Intermittent Fasting Calculator
Map a daily eating window and fasting window from one clock time. Choose whether that time is your first or last calorie, then see both boundaries and the next-day rollover.
A timetable is not a health prediction. This planner does not identify a suitable regimen, estimate weight loss, measure fat burning or tell you when autophagy starts.
Use the calculatorSet Your Daily Eating and Fasting Windows
Choose a time split and one boundary. Presets describe hours; they are not recommendations for you.
Your two windows will appear here
Choose a split and enter a first or last calorie time. The result shows the eating window, fasting window and which boundaries fall on Day 2.
A split is not a meal plan
16:8 describes 16 fasting hours and an 8-hour eating window. It does not set calories, nutrients or meal count.
Midnight does not reset the duration
A window can cross midnight. Each boundary keeps a Day 1 or Day 2 label.
No biological stage labels
The timeline is a clock calculation, not a measurement of ketosis, insulin, fat burning or cellular repair.
Your daily clock-time schedule
Selected time split · not a recommendation
Read the day labels: Day 1 is the day containing your selected first or last calorie time. Day 2 is the following day. Neither label means a saved appointment or a live countdown.
View the full cycle ↓Medical disclaimer: This is an educational clock-time planner for adults, not a diet prescription or a safety assessment. It is not intended for under-18s, pregnancy, breastfeeding or a history of eating disorders. If you have diabetes, take glucose-lowering medicine or have another relevant condition, discuss fasting with your care team first. Do not skip or change medicines to fit a window, or push through concerning symptoms to meet a clock target. Medical, pre-procedure and religious fasting require their own instructions.
Trace one complete daily cycle
| Boundary | Clock time and day |
|---|
This is a repeating wall-clock template without calendar dates or time-zone conversion. Daylight-saving changes or travel can make the real elapsed duration differ from the displayed clock-hour split. Recheck actual dates and your care instructions when that matters.
How one clock time defines both windows
The same duration has different boundary times depending on whether you start from the first or last calorie.
| Calculation | Rule | Example |
|---|---|---|
| Complementary window | Eating minutes = 1,440 − fasting minutes. | 16 hours fasting leaves 8 hours for eating. |
| First-calorie anchor | Add the eating window to find the fasting start, then add the fasting window to reach the next cycle. | 12:00 Day 1 → 20:00 Day 1 → 12:00 Day 2. |
| Last-calorie anchor | Add the fasting window to find the eating start, then add the eating window to reach the next cycle. | 20:00 Day 1 → 12:00 Day 2 → 20:00 Day 2. |
| Midnight rollover | Keep total minutes for order; use the remainder after 1,440 for the clock display. | 22:00 + 8 hours = 06:00 Day 2, not 06:00 Day 1. |
16:8 from 12:00
Eating: 12:00–20:00 on Day 1. Fasting: 20:00 Day 1 to 12:00 Day 2. This is a fictional clock example, not a recommended routine.
14:10 from 19:30
Fasting: 19:30 Day 1 to 09:30 Day 2. Eating: 09:30–19:30 on Day 2. The entered time starts fasting in this example.
16:8 from 22:00
Eating: 22:00 Day 1 to 06:00 Day 2. Fasting: 06:00–22:00 on Day 2. This demonstrates date order; it does not endorse overnight eating.
What a fasting timetable does not decide
Keep daily scheduling, research findings and individual health decisions separate.
Read 16:8 as hours, not a health score
The two numbers describe time within a 24-hour cycle. A 16-hour fast leaves 8 hours for eating. It does not mean 16 hours of measured fat burning or specify a calorie deficit. The default is illustrative, not selected from your health history.
Longer fasting is not a ranked progression
The menu does not label 12:12 as inferior or 23:1 as advanced or optimal. The calculator has no information about your health, nutrition or medicines with which to rank a regimen for you. A shorter eating window is not a goal this tool asks you to reach.
A clock cannot identify an autophagy threshold
There are no entries for tissue measurements, metabolism or biomarkers here. The output therefore does not assign a universal hour when autophagy, ketosis, an insulin reset or a fat-burning phase begins. Time labels stay separate from biological claims.
A schedule does not guarantee weight loss
A 12-month randomised trial in adults with obesity found no significant additional weight-loss benefit from adding an 8-hour eating window to calorie restriction compared with calorie restriction alone. That study does not decide what is suitable for every person, and this planner predicts no weight change.
Meals and medicines need their own plan
The eating window is available time, not permission to ignore nutritional needs. For diabetes, changing meal timing can require professional medication review and monitoring. Do not omit, delay or adjust medicine because a clock label says fasting has begun.
Use the right instructions for the type of fast
This is not a laboratory, anaesthetic, surgery or religious fasting calculator. Those instructions can differ on food, drinks, medicines and exact start times. The page does not declare coffee, supplements, sweeteners or electrolyte products acceptable for every kind of fast.
A 23:1 window does not prescribe OMAD
A one-hour eating window is a time split. The calculator neither counts meals nor assesses whether nutritional needs can be met in that interval. It does not present one meal a day as a method with maximum metabolic benefit.
Recheck midnight, travel and symptoms
Check Day 1 and Day 2 rather than comparing clock numbers alone. For daylight-saving dates or travel, this repeating template is not an elapsed-time tracker. If you feel unwell, the result is not a reason to delay care or keep fasting until a scheduled boundary.
See Johns Hopkins Medicine’s fasting overview and suitability cautions, NIDDK’s discussion of fasting with type 2 diabetes, and the 2022 randomised trial for the evidence behind these limits. These sources do not endorse the schedule you enter.
Helpful resources
Related nutrition tools and primary or official sources for checking the health context.
Useful resources
References do not endorse this website or an individual fasting regimen. Bhavin Sheth is credited for page maintenance, not medical review. This implementation has not been independently clinically validated. Related calculators do not establish that fasting is safe for you.
Questions about clock windows and next-day times
Choose the correct anchor before interpreting the times.
What does 16:8 mean in this calculator?
It means a 16-hour fasting window and an 8-hour eating window in one repeating 24-hour clock-time cycle. It does not specify calories, meal count or health benefits.
Should I enter my first calorie time or my last calorie time?
Choose the matching anchor in the form. First calorie time starts the eating window; last calorie time ends the eating window and starts fasting. The calculator can work from either boundary.
What do Day 1 and Day 2 mean?
Day 1 contains the anchor time you entered. Day 2 is the following day. They preserve the order when a window crosses midnight; they are not stored calendar appointments.
Can I choose a custom fasting duration?
Yes. This implementation accepts 1–23 hours in 0.25-hour, or 15-minute, steps. The rest of the 24-hour cycle becomes the eating window. These are software input limits, not medically approved fasting limits.
Will the planner tell me how much weight I will lose?
No. It does not use food intake, energy needs, body measurements or clinical history to predict weight change. A time window alone does not establish a calorie deficit or guarantee an outcome.
Does autophagy start at the hour shown by the calculator?
No biological threshold is calculated. The tool adds clock minutes; it does not measure autophagy, ketosis, insulin or fat burning and does not assign universal stages to a fast.
Is a longer fasting window always better?
This page makes no such recommendation. Presets are time splits rather than beginner-to-advanced levels. A qualified professional can discuss suitability, especially when health conditions or medicines are involved.
Can I use this for a blood test, surgery or religious fast?
No. Follow the instructions for that specific test, procedure or religious practice. This daily lifestyle clock planner does not determine permitted food, drinks, medicines or required fasting times for those situations.
Does the result account for daylight saving or travel?
No. It is a wall-clock template without dated timestamps or time-zone conversion. A clock change or travel can alter the real elapsed interval. Recheck the actual dates and relevant care instructions when that matters.
Are my times saved or sent anywhere?
This calculator script keeps entries in browser memory without uploading them or saving them to browser storage. It does not track meals or send reminders. Fonts and other website scripts can make their own requests; the statement applies to this calculator script only.