Intermittent Fasting Calculator

It's an eating pattern that alternates between a fasting window and an eating window, rather than a diet that specifies what to eat. This calculator covers daily protocols like 16:8, 18:6, 20:4 and OMAD, which split the 24-hour day, plus the weekly 5:2 protocol, which restricts calories on two non-consecutive days instead of following a daily schedule.

12:12 or 16:8 — both are the gentlest entry points, and 16:8 in particular is the most-studied time-restricted eating protocol in human trials. It's also the easiest to fit into a normal day, commonly by skipping breakfast and eating from around noon to 8pm. More restrictive protocols like 18:6, 20:4 or OMAD are generally attempted only after a daily eating window feels comfortable, not as a starting point.

Not on its own. Meta-analyses of time-restricted eating generally find weight loss in the same range as standard calorie restriction, not a unique metabolic edge from the fasting window itself — if the eating window is filled with enough calories to exceed maintenance, narrowing it further doesn't change that. The main practical benefit for many people is adherence: a restricted window can be easier to sustain than counting calories at every meal.

5:2 doesn't use a daily fasting and eating window at all — you eat normally five days a week and restrict intake to roughly 500-600 calories on two non-consecutive days. That's meaningfully different from a full 24-hour fast, since you're still eating on the restricted days, just far less. Because it works on a weekly rather than daily schedule, the eating/fasting-window fields on this calculator don't apply to it.

Anyone with a current or past eating disorder, since structured restriction can be harmful regardless of protocol; anyone pregnant or breastfeeding; people with type 1 or insulin-treated type 2 diabetes, since changing meal timing changes medication timing and hypoglycemia risk; and anyone on medication that must be taken with food or on a consistent schedule. In all of these cases, talk to a clinician before changing your eating pattern, rather than adjusting it on your own.