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Free Fasting Calculator
Calculate your 16:8, 18:6, OMAD or custom fasting window in seconds. See live biological milestones. Get calorie targets. 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
⏱ Fasting Stages
What Happens Inside Your Body
Live biological stages during your 16-hour fast — each backed by peer-reviewed research.
Last Meal Completed
+0hYour fasting clock starts now. Digestion begins and your body processes the glucose from your last meal.
📄 Mayo ClinicDigestion Phase
+2hBlood glucose peaks and insulin is released. Your body is actively digesting and storing nutrients from the last meal.
📄 NIH — Metabolism OverviewBlood Sugar Stabilizing
+4hInsulin levels begin to drop. Blood sugar returns toward baseline. Your body shifts from storing to maintaining.
📄 PubMed — Insulin & Glucose DynamicsGlycogen Depleting
+8hLiver glycogen stores are running low. Your body begins mobilising free fatty acids as an alternative fuel source.
📄 Cahill GF Jr — NEJM 1970🔥 Ketosis Begins
+12hGlycogen stores are nearly depleted. Your liver starts producing ketone bodies from fat — your brain switches to fat-burning mode.
📄 Cahill GF Jr — NEJM 1970✨ Autophagy Activates
+16hCellular housekeeping begins. Your cells start breaking down damaged proteins and organelles — a key anti-aging mechanism.
📄 Levine & Kroemer — Cell 2008* Biological timelines are approximate and vary by individual. Sources: PubMed, NIH, Cell journal.
⏱ 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.
Science-Backed
Every claim linked to PubMed, Mayo Clinic, or NIH.
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 →Maximize cellular repair
Open Calculator →The Science
What Happens Inside Your Body During a Fast
Most apps just give you times. We show you why those times matter — with live biological milestones backed by peer-reviewed research.
⏱ 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 begins depleting glycogen stores around 8 hours into a fast. Ketosis — where your body switches to burning fat — typically begins at 12–14 hours. Source: Cahill GF Jr, NEJM 1970.
When does autophagy begin?
Autophagy (cellular self-cleaning) activates at approximately 16–18 hours of fasting. This process removes damaged proteins and organelles. Source: Levine & Kroemer, Cell 2008.
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?
No. All calculations run entirely in your browser. We do not collect, store, or transmit any personal information. Your data stays on your device.
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