Intermittent Fasting Timer - 12:12, 16:8, 18:6, OMAD

How to use this tool

1. Choose a daily fasting schedule (12:12, 14:10, 16:8, 18:6, 20:4, or OMAD).
2. Set when your fast began or use the current time as its start.
3. The timer saves your schedule in this browser and shows both windows and a live countdown.

Common fasting protocols

The timer supports the daily schedules above from 12:12 through OMAD. The 5:2 diet uses reduced-calorie days rather than one daily eating window, and the 24- and 36-hour rows are planning references; they do not have timer presets here.

An educational fasting timeline

These broad time ranges describe common metabolic patterns, not switches that happen at exact hours. The timing can differ with the last meal, activity, and individual physiology. This timer counts elapsed time; it cannot measure blood glucose, ketones, or other changes in your body.

• 0–4 hours (fed state): Digestion and absorption from a recent meal continue; glucose is one of the fuels available to the body.
• 4–16 hours (post-absorptive): As absorption from the meal progresses, the body draws on stored fuels, including liver glycogen.
• 16–24 hours (gluconeogenesis): The body makes glucose from non-carbohydrate sources while fuel use continues to shift. The timing and degree of these changes vary.
• 24–48 hours (early ketosis): Ketone production may increase during longer fasts, but levels and timing differ. A timer cannot tell whether a person has entered ketosis or estimate processes such as autophagy.

Electrolytes during fasting

During fasts longer than 24 hours, maintaining electrolytes is important. Low insulin causes the kidneys to excrete more sodium, which can secondarily deplete potassium and magnesium. Symptoms of electrolyte imbalance include headache, fatigue, muscle cramps, and heart palpitations. A pinch of salt in water, a magnesium supplement, or a dilute broth can help prevent these symptoms without breaking the metabolic state of the fast.

How to break a fast

After a long fast, re-introduce food gradually. Start with small, easily digestible foods: bone broth, a small piece of fruit, fermented foods, or soft cooked vegetables. Avoid a large, high-carbohydrate meal immediately after an extended fast, as this can cause refeeding syndrome symptoms (blood glucose and insulin spike) and significant discomfort.

Exercise during fasting

Moderate exercise (walking, light cardio, yoga) is generally safe and well-tolerated in a fasted state. Some research suggests fasted cardio may increase fat oxidation. However, high-intensity training, heavy resistance exercise, and prolonged endurance sessions are better performed in a fed state for both performance and recovery. Listen to your body - dizziness or weakness during fasted exercise is a signal to eat or reduce intensity.

Coffee and tea during a fast

Black coffee and plain tea (without milk, sugar, or cream) typically do not break a metabolic fast. They contain negligible calories and some evidence suggests they may enhance the metabolic effects of fasting. However, strict interpretations of fasting (such as water-only fasts) exclude all beverages except water. Bulletproof coffee (with added fat) does break a fast from a caloric standpoint, even though it minimally affects insulin levels.