Fasting Window Calculator
Your window converted into real clock times, plus the cautions that apply to your sex, cycle phase, training load and stress. The literature is clear that fasting suits some people and backfires for others — this helps you pick honestly.
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Windows covered
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Caution rules
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Emails required
It will talk you out of it if it should
Most fasting calculators just do the arithmetic. This one asks about cycle phase, training and stress first, and says plainly when the window you picked is a poor fit this week.
Fasting Window
Question 1 of 6
Some of the strongest cautions in this tool are sex-specific.
Earlier beats later
An early window improved insulin sensitivity and blood pressure in a crossover trial with no weight loss at all. Late eaters in an identical-calorie study lost markedly less.
Fasting is a stressor
That is not a criticism — it is the mechanism. Stacked on top of high stress, broken sleep or a demanding cycle phase, it stops being free.
The window is not the diet
In head-to-head trials, time-restricted eating produced weight change equivalent to plain calorie restriction. What you eat inside the window still decides the outcome.
Tighter is not better. It is just tighter.
Every rung down this list costs something, and past about 16 hours the cost starts arriving faster than the benefit. The right window is the one you will still be running in three months.
| Window | Fast | Eat | Intensity | Who it suits | What it costs |
|---|---|---|---|---|---|
| 12:12Light | 12 h | 12 h | Anyone starting, and most women in the luteal phase | Almost none — this is close to a normal overnight fast | |
| 14:10Gentle | 14 h | 10 h | A sensible default when stress or sleep is imperfect | Skipping either a late snack or an early breakfast | |
| 16:8Standard | 16 h | 8 h | The most-studied window, and the one most people mean | One meal disappears; protein targets get harder | |
| 18:6Moderate | 18 h | 6 h | Experienced fasters with stable sleep and low stress | Training quality suffers if sessions sit in the fasted block | |
| 20:4Aggressive | 20 h | 4 h | Rarely necessary — adherence usually fails before results arrive | Protein and micronutrients both become genuinely hard | |
| OMADExtreme | 23 h | 1 h | A short-term experiment at most, not a standing protocol | Hitting a protein target in one sitting is close to impossible |
A 16-hour fast that you keep for a year will do more for you than a 20-hour fast you keep for a fortnight. Every trial that compares protocols ends up saying some version of this.
It genuinely suits some people and genuinely doesn't suit others
Usually works for
Good fit
- Adults who already eat two or three meals a day
- People who don't enjoy breakfast and end up overeating later when forced into it
- Adults with stable sleep, moderate stress and settled cortisol
- People who find one structural rule easier than counting every calorie
- Anyone testing an early window for insulin-sensitivity reasons rather than weight reasons
Often backfires for
Poor fit
- Women with cycle disruption or perimenopausal hormonal turbulence
- Adults training at high volumes who need fuel around sessions
- Anyone with a disordered-eating history — a structural rule can reinforce restrictive thinking
- Adults already living in chronic high stress, because fasting adds to cortisol rather than easing it
- Sleep-deprived adults, where fasting and short sleep compound appetite dysregulation
On insulin, a sulfonylurea or a GLP-1? Extending a fast changes how that medication behaves. Speak to your prescriber before you change anything — this page is not a reason to alter a dose.
What the trials actually found
We cite the studies that undercut fasting as readily as the ones that support it, because that is the only way this page is worth reading.
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Length of the NEJM randomised trial that found no benefit over calorie restriction alone
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The early window that improved insulin sensitivity without any weight loss (Sutton 2018)
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Peer-reviewed papers cited on this page
Straight answers
Fasting questions people actually ask
Two things. It converts the window you pick into real clock times, so 16:8 stops being an abstraction and becomes a first meal and a last meal. Then it runs what you told us about sex, cycle phase, training and stress against a set of caution rules and tells you which ones you have tripped.
It is a suitability check rather than a prediction. Nothing here estimates how much weight you will lose, because the honest answer from the trial literature is that the window itself is not what drives that.
Answers are educational. Nothing here replaces advice from a clinician who knows your history and your medication.
A window is a schedule. It isn't a protocol.
The assessment looks at your diet history, your insulin-resistance signals and what has already failed for you, then points at the approach the research supports for that profile — which for plenty of people is not fasting at all.
Educational only, not medical advice. This calculator produces a schedule and a set of general cautions, not a diagnosis, and it cannot see your medication, bloodwork or medical history. Anyone taking insulin, a sulfonylurea or a GLP-1, and anyone pregnant or breastfeeding, should speak to their physician before fasting. Consult your physician before major dietary changes or altering prescribed medication. These statements have not been evaluated by the Food and Drug Administration. Individual results vary; no outcome is guaranteed.