Free Intermittent Fasting Calculator
Choose a plan and enter when you start or finish eating. See when your fast ends and your next eating window opens.
Plan your fast
Your daily rhythm
16 hours fasting · 8 hours eating
- Eating window
- 7:00 AM – 3:00 PM
- Fasting window
- 3:00 PM – 7:00 AM (next day)
Schedule details, sharing and time checkpoints
⏱ Time Checkpoints
Your Fasting Timeline
Elapsed-time checkpoints for a 16-hour schedule. These do not measure metabolic changes.
Fast Started
+0hThe clock measures time since your last calories. It does not measure digestion or metabolic changes.
📄 Mayo ClinicEarly Fasting
+2hDigestion and nutrient absorption vary with the meal and the individual.
📄 NIH — Metabolism OverviewBetween Meals
+4hYour body uses a mixture of fuels. Time alone cannot identify your blood glucose or insulin level.
📄 PubMed — Insulin & Glucose DynamicsOvernight Interval
+8hAn overnight interval may include sleep. Glycogen use varies with diet and activity.
📄 Cahill GF Jr — NEJM 197012 Hours Elapsed
+12hTwelve hours is a time checkpoint, not a diagnosis of ketosis. Ketone production varies between people.
📄 Cahill GF Jr — NEJM 197016 Hours Elapsed
+16hSixteen hours does not establish autophagy activation. There is no validated universal human autophagy timer.
📄 Levine & Kroemer — Cell 2008Time checkpoints are educational, not a recommendation to extend your fast.
If I finish dinner at 8 PM, when can I eat again?
Count from the end of your meal. Here’s how three daily plans work out if you finish eating at 8 PM on Monday.
| Plan | Tuesday’s first meal | Eating window |
|---|---|---|
| 14:10 | 10 AM | 10 AM–8 PM |
| 16:8 | Noon | Noon–8 PM |
| 18:6 | 2 PM | 2–8 PM |
Already know both meal times? Find the fasting schedule that fits your lunch and dinner.
All Tools
Every Fasting Protocol Covered
Most popular — fast 16h, eat 8h
Open Calculator →Intermediate — fast 18h, eat 6h
Open Calculator →Advanced — fast 20h, eat 4h
Open Calculator →One Meal A Day — 23h fast
Open Calculator →2 restricted days per week
Open Calculator →Cross-midnight support
Open Calculator →Suhoor to Iftar windows
Open Calculator →Understand evidence limits
Open Calculator →FAQ
Frequently Asked Questions
What is intermittent fasting?
Intermittent fasting (IF) is an eating pattern that cycles between periods of fasting and eating. It focuses on when you eat, not what you eat. Common protocols include 16:8 (fast 16 hours, eat within 8), 18:6, and OMAD.
When does fat burning start during fasting?
Your body uses both carbohydrate and fat for energy. Fuel use and ketone levels vary with meals, activity, and individual physiology; elapsed fasting time does not confirm ketosis.
When does autophagy begin?
Autophagy is cellular recycling. Human research has not established a universal fasting duration that activates or maximizes it. A timer cannot measure this process.
Is intermittent fasting safe?
For most healthy adults, IF is safe. It is NOT recommended for pregnant women, those with a history of eating disorders, people with diabetes (without medical supervision), or children. Always consult your doctor before starting.
Does this calculator store my data?
Calculator inputs are processed in your browser. Timer state is saved locally. Optional analytics, hosting requests, and contact emails have separate data implications described in our Privacy Policy.
Medical Disclaimer
This tool provides estimates for informational purposes only. Always consult a licensed healthcare provider before beginning any fasting protocol — especially if you are pregnant, have a history of eating disorders, diabetes, or any other medical condition.
Sources: Mayo Clinic · NIH · PubMed
Live fasting timer
Planned fast
16:00:00
16h fasting · 8h eating
Fasting window —
Eating estimate —
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