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Intermittent Fasting Calculator
Schedule your intermittent fasting windows from a protocol (16:8 / 18:6 / 20:4 / OMAD) and your last-meal time — the eating window opens the fasting-hours later and closes at the next last-meal time, shown as exact clock times (last meal 8:00 PM on 16:8 → eat 12:00 PM–8:00 PM). Overlays an approximate fasting-state timeline (fed <12h · glycogen depleting 12–16h · mild ketosis 16–24h · deeper ketosis + autophagy 24h+), plus honest "what breaks a fast" guidance (black coffee/water/electrolytes fine; cream/sugar/sweeteners/BCAAs break it). Prominent safety screen: not without a clinician for Type 1 diabetes or insulin/sulfonylureas (hypoglycemia), pregnancy or breastfeeding, a history of eating disorders, or children and adolescents. Lifestyle information, not medical advice. Accurate, instant and free — for United States.
What these mean:
What these mean:
Fast for 16h, then eat 12:00 PM–8:00 PM.
Eating window · 8h
12:00 PM – 8:00 PM
Fasting window · 16h
8:00 PM – 12:00 PM
Fasting-state timeline
approximate — varies by person
- 0–12hFed
- 12–16hGlycogen depleting
- 16–24hMild ketosisyour 16h fast reaches here
- 24h+Deeper ketosis + autophagy
What breaks a fast
5:2 & eat-stop-eat are day-based
Intermittent fasting isn't for everyone
How we calculate this
Reviewed by Reckonist Editorial · Last reviewed July 2026. Figures follow the methods and sources set out in our editorial standards.
Window times are exact clock math; the fasting-state timeline (ketosis/autophagy) is approximate and varies by person. Lifestyle information, not medical advice — see the contraindications above.
Johns Hopkins / Mayo · fasting-state literature
Plan what to eat in your window with the Macro Calculator, stay hydrated during the fast with the Water Intake Calculator, and track progress with the Weight Loss Percentage Calculator.
The protocols — 16:8, 18:6, 20:4, OMAD & 5:2
Intermittent fasting splits the day into a fasting window and an eating window. The presets differ only in how long the fast lasts — the eating window is simply 24 − fasting hours.
Daily windows
16:8 · 18:6 · 20:4 · OMAD
eat_hours = 24 − fast_hours
16:8 = 8h eat (beginner default) · 18:6 · 20:4 (Warrior) · OMAD ≈ 23h fast, ~1h eat.
Day-based patterns
5:2 · eat-stop-eat
whole fasting days, not clock windows
5:2 = ~500–600 kcal on 2 days/week; eat-stop-eat = a full 24h fast once or twice a week.
The window scheduler — from last meal to clock times
Enter the time of your last meal and pick a protocol. The eating window opens the fasting-hours later and closes at the next last-meal time — this tool renders both windows as clock times.
- 1Last meal: 8:00 PM = 20:00.
- 2Eating window opens (fast 16h later): 20:00 + 16h = 12:00 PM (noon) next day.
- 3Eating window (8h): 12:00 PM – 8:00 PM; the rest is the fasting window.
The fasting-state timeline — approximate, not a guarantee
As the fast lengthens, the body shifts fuel sources. The hour markers below are approximate and vary by person, diet and activity — treat them as a general map:
- 0–12h — fed / glycogen: running on your last meal.
- 12–16h — glycogen depleting: stored carbohydrate runs low.
- 16–24h — mild ketosis: fat-derived ketones rise (milder than keto-diet ketosis).
- 24h+ — deeper ketosis + more autophagy is associated with longer fasts.
These timings are estimates
What breaks a fast (and what doesn't)
Anything with calories breaks the fast. During the fasting window, black coffee, plain water and unsweetened tea are fine, and electrolytes (sodium, potassium, magnesium with no sugar) are generally accepted.
Breaks it
Who should NOT fast without a clinician
Intermittent fasting isn't for everyone
These contraindications are verified against Johns Hopkins Medicine and Mayo Clinic guidance. The window math on this page is exact; the fasting-state timings are approximate. This is lifestyle information, not medical advice.
Frequently asked questions
Pick the time of your last meal, then fast for 16 hours — the eating window opens 16 hours later and stays open for 8 hours until the same time next day. If your last meal is at 8:00 PM, you fast until 12:00 PM (noon) the next day, and your eating window is 12:00 PM–8:00 PM. This calculator does that clock math for every preset (16:8, 18:6, 20:4, OMAD) automatically, and shows both the eating and fasting windows as clock times.
These timings are approximate and vary a lot by person, diet and activity. As a rough guide: you stay in the fed/glycogen state for roughly the first 12 hours, glycogen depletes from about 12–16 hours, mild ketosis tends to set in around 16–24 hours, and deeper ketosis with more autophagy is associated with 24 hours and beyond. Fasting ketosis is milder than a ketogenic-diet ketosis. Treat the timeline as a general map, not a guarantee.
OMAD means "one meal a day" — you eat a single meal within a short window (about 1 hour) and fast for the rest of the day, roughly a 23-hour fast. It is one of the more aggressive schedules. This tool models OMAD as a 23:1 protocol. Because the eating window is so short, it is easier to under-eat protein and micronutrients, so it is not a good starting point for beginners.
Anything with calories breaks the fast. During the fasting window, black coffee, plain water and unsweetened tea are fine, and electrolytes (sodium, potassium, magnesium with no sugar) are generally accepted. What breaks it: cream or milk in coffee, sugar, honey, juice, artificial sweeteners, and BCAAs or protein. If you are fasting for the metabolic effects, keep the fasting window to zero-calorie drinks only.
They are day-based patterns rather than daily windows. 5:2 means eating normally five days a week and cutting to about 500–600 calories on two non-consecutive days (roughly 500 for women, 600 for men). Eat-stop-eat means one or two full 24-hour fasts per week. This calculator schedules daily windows (16:8, 18:6, 20:4, OMAD); 5:2 and eat-stop-eat are planned as whole fasting days instead.
Intermittent fasting is not safe for everyone. Do not start without a clinician’s supervision if you have Type 1 diabetes or take insulin or sulfonylureas (risk of hypoglycemia — the medication must be adjusted by a clinician), if you are pregnant or breastfeeding, if you have a history of an eating disorder, or for children and adolescents. If any of these apply, talk to a clinician before changing your eating pattern. This is lifestyle information, not medical advice.
Methods, formulas & references
Methodology: fasting hours per protocol (16:8 → 16, 18:6 → 18, 20:4 → 20, OMAD → 23); eating hours = 24 − fasting hours; eating window opens fasting-hours after the last meal and closes at the next last-meal time (clock wrap handled mod 24h). The fasting-state timeline (fed <12h · glycogen depleting 12–16h · mild ketosis 16–24h · deeper ketosis + autophagy 24h+) is approximate and varies by person. All calculations run client-side and nothing is stored. Lifestyle information, not medical advice — see the contraindications.
Cross-links
How we calculate this
Reviewed by Reckonist Editorial · Last reviewed July 2026. Figures follow the methods and sources set out in our editorial standards.
Window times are exact clock math from your chosen protocol and last-meal time; the fasting-state timeline (ketosis/autophagy) is approximate and varies by person. Lifestyle information, not medical advice — see the contraindications above.
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