Intermittent Fasting Calculator

Intermittent fasting is not a diet in the sense of telling you what to eat. It is a schedule that tells you when. The most popular form, 16:8, means fasting for sixteen hours and eating within an eight-hour window. Others include 18:6, 20:4 and the gentler 14:10 and 12:12. The idea is that compressing your eating into a shorter window makes it easier to eat less overall and gives your digestive system a longer break.

Mechanically, the weight loss that often comes with it still reduces to calories. Many people eat less simply because a shorter window contains fewer meals and fewer opportunities to snack, and because late-night eating — often the most calorie-dense and least intentional food of the day — disappears. Studies comparing intermittent fasting with ordinary calorie restriction generally find similar weight loss when calories are matched, which is worth knowing before choosing a protocol for reasons other than convenience.

This calculator is about the logistics. Pick a protocol and the time of your last meal, and it lays out when your fasting window formally begins, when you may eat again the next day, and on which side of midnight each boundary falls. That is the part people get wrong: a 16-hour fast that starts at 8 PM does not mean you eat at noon the next day by accident — it means 12 PM, and if you sleep late it is easy to overshoot or undershoot without a plan.

A few sensible cautions. Fasting is not appropriate for everyone — it is generally not advised during pregnancy, for people with a history of eating disorders, for those with diabetes on glucose-lowering medication, or for anyone underweight. During the fast, water, plain tea and black coffee are fine. And a feeding window is not a licence: eating the same number of calories in eight hours has little advantage over eating them in twelve, so quality still matters. If you feel dizzy, weak or unwell, stop and eat.

This calculator returns estimates for general information only. It is not medical advice, diagnosis or treatment, and it cannot replace an assessment by a qualified health professional. Individual results vary, and figures such as calorie needs, body-fat percentage and heart-rate zones are approximations based on population averages rather than measurements of you.