Intermittent Fasting Calculator
Plan any intermittent fasting schedule — 16:8, 18:6, 20:4, OMAD, 5:2 or ADF. Enter your protocol and fast start time to get your exact eating window, a 24-hour fasting timeline with stage markers, an intermittent fasting weekly plan, and a fasting tracker for your progress.
Not medical advice. This is an educational scheduling tool. Fasting affects medication, blood sugar and health conditions — talk to your doctor before starting, especially if you are pregnant, breastfeeding, under 18, have a history of eating disorders, or take prescription medication.
Scope note: this is an intermittent fasting calculator — it plans when you eat (fasting windows, eating windows, weekly schedules). It is not a blood-test tool. For fasting glucose / fasting insulin biomarkers (HOMA-IR, unit conversion), see the related calculators below — different intent, different tools.
Fasting schedule planner — your eating window calculator
Weekly plan — 5:2, ADF and daily protocols
Fasting stages timeline — what the hours mean
Typical ranges reported in fasting literature for a water-only fast. Individual timing varies widely with your last meal, activity, sleep and metabolic health — treat these as guideposts, not precise predictions. Select a protocol in the planner to see which markers your fast reaches.
| Hour | Stage | What is generally reported |
|---|---|---|
| ~12h | Metabolic switch begins | Liver glycogen runs low for many people and the body starts making ketones. Partial ketosis may begin around here; timing varies. |
| ~16h | Fat burning ramps up | Ketone levels rise and stored fat becomes a larger share of fuel for many fasters. Insulin is typically low by this point. |
| ~18h | Ketosis deepens | Many people report steadier energy and smaller hunger waves here as the body relies mostly on fat-derived fuel. |
| ~24h | Cellular cleanup signaling | Markers of autophagy — the cell-recycling process — have been observed from around this point in published research (mostly animal and cellular studies). Autophagy is continuous, not a switch that flips at an exact hour. |
Why ranges, not exact times? Autophagy, for example, is continuous — fasting may increase it, but human research does not pin it to an exact hour. Charts that present stage unlocks like video-game achievements are selling fake precision.
Progress tracker — log your fasts
Everything stays in this browser (localStorage). No account, no server, nothing leaves your device.
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Intermittent fasting methods compared
| Protocol | Pattern | Level | Notes |
|---|---|---|---|
| 14:10 | 14h fast / 10h eating | Gentle start | A gentle entry point. Many beginners start here for 1-2 weeks before trying 16:8. |
| 16:8 | 16h fast / 8h eating | Most popular | The most widely used pattern: skip breakfast, eat lunch and dinner inside 8 hours. |
| 18:6 | 18h fast / 6h eating | Intermediate | A step up from 16:8 with a 6-hour eating window — two meals, no snacks for most people. |
| 20:4 | 20h fast / 4h eating | Advanced | Warrior-style: one large meal plus a small meal or snack inside 4 hours. |
| OMAD | 23h fast / 1h eating | Advanced | One meal a day: a single ~1-hour eating window. Only for experienced fasters. |
| 5:2 | 2 full days/week fast | Flexible | Eat normally 5 days a week; on 2 non-consecutive days limit intake to about 500-600 kcal. |
| ADF | alternating 24h days | Advanced | Alternate-day fasting: fast days and normal eating days alternate all week. |
16:8 eating window chart — four placements
The same 16:8 fasting schedule fits different lives. Pick the eating window that captures your main social meals:
| Fast starts | Fasting window | Eating window |
|---|---|---|
| 8:00 PM | 8:00 PM – 12:00 PM | 12:00 PM – 8:00 PM |
| 6:00 PM | 6:00 PM – 10:00 AM | 10:00 AM – 6:00 PM |
| 3:00 PM | 3:00 PM – 7:00 AM | 7:00 AM – 3:00 PM |
| 10:00 PM | 10:00 PM – 2:00 PM | 2:00 PM – 10:00 PM |
The most common placement is the 12:00 PM – 8:00 PM eating window (fast from 8:00 PM): it skips breakfast, keeps lunch and dinner, and puts half the fast inside sleep.
Worked example: a full 16:8 day
You finish dinner and start the fast at 8:00 PM on a 16:8 fasting schedule:
- Eating window: 12:00 PM – 8:00 PM the next day (8 hours).
- 12-hour marker (8:00 AM): metabolic switch begins — ketone production ramps up.
- 16-hour marker (12:00 PM): fat burning ramps up — time for your first meal.
- Water, black coffee and plain tea are fine through the fast; your first meal at noon ends it.
All times above are computed by the planner engine on this page — try your own start time in the fasting schedule planner.
What breaks a fast — and what you can drink
Generally fine while fasting
- Water and sparkling water
- Black coffee (plain, ~2 kcal)
- Plain tea, unsweetened
- Calorie-free electrolytes
Breaks the fast
- Any food or snacks
- Sugary drinks, juice, soda, alcohol
- Milk, cream or sugar in coffee
- Smoothies and shakes
How to start (and who should skip it)
For intermittent fasting for beginners: run 12:12 for a week, then a 14:10 fasting schedule, then 16:8. Keep the same window every day, hydrate through the fast, and do not attempt OMAD or ADF in your first month. Consistency beats intensity — a sustainable 16:8 held for months outperforms sporadic heroic fasts.
Skip fasting or get medical guidance first if you are pregnant or breastfeeding, under 18, have a history of eating disorders, are underweight, take diabetes or blood-pressure medication, or have a chronic condition. How long should I fast if none of those apply? Start at 14:10 or 16:8 and extend only when it feels easy.
Related calculators
Boundary note: the HOMA-IR, mmol/L↔mg/dL and VLDL tools work with fasting blood values (glucose, insulin, lipids). This page plans fasting schedules (when to eat). Use each for its own intent.
Frequently Asked Questions
How do I calculate my intermittent fasting schedule?
Pick a protocol and a fast start time (when you finish your last meal), then add the fasting hours. For example, on a 16:8 fasting schedule starting the fast at 8:00 PM, the eating window opens 16 hours later at 12:00 PM and closes at 8:00 PM the next day. The planner above does this arithmetic for 16:8, 18:6, 20:4, OMAD, 5:2 and ADF, and draws your full 24-hour fasting timeline.
What is a 16:8 fasting schedule?
You fast for 16 hours and eat all meals inside an 8-hour eating window, every day. If you finish dinner at 8:00 PM, what time should I eat on 16:8? Your first meal is at 12:00 PM the next day and your last at 8:00 PM. It is the most popular pattern because roughly half the fast happens while you sleep.
What is an 18:6 fasting schedule?
18 hours fasting, 6 hours eating. Start the fast at 7:00 PM and the eating window is 1:00 PM to 7:00 PM the next day. It suits people who found 16:8 easy and want a smaller window without going full OMAD.
What is the 20:4 fasting schedule (Warrior diet)?
20 hours fasting with a 4-hour eating window. Starting the fast at 8:00 PM gives an eating window of 4:00 PM to 8:00 PM the next day — typically one large meal plus a small meal or snack. It is an advanced pattern; build up through 16:8 and 18:6 first.
What is an OMAD fasting schedule?
OMAD (one meal a day) is a 23-hour fast with a 1-hour eating window. Finish dinner at 6:00 PM and you eat again from 5:00 PM to 6:00 PM the next day. Because the window is so short, getting enough protein and nutrients into a single meal takes planning.
How does the 5:2 diet plan work?
You eat normally 5 days a week and limit intake to about 500–600 kcal on 2 non-consecutive fast days. A common setup is fasting on Monday and Thursday with normal eating the other five days — the intermittent fasting weekly plan above builds exactly this layout from whichever two days you pick.
What is an alternate day fasting schedule?
Fast days and normal eating days alternate: Monday fast, Tuesday eat, Wednesday fast, and so on. Classic ADF fast days are water-only; modified ADF allows about 500 kcal. The weekly planner alternates the pattern automatically from whichever day you start.
What are the stages of fasting by hour?
Typical ranges reported in fasting literature: around 12 hours the metabolic switch begins and ketone production ramps up; around 16 hours fat burning is elevated; around 18 hours ketosis deepens; around 24 hours markers of autophagy (cell recycling) have been observed, mostly in animal and cellular studies. These are guideposts, not exact switch times — individual timing varies.
When does ketosis start during fasting?
Partial ketosis often begins around 12 hours as liver glycogen runs low, with fuller ketosis commonly reported around 16–18 hours. Your last meal, activity level and metabolic health shift this window, so treat the hours as a range, not a promise.
How many hours of fasting until autophagy?
Autophagy is continuous — fasting may increase it, but research does not pin it to an exact hour in humans. Markers have been observed from around 24 hours in published studies, mostly animal and cellular work. Anyone selling you an exact autophagy start time is selling fake precision.
What breaks a fast?
Anything with meaningful calories — food, sugary drinks, milk or cream in coffee. Plain water, sparkling water, black coffee and unsweetened tea are generally considered fine. For strict fasts many people keep intake at effectively zero; a 50 kcal rule of thumb is sometimes used for weight-loss fasting, but it is a convention, not a law of nature.
Does black coffee break a fast?
Plain black coffee has about 2 kcal per cup and is generally considered fasting-friendly. What breaks the fast is what you add: sugar, milk, cream or butter turn it into a small meal. Keep caffeine moderate and skip it if it disturbs your sleep.
What can I drink while fasting?
Water, sparkling water, black coffee, plain tea and calorie-free electrolytes. Avoid juice, soda, alcohol, smoothies and anything with sugar, milk or cream. Staying hydrated makes the fasting window far easier.
Who should not do intermittent fasting?
Talk to a doctor first if you are pregnant or breastfeeding, under 18, have a history of eating disorders, are underweight, take diabetes or blood-pressure medication (fasting can change dosing needs), or have a chronic condition. Stop fasting and seek care if you feel dizzy, faint or unwell. This page is educational, not medical advice.
Can I exercise while fasting?
Light to moderate exercise — walking, easy cycling, mobility work — is fine for most people during the fast. Schedule intense or long sessions inside your eating window when possible, stay hydrated, and back off if you feel dizzy. If you train for performance, talk to a coach or clinician.
Is intermittent fasting safe?
For healthy adults, common patterns like 14:10 and 16:8 are widely practiced and studied. Safety depends on the person: the groups listed above should get medical guidance first, and fasts beyond 24 hours deserve extra caution. Nothing on this page treats, prevents or heals any disease — it is a scheduling tool, not medical advice.
How do I start intermittent fasting as a beginner?
Start with intermittent fasting for beginners the easy way: 12:12 for a week (finish dinner at 8:00 PM, breakfast at 8:00 AM), then 14:10, then 16:8. Keep the same window daily, drink water through the fast, and do not try OMAD or ADF in your first month.
Is this the same as the fasting glucose or HOMA-IR tools?
No. This page plans intermittent fasting schedules — when to eat and when to fast. The HOMA-IR calculator and the mmol/L to mg/dL converter deal with fasting blood biomarkers (glucose, insulin) measured in a lab. Different intent: meal timing here, blood values there.
How long should I fast?
Match the protocol to your experience: 14:10 or 16:8 to start, 18:6 once 16:8 feels easy, 20:4 or OMAD only after months of practice, 5:2 if you prefer whole fast days to daily windows. Longer is not automatically better — the best protocol is the one you can repeat for months.
What 16:8 fasting results do people report?
Commonly reported experiences include easier calorie control, less evening snacking and steadier appetite after a few weeks. Actual fat loss still comes from a sustained calorie deficit and food quality — fasting is a timing tool. Track your own trend with the fasting tracker above instead of comparing yourself to anecdotes. Fasting changes when you eat; it does not promise fat loss by itself.
Medical disclaimer. This intermittent fasting calculator is for education only and is not medical advice, diagnosis or treatment. Fasting can interact with medications and medical conditions. Consult a qualified healthcare professional before beginning any fasting protocol, and stop fasting and seek care if you feel unwell. No statement on this page claims that fasting treats, prevents or heals any disease.
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