FastWiselyIntermittent fasting is less about strict rules and more about rhythm: a stretch of hours when you don't eat, followed by a window when you do. The free calculator above does the clock math for you. Choose a schedule, enter when you finished your last meal and you'll instantly see when your next eating window opens, when it closes and how long you have to go, with a countdown that updates every second.
What the calculator shows you
- Fast started: the moment your last meal ended.
- Next meal: the time your fasting hours are complete and your eating window opens.
- Window closes: when to finish eating so the next fast can begin on schedule.
- Countdown: a live timer until you can eat again.
- Timeline estimate: if you add a current and goal weight, a cautious range based on 0.5–1 lb per week.
Choosing a method
Schedules are written as fasting hours followed by eating hours. 12:12 is a gentle introduction that many people already follow without noticing. 14:10 adds a little challenge. 16:8 is the most widely used option and fits neatly around a late breakfast and an early dinner. 18:6 and 20:4 are more demanding and better suited to people who already fast comfortably. If you aren't sure, start short. Adding an hour later is easy; recovering from an overly ambitious first week is harder.
How to read your results
The "next meal" time is a target, not a deadline. If you're genuinely unwell or light-headed, eat. On the other hand, a little hunger in the final hour is normal and usually passes. Many people find it helpful to plan the first meal in advance so they break the fast with something balanced rather than whatever is closest.
Fitting fasting into a real schedule
The most sustainable plan works around the parts of your life you care about. If family dinner is important, let that be your last meal and push your first meal later. If you train in the morning, consider an earlier window so you can refuel afterwards. Weekends can flex by an hour or two without undoing your progress; consistency over weeks matters far more than any single day.
Making the hours easier
Hydration is the simplest lever: keep water or plain tea nearby throughout the fast. Protect your sleep, since short nights tend to increase appetite the next day. Fill your eating window with protein, vegetables, whole grains and healthy fats so you stay satisfied. And track a few fasts with our timer so you can spot patterns, such as which days are hard and which hours drag.
A note on expectations
Fasting changes when you eat; it doesn't override what you eat, how much you move or how well you sleep. That's why our estimate is conservative and clearly labeled. Use it as a planning tool, check in with a healthcare professional if you have any medical conditions, and judge progress over months rather than days.
Combining the tools
The calculator is a good starting point, and the other FastWisely tools fill in the rest. Use the fasting timer to log real fasts and see how close you get to your target. The meal plan generator suggests balanced meals that fit your window, and the water intake calculator gives you a daily hydration target, which is useful because you miss the water normally found in food. If you're unsure which method to pick, our one-minute quiz looks at your routine, sleep and experience and suggests a sensible place to begin.