Free Tool
Intermittent Fasting Timer
Free here • Paid in Zero, Fasted, Fastly (most charge for protocol picker + phase tracking)
Pick a protocol, hit start, and we will track your fast with a sound and notification when you hit your goal. The timer keeps running even if you close the tab. Nothing leaves your device.
New to intermittent fasting? Read the plain-English definition →
Pick a protocol
Fat loss + metabolic flexibility, well-studied
00:00:00
0% of goal
Ready
Fed state
Insulin elevated, glucose primary fuel, digestion ongoing.
Elapsed
0h 0m
Remaining
16h 0m
Metabolic phases
Approximate timing for most people. Your actual response depends on insulin sensitivity, last meal, and training.
Fed state
Insulin elevated, glucose primary fuel, digestion ongoing.
0h–4h
Post-absorptive
Blood glucose stabilizing, glycogen primary fuel.
4h–12h
Fat-burning (lipolysis)
Glycogen depleting, fat becoming primary fuel.
12h–16h
Ketosis
Ketone production rising. Brain switches partially to ketones.
16h–24h
Autophagy onset
Cellular cleanup processes increasing (Sinclair 2019; Mizushima 2011).
24h–36h
Deep autophagy + GH spike
Growth hormone peaks. Use under medical supervision.
36h+
Your local stats
Fasts completed
0
Longest fast
0h 0m
These stats are saved in this browser only. Switch devices or clear cookies, and they reset. To track fasts across devices, use Zealova.
Track every fast across devices in Zealova
Zealova logs every fast you complete, shows your streak across devices, syncs with your training (fasted vs fed sessions), and includes fasting in your monthly Wrapped recap.
Free 7-day trial. $7.99/mo or $59.99/yr after. Cancel anytime.
Methodology + Sources
- Anton SD et al. (2018). Flipping the metabolic switch: understanding and applying the health benefits of fasting. Obesity 26(2):254-268.
- Mattson MP et al. (2017). Impact of intermittent fasting on health and disease processes. Ageing Research Reviews 39:46-58.
- Mizushima N, Komatsu M (2011). Autophagy: renovation of cells and tissues. Cell 147(4):728-741.
- Phase timing references: Cahill GF Jr (2006). Fuel metabolism in starvation. Annual Review of Nutrition 26:1-22.
Last updated: 2026-05-14
Built by Sai, founder of Zealova. Not a neutral third party. Calculator results are estimates. For medical decisions, consult a qualified professional.
Frequently asked questions
Will the timer keep running if I close the tab?+
Yes. Your start time is saved to your browser. Reopen this page anytime and the timer picks up where you left off. The clock keeps ticking based on real elapsed wall-clock time, not just while the tab is open.
How does the sound and notification work?+
When you hit your fasting goal, a 3-tone chime plays through your speakers and (if you allow it) a browser notification appears. Both work even if the tab is in the background. Phones vibrate too.
Which fasting protocol should I start with?+
12:12 if you are new to fasting. 16:8 is the most-studied and most-popular protocol. 18:6 and 20:4 are for experienced fasters. Extended 36-hour fasts should be done under medical supervision if you have any health conditions.
Are the metabolic phases accurate?+
The hour markers (fed, lipolysis, ketosis, autophagy onset) are based on average response curves from clinical fasting research (Anton et al. 2018; Mattson et al. 2017). Individual response varies based on insulin sensitivity, last meal composition, and exercise.
Can I log this fast somewhere?+
In Zealova. The app saves every fast you complete, shows your streak and longest fast, integrates with your training (training fasted vs fed), and shows your fasting data in your monthly Wrapped recap.
Is fasting safe for me?+
Most people can safely do 12-16 hour fasts. Talk to a doctor first if you are pregnant, have diabetes, take medications that require food, have a history of disordered eating, or are under 18. This tool is not medical advice.
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