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Intermittent Fasting Window Calculator

Calculate your fasting and eating windows for 16:8, 18:6, 20:4, or OMAD protocols. Get exact start/end times and estimate when autophagy and ketosis begin.

🗓️ Updated June 2026 Reviewed by
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How many calories to eat on 16/8 intermittent fasting?Find your daily calorie target and how to split it across your 8-hour eating window — using the Mifflin-St. Jeor equation (AND gold standard). Free, instant, no sign-up.16:8 Intermittent Fasting Macro Calculator8 intermittent fasting macros in seconds: 30% protein, 30% fat, 40% carbs split across 2 meals. Free macro table for 1500–2500 kcal.16/8 Intermittent Fasting Calculator — Find Your Eating WindowEnter your last meal time and get your exact break-fast time and eating window. Supports 16/8, 18/6, and OMAD protocols. Free, instant, no signup.What Are Normal Fasting Blood Glucose Levels?Normal fasting glucose <100 mg/dL. Prediabetes 100-125. Diabetes ≥126. Enter your reading in mg/dL or mmol/L — instant ADA 2026 classification.
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Intermittent fasting (IF) cycles between defined eating and fasting periods. Getting the timing right matters: ketosis typically begins 12–18 hours into a fast, and autophagy (cellular cleanup) ramps up around 16–24 hours. This calculator takes your chosen protocol and eating window start time, then outputs your exact fast start, fast end, and eating window — plus a plain-English summary of what's happening in your body at each phase.

When to use this calculator

  • Planning your first intermittent fasting schedule around work hours
  • Shifting your eating window earlier or later to fit social meals
  • Understanding when cellular autophagy and fat-burning (ketosis) begin
  • Comparing 16:8 vs 18:6 vs 20:4 protocols side by side
  • Aligning your fasting window with sleep to maximize fasted hours
  • Checking whether your current eating habits already match an IF protocol

Intermittent Fasting Protocols: Hours & Metabolic Milestones

ProtocolFasting HoursEating HoursKetosis Onset (hrs into fast)Autophagy Onset (hrs into fast)
16:816812–18 h16–24 h
18:618612–18 h16–24 h
20:420412–18 h16–24 h
OMAD23112–18 h16–24 h

Fuente: New England Journal of Medicine (2019) — Mattson et al., 'Effects of Intermittent Fasting on Health, Aging, and Disease'; NIH/NLM, 'Autophagy: Regulation and Role in Disease' (2014). Ketosis and autophagy ranges are population-level estimates; individual timing varies.

How it works

What is intermittent fasting?

Intermittent fasting is an eating pattern that cycles between defined periods of eating and fasting. Ketosis typically begins 12–18 hours into a fast, while autophagy (cellular cleanup) ramps up around 16–24 hours. Common protocols include 16:8 (16 hours fasting, 8 hours eating) and 20:4, each triggering different metabolic shifts.

How It Works

Intermittent fasting protocols are defined by a ratio of fasting hours : eating hours. This calculator accepts your chosen protocol and the clock time you plan to break your fast (eating window start), then computes every boundary time and estimates key metabolic milestones.

Formula

Eating window end = Eating start + Eating hours
Fast start       = Eating window end  (mod 24h)
Fast end         = Fast start + Fasting hours  (mod 24h)
                 = Eating window start  (next day if overnight)

Ketosis onset    ≈ Fast start + 12–18 h
Autophagy onset  ≈ Fast start + 16–24 h

Protocol breakdown:

ProtocolFasting HoursEating Hours
16:8168
18:6186
20:4204
OMAD231

Worked Example

Protocol: 16:8, Eating window start: 12:00 PM

  • Eating window: 12:00 PM → 8:00 PM (8 hours)

  • Fast begins: 8:00 PM

  • Fast ends: 12:00 PM next day (16 hours later)

  • Ketosis estimate: ~8:00 AM – 2:00 PM (hours 12–18 of fast)

  • Autophagy estimate: ~12:00 PM – 8:00 PM (hours 16–24 of fast)

    • Protocol: OMAD, Eating window start: 6:00 PM

  • Eating window: 6:00 PM → 7:00 PM (1 hour)

  • Fast begins: 7:00 PM

  • Fast ends: 6:00 PM next day (23 hours later)

  • Ketosis estimate: ~7:00 AM – 1:00 PM

  • Autophagy estimate: ~11:00 AM – 7:00 PM

    • Metabolic Phase Notes

    Ketosis occurs when liver glycogen is depleted and the body begins oxidizing fatty acids into ketone bodies. This typically happens 12–18 hours into a fast for most adults, though exact timing varies with pre-fast carbohydrate intake, activity level, and individual metabolism.

    Autophagy is a cellular recycling process that increases during extended fasting. Research (primarily in animal models and limited human studies) suggests meaningful upregulation begins around 16–24 hours. The calculator marks a conservative range; actual induction depends on baseline metabolic state.

    Permitted During the Fasting Window

  • Water (plain, sparkling — no sweeteners)

  • Black coffee (no milk, cream, or sugar)

  • Plain black or green tea

  • Electrolytes without calories

    • Anything with calories — including bulletproof coffee, bone broth, or diet sodas containing insulin-spiking sweeteners — is generally considered to break a metabolic fast.

    Limitations

  • Ketosis and autophagy estimates are population-level ranges, not individual predictions.

  • Time zones, shift work, and highly variable meal timing reduce precision.

  • This calculator does not account for exercise timing, macro composition, or medical conditions.

  • Consult a physician before starting IF if you have diabetes, a history of disordered eating, are pregnant, or take medications requiring food.
    • Disclaimer: Los resultados son orientativos y no reemplazan la consulta médica profesional. Antes de tomar decisiones con impacto, consultá con un médico, nutricionista o profesional de la salud matriculado.

    Frequently asked questions

    What is the 16:8 protocol?
    You fast for 16 consecutive hours and eat within an 8-hour window each day. A common schedule is noon to 8 PM. It is the most studied and widely practiced IF pattern and is generally well-tolerated for beginners.
    Does black coffee break a fast?
    Black coffee (no milk, sugar, or creamer) contains roughly 5 calories per cup and does not significantly raise insulin in most people. The majority of IF practitioners and researchers consider it acceptable during the fasting window without meaningfully disrupting metabolic benefits.
    When does ketosis start during a fast?
    For most adults eating a typical Western diet, ketosis begins approximately 12–18 hours into a fast once liver glycogen stores are depleted. If you ate low-carb before your fast, ketosis can begin earlier — sometimes within 8–10 hours.
    What is autophagy and when does it begin?
    Autophagy is a cellular process that breaks down damaged proteins and organelles. Animal studies and limited human research suggest it increases meaningfully around 16–24 hours of fasting. Extended fasts (24–72 h) show the strongest autophagy signals, but even 16-hour fasts show upregulation compared to fed states.
    Is OMAD (one meal a day) safe?
    OMAD (23:1) is practiced by many healthy adults but is more demanding than 16:8. Potential downsides include difficulty meeting caloric and micronutrient needs in one hour, higher risk of muscle loss, and potential blood sugar volatility. It is not recommended for people with diabetes, a history of eating disorders, or high athletic training loads without professional guidance.
    Can I exercise during the fasting window?
    Yes. Fasted exercise, particularly low-to-moderate intensity cardio, is compatible with IF and may enhance fat oxidation. High-intensity or strength training sessions are often better placed just before or during the eating window to support recovery and muscle protein synthesis.
    Does the eating window start time matter?
    Yes. Chrono-nutrition research suggests earlier eating windows (e.g., 8 AM–4 PM) align better with circadian rhythms and may improve insulin sensitivity and metabolic markers compared to late-night eating windows, even with identical calorie totals.
    How do I shift my eating window earlier or later?
    Move your eating window start by 30–60 minutes every few days rather than shifting it abruptly. Hunger cues will adapt within 1–2 weeks. Keeping consistent sleep/wake times helps anchor the transition.
    Does intermittent fasting work without calorie restriction?
    Evidence is mixed. Several controlled trials show IF produces weight loss primarily because it reduces total calorie intake by shrinking the eating window. When calories are matched between IF and continuous restriction, outcomes are similar. IF may offer additional metabolic benefits independent of weight loss, but research is ongoing.
    Who should NOT do intermittent fasting?
    People who are pregnant or breastfeeding, children and adolescents, individuals with type 1 diabetes or unstable type 2 diabetes, those with a history of anorexia or bulimia, and anyone on medications that require food should avoid or closely supervise IF with a healthcare provider.

    Sources & references

    Methodology & trust

    Editorial

    Calculadora de salud revisada por el equipo editorial de Hacé Cuentas, contrastada con Effects of Intermittent Fasting on Health, Aging, and Disease — NEJM 2019, según nuestra política editorial y metodología.

    Updates

    Última revisión: June 20, 2026. Los parámetros se verifican periódicamente con las fuentes citadas.

    Privacy

    Calculations run 100% in your browser. We do not store or transmit your data.

    Limitations

    Indicative results. For critical decisions, consult a professional.

    📌 How to cite this calculator

    Rodríguez, M. (2026). Intermittent Fasting Window Calculator. Hacé Cuentas. https://hacecuentas.com/intermittent-fasting-window-calculator

    Contenido bajo licencia CC-BY 4.0 — reutilizable citando la fuente con enlace a Hacé Cuentas.

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