Fasting Calculator

Most people find 16:8 the easiest entry point — a 16-hour fast with an 8-hour eating window, which in practice usually means skipping breakfast and eating between roughly midday and 8pm. It requires no change to what you eat, only when, and the overnight sleep period covers half the fasting window. If 16 hours feels difficult at first, 14:10 is a legitimate starting point and still produces meaningful results; there is nothing magic about the number 16. Build up gradually over two to three weeks rather than jumping straight to a long fast, since the initial hunger and energy dip usually settle within that time as your body adjusts to the new meal timing.

For fat loss specifically, the evidence suggests intermittent fasting works about as well as continuous calorie restriction — not better, not worse — when total calorie intake is matched. Controlled trials comparing the two consistently find similar results over periods of several months. What fasting does offer some people is a practical advantage: a shorter eating window naturally limits snacking and makes calorie control easier without counting anything. That is a genuine benefit, but it is a behavioural one rather than a metabolic one. If you find it easier to eat within a window, fasting is a good tool for you; if it makes you binge in the evening, conventional portion control will serve you better.

Water, plain black coffee and unsweetened tea are all fine and will not meaningfully affect a fast. Staying well hydrated is genuinely important, particularly in the morning when many people mistake thirst for hunger. What breaks a fast is calories: milk in coffee, sugar, juice, and anything sweetened will trigger an insulin response. Artificial sweeteners are debated and the evidence is mixed, so if you are strict about it, keep drinks plain. Black coffee in particular tends to blunt appetite, which is why many people find morning fasting easier with it than without.

Extended water fasting — going beyond roughly 24 hours with only water — carries real risks and is not something to attempt casually or based on an online calculator. Potential complications include electrolyte imbalance, dangerous drops in blood pressure, fainting, cardiac arrhythmia, and in people refeeding after a long fast, refeeding syndrome, which can be life-threatening. Risk is substantially higher for anyone with diabetes, an eating disorder history, low body weight, cardiovascular conditions, or who is pregnant, breastfeeding, or taking medication that requires food. Multi-day fasts should only be undertaken with medical supervision. This calculator covers time-restricted eating and single-day fasts; it is not a protocol for extended fasting.

Short fasting windows of 16–24 hours are unlikely to cost you meaningful muscle, particularly if you keep resistance training and hit adequate protein during your eating window. The commonly cited fear that muscle breaks down after a few hours without food is not supported by the evidence — the body preferentially uses stored glycogen and fat over that timescale. The genuine risks come from two directions: very aggressive calorie deficits sustained over months, and inadequate protein intake. Aim for roughly 1.6–2.2 g of protein per kilogram of bodyweight spread across the meals in your window, keep lifting, and muscle retention during fasting-based fat loss is comparable to any other approach.

Weight loss depends on the calorie deficit you create, not on the fasting schedule itself. A deficit of roughly 500 calories per day produces about 0.5 kg of fat loss per week, which is a sustainable and commonly recommended rate. Fasting helps only insofar as it makes that deficit easier to maintain. Expect a faster drop in the first week or two — a substantial portion of that is water and glycogen rather than fat, since each gram of stored glycogen holds several grams of water. That initial drop is not a sign that fat loss is happening at that rate, and the pace settling down afterwards is normal rather than a plateau.

Fasting is not appropriate for everyone. Avoid it, or seek medical guidance first, if you are pregnant or breastfeeding, under 18, have a history of disordered eating, have type 1 diabetes or take insulin or other glucose-lowering medication, are underweight, or take medication that must be consumed with food. People with type 2 diabetes can sometimes fast safely but require medical supervision because of hypoglycaemia risk. If you experience dizziness, persistent fatigue, heart palpitations or difficulty concentrating during a fast, stop and eat — those are signals worth listening to rather than pushing through.

In the first four to eight hours your body is still digesting and absorbing your last meal, running primarily on that glucose. Between roughly eight and twelve hours, blood glucose falls and the body increasingly draws on stored liver glycogen. Around twelve to sixteen hours, glycogen stores are becoming depleted and fat oxidation rises noticeably. Beyond sixteen to twenty-four hours, ketone production increases as fat becomes the dominant fuel source, and autophagy — the cellular recycling process often cited as a benefit of fasting — is thought to increase, though the human evidence on timing is much less precise than popular accounts suggest. These are approximate transitions rather than sharp switches, and they vary considerably with your last meal, activity level and individual metabolism.