Intermittent Fasting Planner

Preset:

Pick 12:12 through 20:4, circadian 13:11, or Custom for a 24-hour split.

Fasting window: {{ formatNumber(fastingHours, 2) }} h

Use 0-24 hours in 0.25-hour steps; common custom values are 14, 16, or 18.

Eating window: {{ formatNumber(eatingHours, 2) }} h

Enter the remaining daily hours, such as 8 for a 16:8 plan.

First meal time:

Use 24-hour time such as 12:00 or 18:30.

Safety profile:

Medical complexity applies the strictest fasting-window guardrail.

Check these items:

{{ err }}

Waiting for input. Enter your routine above to see the plan.

Wake time:

Use HH:MM, for example 06:30.

Sleep time:

Use HH:MM, for example 22:30 or 00:30.

Goal focus:

Choose weight loss, metabolic health, or muscle maintenance.

Start day:

Pick the weekday for row 1 of the schedule.

Plan days: {{ plan_days }} d

Enter 1-30 days.

Hydration interval: {{ hydration_interval_minutes }} min

Use 0, or 30-360 minutes in 15-minute steps.

Workout preference:

Choose no workout, fasted morning, midday strength, or evening mobility.

Weekend shift:

Enter -180 to 180 minutes; positive shifts weekends later.

Shift amount {{ weekend_shift_minutes }} min

Daily meal pattern:

Choose 2, 3, or 4 planned feedings per eating window.

Fasting safety ceiling: {{ formatNumber(max_fast_hours, 1) }} h

Enter 12-24 hours in 0.5-hour steps.

Day	Eating window	Fasting window	Meal structure	Focus points
{{ row.dayLabel }} {{ badge.text }}	{{ row.eatingStart }} → {{ row.eatingEnd }} {{ row.eatingEndNote }} {{ row.eatingCue }}	{{ row.fastingStart }} → {{ row.fastingEnd }} {{ row.fastingEndNote }} {{ row.fastingCue }}	{{ row.mealPlan }} {{ row.mealSpacing }}	{{ row.workout }} {{ row.note }}
Hydration reminder	{{ slot }}	Daily reminder	Applies each day	Adjust manually around workouts, heat, caffeine, or medication guidance.
{{ goalInsights.title }}	Goal insight	{{ goalInsights.summary }}	Use during adherence checks	{{ goalInsights.tips.join(' ') }}

Safeguard	Value	Copy
{{ row[0] }}	{{ row[1] }}

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Time-restricted eating is the daily form of intermittent fasting where meals are kept inside a planned eating window and the remaining hours are treated as a fasting window. The clock matters because a small timing change can move the last meal closer to sleep, push the first meal later than expected, or turn an ordinary social weekend into a longer fast than the name of the plan suggests.

People often describe fasting plans with ratios such as 14:10 or 16:8. The first number is fasting hours, and the second number is eating hours. That shorthand is useful, but it hides the practical schedule: a 16-hour fast anchored by a noon first meal is very different from the same 16-hour fast anchored at 8 a.m. The daily pattern also differs from alternate-day fasting, periodic fasting, religious fasting, and medical fasts before procedures, which have different risks and different rules.

Eating window: The span when meals or planned feedings happen.
Fasting window: The span between the end of one eating window and the next first meal.
Time-restricted eating: A repeating daily schedule that limits food timing without necessarily prescribing calories.
OMAD: A one-meal-a-day pattern with a very short eating window, which can be hard to fuel safely.

The anchor time is the first meal, not just the total fasting duration. From that anchor, the last meal, overnight fast, next first meal, workout timing, and waking-hour hydration cues all fall into place. Sleep timing matters too. Eating close to bedtime can make a schedule harder to follow and may conflict with circadian rhythm research that studies how meal timing, metabolism, and sleep interact.

The same fasting ratio can feel very different when the first meal, last meal, sleep, or weekend timing changes.

Fasting windows do not make food quality, total intake, protein, medications, training load, or hydration disappear. A shorter repeatable schedule with enough room for balanced meals is often more useful than an aggressive window that compresses food into one rushed sitting or encourages overeating when the fast ends.

Intermittent fasting is also not suitable for everyone. Children and teens, people who are pregnant or breastfeeding, people with a history of eating disorders, people using insulin or glucose-lowering medicines, and anyone with medical complexity should treat fasting as a clinician-guided decision rather than a casual calendar change.

How to Use This Tool:

Start with the rhythm you could repeat on an ordinary week. A plan that looks clean on paper can fail if it crowds dinner, training, medication timing, social meals, or sleep.

Choose a fasting preset, or use Custom when the built-in ratios do not match your routine. Custom fasting and eating hours must add up to one day within the tool's rounding tolerance.
Set the First meal time. The eating window starts there, the fast begins when that window ends, and the next first meal closes the cycle.
Select the Safety profile. The active-training option changes guidance around workouts, while the medical-complexity option blocks longer fasting windows.
Open Advanced to tune wake time, sleep time, goal focus, start day, plan length, hydration interval, workout preference, weekend shift, meal pattern, and fasting safety ceiling.
Use the weekend shift only for a real Saturday or Sunday timing difference. Positive shifts move weekend eating windows later; negative shifts move them earlier.
Review the result tabs. Fasting Schedule gives the day-by-day table, Adherence Safeguards shows caution checks, Rhythm Flow charts eating and fasting hours, and JSON gives a structured export.

If a red warning appears, resolve it before using the schedule. Common causes are invalid times, custom hours that do not total 24 hours, a fast above the chosen ceiling, a medical profile paired with a fast over 16 hours, too many or too few plan days, or a hydration interval below 30 minutes.

Interpreting Results:

The summary badges confirm the plan at a glance: fasting hours, eating hours, first meal time, plan length, start day, hydration cadence, meal count, safety profile, and goal focus. Use those badges as a quick error check before reading the full schedule.

The live window band shows where the current local time falls in the configured daily rhythm. It labels the current window as eating or fasting and names the next boundary. That readout is a timing guide only; it does not track actual meals, symptoms, calories, glucose, or hydration.

The schedule table lists each day, the eating window, the fasting window, meal structure, workout cue, and focus note. Rows can include weekend badges when a Saturday or Sunday shift is active. If an eating or fasting end time crosses midnight, the row adds a next-day note so the clock time is not mistaken for the same calendar day.

The safeguards tab is deliberately conservative. It reports weekly fasting exposure, the estimated longest fasting stretch, the selected safety ceiling, average meal spacing, hydration cadence, and a profile-specific note. The longest-stretch estimate adds the absolute weekend shift to the named fasting window because shifting a window earlier or later can create one longer transition.

The Rhythm Flow chart stacks eating and fasting hours for each scheduled day. A weekend shift is highlighted as a timing change, not as a different bar height, because the ratio itself remains the same. Use downloads, copy buttons, and the JSON view for planning notes, not as a substitute for professional nutrition or medical advice.

Technical Details:

A daily fasting plan is a circular time calculation. The day has 1,440 minutes, the first meal places the eating window on that circle, and every other boundary is measured from that anchor. When a window runs past midnight, the clock time wraps around while the underlying interval remains continuous.

Preset ratios supply the fasting and eating durations directly. Custom mode rounds entered durations to the nearest quarter hour and accepts the split only when fasting plus eating equals 24 hours within a ±0.25 hour tolerance. The circadian preset is a fixed 13:11 ratio anchored to the chosen first meal; it does not calculate local sunrise or sunset.

Formula Core

Let M_start be the first-meal minute after midnight, H_eat the eating-window hours, H_fast the fasting-window hours, S_day the weekend shift in minutes for that day, and N the planned meal count.

M_{end} = (M_{start} + S_{day} + 60 \times H_{eat}) mod 1440

F_{weekly} = 7 \times H_{fast}

F_{longest} = H_{fast} + \frac{| S_{weekend} |}{60}

M_{spacing} = \frac{H_{eat}}{N - 1} when N > 1

For example, a 16:8 plan with a first meal at 12:00 starts eating at minute 720. The eating window ends 480 minutes later at 20:00, the fasting window runs from 20:00 to the next 12:00, weekly fasting exposure is 112 hours, and a 90-minute weekend shift raises the longest-fast estimate to 17.5 hours.

Rule	Boundary	Effect
Custom total	Fasting hours plus eating hours must be 24 within ±0.25 hour.	Blocks impossible daily cycles and accidental drifting schedules.
Fasting ceiling	Selected fasting hours cannot exceed the configured maximum, from 12 to 24 hours.	Forces longer plans to be intentional before results appear.
Medical caution	The medical-complexity profile allows fasting windows of 16 hours or less.	Keeps aggressive ratios out of the most cautious profile.
Hydration interval	Reminder spacing is either off or at least 30 minutes, up to 360 minutes.	Avoids unrealistic reminder density during waking hours.
Plan length	The schedule covers 1 to 30 days.	Keeps the output readable for short trials and month-style planning.
Weekend shift	Saturday and Sunday anchors can move from 180 minutes earlier to 180 minutes later.	Shows social or sleep-schedule changes without changing the underlying ratio.

Output	How to Read It
Eating and fasting windows	Clock ranges derived from the first-meal anchor and the selected ratio, with notes when a boundary crosses into another day.
Meal structure	The selected meal count spread across the eating window; spacing is approximate and does not guarantee nutrition adequacy.
Workout cue	A suggested time relative to the eating window, such as before the first meal, after the first meal, or near the end of the eating window.
Goal insight	Plain-language reminders for weight loss, metabolic health, or muscle maintenance. The goal focus changes notes, not the fasting math.
Hydration reminders	Waking-hour clock prompts based on the chosen interval; they are reminders, not fluid prescriptions.

Limitations, Privacy, and Safety:

The planner does not decide whether intermittent fasting is medically appropriate for a specific person.
It does not evaluate calories, protein, micronutrients, caffeine, alcohol, symptoms, sleep quality, blood glucose, or medication timing.
It is built for repeating daily time-restricted eating, not alternate-day fasting, prolonged fasting, religious fasting, or procedure-related fasting.
Hydration prompts are schedule reminders only. They do not account for heat, illness, sweating, kidney disease, diuretics, or clinician instructions.
Schedule data is generated from values entered in the page. Copied or downloaded plans may reveal personal health routines, so store and share them with care.

Worked Examples:

Scenario	Setup	What to Check
First 16:8 week	16:8 preset, first meal at 12:00, three meals, general adult profile, seven plan days.	Confirm the eating end time, bedtime spacing, and whether the final fasting hours are realistic before work or school.
Training day schedule	14:10 preset, active-training profile, midday strength preference, two or three structured meals.	Check whether the workout cue lands near enough to the first substantial meal and whether meal spacing supports recovery.
Late weekend brunch	16:8 preset with the weekend shift enabled and Saturday/Sunday first meals moved 90 minutes later.	Review the longest-fast estimate because the transition into or out of the weekend can exceed the named 16-hour fast.
Medical caution profile	Any plan with medication or medical-complexity concerns selected.	Keep fasting windows at 16 hours or less and use the output as a discussion aid, not as clearance to fast.

FAQ:

Does a valid schedule mean fasting is safe for me?

No. A valid schedule means the timing rules fit the selected inputs. Safety depends on age, pregnancy status, medical history, medications, eating history, symptoms, training load, and nutrition needs.

Why does the planner start from the first meal?

The first meal gives the day a clear anchor. From that one clock time, the eating end, fasting start, overnight fast, and next first meal can be calculated without guessing where the cycle begins.

Why does a longer preset show an error?

Longer presets must fit both the fasting safety ceiling and the selected safety profile. Raise the ceiling only if that longer fast is intentional and appropriate for your situation.

Can I use this for alternate-day fasting?

No. The schedule is based on a repeating 24-hour eating and fasting cycle. Alternate-day fasting and very-low-calorie fasting patterns need different planning and medical safeguards.

What should I change if the plan feels too restrictive?

Choose a shorter fast, widen the eating window, reduce weekend shifting, move the first meal earlier, or use a meal pattern that leaves enough time for balanced food. A repeatable plan is more useful than a strict plan that breaks after a few days.