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Free Fasting Calculator
Calculate your 16:8, 18:6, OMAD or custom fasting window in seconds. Track elapsed time. Get estimated calorie needs. No account, no paywall, ever.
Free · Instant · No sign-up required
Step 1 — Choose Your Protocol
16:8 is the most popular protocol — fast 16h, eat within 8h.
Step 2 — Personalise Your Schedule
Your eating window starts from this time
🎯 Your Goal
⏱ 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.
⏱ Live Timer
Track Your Fast in Real Time
Your progress saves automatically — even if you close this tab.
⏸
Ready
No account needed. Your timer is saved locally in your browser.
Why FastingCalc?
Built Different
We got tired of fasting apps charging $30/month and locking your own data behind a paywall. So we built this.
Permanently Free
No paywalls. No 7-day trial. Every feature, free forever.
Instant Results
Your schedule calculated the moment you touch the inputs.
Transparent Estimates
Schedule calculations with explanations and source links.
Night-Shift Support
The only calculator that handles cross-midnight fasting windows.
No Account Needed
Zero sign-up, zero data stored on our servers. Ever.
Calorie + Macro Targets
BMR-based nutrition targets for your specific fasting goal.
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 →The Science
What Happens Inside Your Body During a Fast
Our timeline explains elapsed fasting time and the limits of predicting biological changes from a clock.
⏱ Fasting Stages Timeline
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