You can find your fasting window for free by counting the hours between your last food or calorie-containing drink and your next one. Finish dinner at 8 pm and eat again at noon the next day, and your fasting time is 16 hours. You do not need a paid quiz, subscription, or wearable to calculate that interval.
Use the free live fasting timer to count a fast from now, or the 16:8 calculator to plan a start and finish time.
What is my fasting window?
Your fasting window starts after your last calorie-containing food or drink and ends when you next consume calories. Your eating window is the part of the day you set aside for meals. Neither number tells you exactly when your body enters a metabolic stage.
How to find your fasting window in three steps
- Record your last calories. Include snacks and drinks: a snack at 9 pm moves the start from an 8 pm dinner to 9 pm.
- Choose the interval you want to calculate. Add that duration to your start time, including the hours after midnight.
- Write down your next eating time. For a repeating daily schedule, eating-window hours equal 24 minus fasting hours.
Formula: planned fast end = last-calorie time + fasting duration. To measure a completed fast, subtract the start date and time from the end date and time.
Overnight fasting-window examples
These examples use an 8 pm finish time. They illustrate the arithmetic, not a recommendation to follow a particular plan.
| Schedule | Fasting window | Eating window |
|---|---|---|
| 12:12 | 8 pm to 8 am next day | 8 am to 8 pm |
| 14:10 | 8 pm to 10 am next day | 10 am to 8 pm |
| 16:8 | 8 pm to noon next day | Noon to 8 pm |
How to track your fasting time without paying
Press “Start Fasting Now” on the live timer when you finish eating, then “End Fast” when you eat again. The timer saves your start time in this browser. Return using the same browser and device; clearing browser storage removes the saved session.
Already started earlier? Calculate from your recorded last-meal time with a clock or meal log. The live timer currently starts from the moment you press its button, so it cannot reconstruct an earlier start for you.
Can a free calculator find my ideal fasting window?
A calculator can work out times, but it cannot determine the medically best schedule for you. Consider your meals, sleep, work, and any medication requirements with your clinician. There is no need to lengthen a fast just to reach a timer milestone.
Johns Hopkins Medicine advises discussing fasting with a doctor and identifies groups who should avoid it, including children, people who are pregnant or breastfeeding, people with type 1 diabetes taking insulin, and people with a history of eating disorders.
What can I drink during the fasting window?
For ordinary intermittent fasting, water and zero-calorie drinks are generally allowed. Check our sparkling water and Waterloo fasting guide for label checks and the distinction between a dietary fast and medical or religious fasting rules.
Frequently asked questions
How do I know my fasting time for free?
Count from your last food or calorie-containing drink to your next one. For example, 8 pm to noon the next day is 16 hours. A clock, a written meal log, or our free live fasting timer can track the time without a subscription.
How do I find my fasting window for free?
Write down when you finish eating, choose a fasting duration, and add those hours to your finish time. The result is your next planned eating time. For a daily schedule, subtract the fasting hours from 24 to find the eating-window length.
Is my fasting window the same as my eating window?
No. Your fasting window is the interval without food or calorie-containing drinks. Your eating window is the time set aside for meals. A 16:8 schedule has a 16-hour fasting window and an 8-hour eating window.
Does sleep count toward my fasting time?
Yes. Count overnight hours as long as you do not eat or have calorie-containing drinks during that interval.