Fasting Window Calculator
Map out your eating and fasting windows so the day is planned before you are hungry.
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Learn the details
Intermittent fasting protocols are defined by the ratio of fasting to eating hours, like 16:8 (16 hours fasting, 8 hours eating), 18:6, and 20:4. The fasting period typically includes sleep.
Fasting usually gets sold as a switch that flips your metabolism. The honest version is quieter and more useful: it is a schedule that helps some people eat less without thinking about it, and does very little for the people it does not suit.
What fasting does and does not do
When time-restricted eating is compared against a normal eating pattern with calories and protein matched between the groups, fat loss tends to land in much the same place. That is the finding worth keeping. Fasting is not burning fat you would not otherwise burn. What it does is remove eating occasions: fewer chances to snack, fewer decisions, and for a lot of people a smaller total intake without counting a thing. That is a real advantage. It is an adherence advantage, not a metabolic one.
The common windows and who they suit
16:8 is the gentle entry point here, and where most people settle, because a good half of the fast is spent asleep and it mostly means closing the kitchen after dinner. 18:6 and 20:4 suit people with a steady appetite and a day that tolerates one main eating block. OMAD 23:1, one meal a day, makes hitting protein genuinely hard, so I rarely point people there. 5:2 works differently, five normal days and two very low ones, which fits anyone who would rather not think about timing most of the week.
A worked example
Choose 16:8 with a first meal at 12:00 PM. Your eating window runs 12:00 PM to 8:00 PM and the fast runs 8:00 PM through to 12:00 PM the next day. Sixteen hours sounds severe until you notice that seven or eight of them are sleep. In practice you are skipping breakfast and not eating after dinner. If you train in the morning, sliding the whole window earlier, say 9:00 AM to 5:00 PM, works better than forcing your food around a number.
What breaks a fast in practice
Calories break a fast. That is the workable rule. Water, black coffee, plain tea, and sparkling water do not. A splash of milk in your coffee technically does, and for fat loss purposes it does not matter. The endless online arguments are really about autophagy, and that work comes largely from animal and cell studies, so the human picture is far less settled than the confidence of the arguing suggests. Judge the rule by whether it keeps your intake where you want it.
The honest limits
Total intake and protein still decide the outcome. A short window with too much food in it is still a surplus, and a short window with too little protein still costs you muscle. Find your maintenance calories and your daily protein target first, then pick the window that makes hitting them easier. Be honest about fit, too. Some people feel steady and clear-headed fasting. Others get irritable, ravenous, and eat more at night than they saved all day. Neither response is a discipline problem. And if you are pregnant or nursing, live with diabetes or another condition, take medication that has to be taken with food, or have any history of disordered eating, talk to a doctor before changing when you eat. This is general information, not a plan for you.
Sources: de Cabo and Mattson 2019, NEJM
Start with the 16:8 window, the gentlest option here; black coffee and plain tea don't break a fast. Fewer eating hours often means fewer calories, and for most people that is the whole mechanism.