Which fasting protocols are supported?

This planner supports 14:10, 16:8, 18:6, 20:4, OMAD (23:1), and 5:2 scheduling.

How are eating and fasting windows calculated?

Your first meal time sets the start of the eating window. The selected protocol determines eating and fasting durations across a 24-hour cycle.

How does the 5:2 option work here?

For 5:2, you pick two low-calorie fasting days each week. The planner highlights those days and shows a reference 24-hour fasting-day timeline.

Can agents run this deterministically?

Yes. Supply protocol, first_meal_time, and optional fast_days as JSON to receive eating/fasting windows and day-based guidance deterministically.

Is this tool free and private to use?

Yes. AI Fit Hub tools are free, no-signup browser tools. Inputs stay in your browser unless you choose to share a URL.

Do these tools replace medical guidance?

No. These outputs are general fitness estimates — not medical advice.

Methodology: Intermittent Fasting Window Planner

Scope

Picks eating and fasting windows for common protocols (16:8, 18:6, 20:4, OMAD, 5:2) and renders a 24-hour timeline.

Formula

eating_window_start = wake_time + fast_extension. eating_window_end = start + eating_window_hours.

Coefficients

Parameter	Value	Note
16:8	`16 h fast / 8 h eat`
18:6	`18 h fast / 6 h eat`
20:4	`20 h fast / 4 h eat`
OMAD	`~23 h fast / 1 h eat`
5:2	`5 normal days + 2 ~500 kcal days`

Data sources

Patterson RE, Laughlin GA, LaCroix AZ, et al. Intermittent fasting and human metabolic health. J Acad Nutr Diet. 2015;115(8):1203-1212. — PMID 25857868.
de Cabo R, Mattson MP. Effects of intermittent fasting on health, aging, and disease. N Engl J Med. 2019;381(26):2541-2551. — PMID 31881139.
Varady KA, Cienfuegos S, Ezpeleta M, Gabel K. Clinical application of intermittent fasting for weight loss: progress and future directions. Nat Rev Endocrinol. 2022;18(5):309-321. — PMID 35194176. Contemporary review of IF weight-loss outcomes.

Assumptions

Total daily calories and macros are controlled; fasting schedules without calorie awareness do not automatically cause weight loss.

Approximation range

Meta-analyses find IF matches continuous calorie restriction for weight loss when calories are matched.

Limitations

Not appropriate during pregnancy, lactation, or for people with a history of eating disorders.
Long fasts can impair hypertrophy adaptation in lifters — protein timing still matters.

Reproducibility

Wake 7:00, 16:8 with 2 h post-wake fast extension: eating window 9:00–17:00.

Change log

2026-04-24: methodology page first published.

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Macro Calculator — Convert calorie targets into protein, carbs, and fat grams for your goal.
Protein Intake Calculator — Get daily protein targets based on training level and goal.

Worked example

Computed by the same engine bundle served at /engines/intermittent-fasting-window-planner.js. Re-runnable: the values below are the literal output of compute(engineInput).

Input

tool: intermittent_fasting_window_planner
protocol: 16:8
first_meal_time: 12:00

Output

protocol: 16:8
protocolLabel: 16:8
eatingHours: 8
fastingHours: 16
isDayBased: false
tip: Start with 16:8 if you are new to fasting. Keep meal timing consistent for 1-2 weeks before tightening the window.
eatingStartMinute: 720
eatingWindowStart: {"hours":12,"minutes":0,"dayOffset":0}
eatingWindowEnd: {"hours":20,"minutes":0,"dayOffset":0}
fastingWindowStart: {"hours":20,"minutes":0,"dayOffset":0}
fastingWindowEnd: {"hours":12,"minutes":0,"dayOffset":1}
fastDays: []
mealSchedule: [{"mealNumber":1,"time":{"hours":12,"minutes":0,"dayOffset":0},"calories":0},{"mealNumber":2,"time":{"hours":16,"minutes":0,"dayOffset":0},"calories":0},{"mealNumber":3,"time":{"hours":19,"minutes":45,"dayOffset":0},"calories":0}]
coachSummary: 16:8: eat between 12:00 PM and 8:00 PM, fast for 16 hours. Fit 3 meals in the window. Start with 16:8 if you are new to fasting. Keep meal timing consistent for 1-2 weeks before tightening the window.

FAQ

Which fasting protocols are supported?: This planner supports 14:10, 16:8, 18:6, 20:4, OMAD (23:1), and 5:2 scheduling.
How are eating and fasting windows calculated?: Your first meal time sets the start of the eating window. The selected protocol determines eating and fasting durations across a 24-hour cycle.
How does the 5:2 option work here?: For 5:2, you pick two low-calorie fasting days each week. The planner highlights those days and shows a reference 24-hour fasting-day timeline.
Can agents run this deterministically?: Yes. Supply protocol, first_meal_time, and optional fast_days as JSON to receive eating/fasting windows and day-based guidance deterministically.
Is this tool free and private to use?: Yes. AI Fit Hub tools are free, no-signup browser tools. Inputs stay in your browser unless you choose to share a URL.
Do these tools replace medical guidance?: No. These outputs are general fitness estimates — not medical advice.