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Intermittent Fasting Window Calculator

Plan your 16:8, 18:6, or 20:4 fasting and eating windows around your schedule, workouts, and meals for sustainable fat loss.

Time-restricted eating schedules explained

Intermittent fasting is an umbrella term for eating patterns that cycle between periods of eating and not eating. Time-restricted eating compresses all daily food intake into a window of several hours, typically 4 to 10 hours, with the remainder spent fasting. The most common schedule is 16:8, meaning 16 hours fasting and 8 hours eating, often framed as skipping breakfast and eating between noon and 8 pm. Tighter windows include 18:6 and 20:4, while the extreme end is OMAD or one meal a day at roughly 23:1. Neuroscientist Mark Mattson published an influential 2017 review in the New England Journal of Medicine summarizing evidence that time-restricted eating can improve insulin sensitivity, blood pressure, LDL cholesterol and markers of inflammation, largely through metabolic switching between glucose and ketone fuel. This calculator helps you pick a window based on lifestyle and work schedule rather than dogma.

Does intermittent fasting work for weight loss?

Intermittent fasting can produce meaningful weight loss, but the mechanism is primarily calorie restriction, not metabolic magic. Most randomized trials comparing time-restricted eating to isocaloric continuous energy restriction show similar weight loss when protein and total calories are matched. The benefit for many people is adherence: eating within a shorter window tends to reduce total intake by 200 to 400 kilocalories per day without conscious tracking, simply because there is less time to consume food. Some additional benefits have been documented for insulin sensitivity and markers of cellular stress response, though evidence in humans is still mixed. Downsides include loss of social meals, potential for overeating at the first meal if fasts are too aggressive, and incompatibility with some medications or medical conditions. Pregnant women, people with a history of eating disorders and those on insulin should consult a clinician before starting.

Frequently Asked Questions

Is 16:8 better than 18:6 or OMAD?▼
For most people no. The best window is the one you can sustain consistently over months. Longer fasts like OMAD show no clear metabolic advantage over 16:8 in controlled trials, and they increase risk of undereating protein and micronutrients, poor sleep and social friction. Start at 12:12, progress to 14:10 over two weeks, then 16:8 if comfortable. Only experiment with tighter windows after at least a month of successful 16:8 practice.
Can I drink coffee or tea during the fasting window?▼
Yes, plain coffee, tea and water are fine during a fast. Small amounts of cream or a splash of milk, perhaps 20 kilocalories or less, are unlikely to meaningfully disrupt metabolic benefits. Sweeteners, even zero calorie ones, can trigger insulin response in some people and are best avoided. Bone broth and electrolyte drinks are acceptable on longer fasts. Any caloric intake above roughly 50 kilocalories technically ends the fast.
Will fasting slow my metabolism?▼
Not within typical time-restricted eating windows. Short-term fasts up to 72 hours actually increase resting metabolic rate slightly via catecholamine release. Metabolic adaptation becomes relevant with prolonged severe calorie restriction regardless of meal timing, so the concern applies to how much you eat more than when. If you are losing weight rapidly for weeks, some adaptation occurs, but this is calorie-driven rather than fasting-driven.
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