivity, are often advised to start with a gentler 14:10 or 12:12 window rather than jumping to 16:8 or longer, and to adjust around heavy training days.

Glossary

Fasting window

The hours each day when you do not eat and take in only water, black coffee or plain tea.

Eating window

The hours when all of your meals and caloric drinks happen, the second number in the ratio.

16:8

A method with 16 hours of fasting and an 8 hour eating window, the most popular starting point.

OMAD

One Meal A Day, an advanced pattern with roughly a 23 hour fast and a single short eating window.

Time-restricted eating

The clinical name for limiting daily food intake to a set window of hours, the basis of methods like 16:8.

Autophagy

A cellular cleanup process where the body breaks down and recycles damaged components, reported to increase during longer fasting windows.

5:2 diet

A different style of intermittent fasting based on days rather than hours: eating normally 5 days a week and sharply reducing calories on 2 non-consecutive days.

Warrior diet

An informal name for a roughly 20:4 schedule, a long fasting window with one large eating period, often in the evening.

Frequently asked questions

Sources

• Intermittent Fasting: What is it, and how does it work? , Johns Hopkins Medicine
• Diet Review: Intermittent Fasting for Weight Loss , Harvard T.H. Chan School of Public Health