Intermittent Fasting Calculator

How Intermittent Fasting Windows Work

Intermittent fasting splits your day into an eating window, when you consume all your calories, and a fasting window, when you don't eat at all (though water, black coffee, and plain tea are generally fine). Protocols are usually named after the split — 16:8 means 16 hours fasting and 8 hours eating, 18:6 means 18 fasting and 6 eating, and so on. None of these require eating at specific clock times; what matters is the length of the gap between your last meal and your next one.

Because the schedule is relative to when you stop eating, not a fixed clock time, your personal window shifts depending on when your day actually ends — which is exactly what this calculator works out for you.

How to Use This Calculator

Enter the time you plan to finish eating today, and choose your fasting protocol (or enter a custom number of fasting hours). The calculator shows you exactly when your fasting window ends and your next eating window opens, along with the total hours in each.

How We Calculate It

We take your last meal time and add your protocol's fasting duration to find the moment your fast ends and your eating window begins. The eating window then runs from that time until your next planned last meal, giving you the remaining hours in a 24-hour day (24 minus your fasting hours).

A Few Notes

This calculator only handles the timing side of intermittent fasting — it doesn't calculate calories or tell you whether a given protocol is right for you. Longer fasting windows aren't automatically better; the right length depends on your goals, how your body responds, and simple day-to-day practicality. If you have diabetes, a history of disordered eating, are pregnant or breastfeeding, or take medication that requires food, talk to a doctor before starting any fasting protocol.