Fasting Calculator — Fasting Window & Stage Tracker

Calculate your intermittent fasting window for 16:8, 18:6, 20:4, OMAD, or custom protocols, with a live stage tracker showing ketosis and autophagy timing.

July 2, 2026

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Reference Values

Last verified: Jul 2, 2026

Category	Range	What It Means	Status
Fed State	0–4 hours	Body is digesting and absorbing nutrients from the last meal; insulin is elevated and energy comes primarily from recently eaten carbohydrates.	Okay
Post-Absorptive / Glycogen Use	4–12 hours	Insulin falls and the liver begins releasing stored glycogen for energy as digestion completes.	Good
Glycogen Depletion / Ketosis Onset	12–16 hours	Liver glycogen stores become depleted; the body begins shifting toward fat-derived ketones for fuel. This is the point most 16:8-style protocols target.	Good
Fat-Burning / Ketosis ★	16–24 hours	Reliable fat-burning state — most people reach a measurable ketosis threshold in this window (individual timing varies significantly).	★ Best
Autophagy (Increasing)	24+ hours	Cellular cleanup processes (autophagy) are believed to increase in this range, though the exact onset and rate are still debated in nutrition science and vary by individual.	Okay

Source: General fasting-physiology timeline aggregated from nutrition science reporting — individual timing varies substantially by metabolism, prior meal composition, activity level, and other factors. Not medical advice; consult a healthcare provider before extended fasting, especially with any underlying health condition.

Worked Examples

16:8 Protocol, Last Meal Ends 8:00 PM

Last Meal Ended: 8:00 PM
Protocol: 16:8 (16 hr fast / 8 hr eating)

Eating window opens 12:00 PM next day · closes 8:00 PM

8:00 PM + 16 hours = 12:00 PM the next day. The 8-hour eating window then runs from 12:00 PM to 8:00 PM.

18:6 Protocol, Last Meal Ends 7:00 PM

Last Meal Ended: 7:00 PM
Protocol: 18:6 (18 hr fast / 6 hr eating)

Eating window opens 1:00 PM next day · closes 7:00 PM

7:00 PM + 18 hours = 1:00 PM the next day, opening a 6-hour eating window until 7:00 PM.

OMAD (23:1), Last Meal Ends 6:00 PM

Last Meal Ended: 6:00 PM
Protocol: OMAD (23 hr fast / 1 hr eating)

Eating window opens 5:00 PM next day · closes 6:00 PM

6:00 PM + 23 hours = 5:00 PM the next day, leaving a single 1-hour eating window before the next fast begins.

How to Use This Calculator

1

Enter the time your last meal ended

Select the date and time you finished eating.
2

Choose a fasting protocol

16:8, 18:6, 20:4, OMAD, or enter custom fasting and eating window hours.
3

Read your eating window times and current stage

Results show exactly when your eating window opens and closes, plus a live-updating progress bar showing your current fasting stage.

What Each Value Means

Fasting Window (hours): The period of time between your last meal and your next meal, during which no calories are consumed. Length varies by protocol — 16 hours for 16:8, up to 23 hours for OMAD.
Eating Window (hours): The remaining hours of the day during which meals can be eaten, calculated as 24 minus the fasting window hours (or set independently for custom protocols).
Fasting Stage (stage label): A general physiological milestone (fed state, glycogen depletion, ketosis, autophagy) associated with elapsed fasting time — approximate and individually variable, not a precise biomarker measurement.

Frequently Asked Questions

How do you calculate your intermittent fasting window?

Add your fasting protocol's hours to the time your last meal ended. For 16:8 fasting starting at 8:00 PM: 8:00 PM + 16 hours = 12:00 PM the next day, when your 8-hour eating window opens. The eating window then closes 8 hours later, at 8:00 PM, restarting the fasting clock.

What's the difference between 16:8, 18:6, 20:4, and OMAD?

These describe the fasting-hours:eating-hours split within a 24-hour day. 16:8 fasts 16 hours with an 8-hour eating window (the most common starting protocol). 18:6 and 20:4 progressively shorten the eating window. OMAD (One Meal A Day) fasts roughly 23 hours with just a 1-hour eating window, typically for a single meal.

What happens in each stage of a fast?

Roughly: 0-4 hours is the fed state (digesting your last meal). 4-12 hours is post-absorptive, as insulin falls and liver glycogen starts being used. 12-16 hours brings glycogen depletion and the onset of ketosis. 16-24 hours is a reliable fat-burning/ketosis state, which is why many protocols target at least 16 hours. Beyond 24 hours, autophagy (cellular cleanup) is believed to increase, though the exact onset and rate are still debated and vary by individual.

Is intermittent fasting the same as calorie restriction?

No — intermittent fasting restricts WHEN you eat (a time window) rather than directly restricting how much you eat, though many people do end up eating less overall due to a shorter eating window. Some people maintain the same total daily calories within a compressed window; others naturally reduce intake. The metabolic effects of fasting itself (ketosis, autophagy) are distinct from simple calorie counting.

Is intermittent fasting safe for everyone?

No — it's not appropriate for everyone. People who are pregnant or breastfeeding, have a history of disordered eating, have diabetes or take blood-sugar-affecting medications, or have certain other health conditions should consult a healthcare provider before starting any fasting protocol. This calculator provides general timing information only, not personalized medical guidance.