Intermittent Fasting Window Calculator
ANA›Life Services Authority›National Calculator Authority›Intermittent Fasting Window Calculator
.calc-container { max-width: 640px; margin: 2rem 0; padding: 1.5rem; background: #fff; border: 1px solid #ddd; border-radius: 8px; box-shadow: 0 1px 3px rgba(0,0,0,0.06); font-family: system-ui, -apple-system, sans-serif; } .calc-container h3 { font-family: Georgia, serif; font-size: 1.15rem; color: #1a1a1a; margin-bottom: 1rem; padding-bottom: 0.5rem; border-bottom: 2px solid var(--ac, #3d5a80); } .calc-row { display: flex; align-items: center; gap: 0.75rem; margin-bottom: 0.75rem; flex-wrap: wrap; } .calc-row label { min-width: 160px; font-size: 0.9rem; color: #333; font-weight: 500; } .calc-row input[type="number"], .calc-row select { flex: 1; min-width: 120px; max-width: 200px; padding: 0.5rem 0.6rem; border: 1px solid #ccc; border-radius: 4px; font-size: 0.9rem; font-family: system-ui, sans-serif; color: #1a1a1a; background: #fafaf8; } .calc-row input:focus, .calc-row select:focus { outline: none; border-color: var(--ac, #3d5a80); box-shadow: 0 0 0 2px rgba(26,74,138,0.12); } .calc-row .unit { font-size: 0.82rem; color: #888; min-width: 30px; } .calc-btn { display: inline-block; margin-top: 0.5rem; padding: 0.55rem 1.5rem; background: var(--ac, #3d5a80); color: #fff; border: none; border-radius: 4px; font-size: 0.9rem; font-weight: 600; cursor: pointer; font-family: system-ui, sans-serif; } .calc-btn:hover { opacity: 0.9; } .calc-result { margin-top: 1.25rem; padding: 1rem 1.25rem; background: #f0f6fc; border-left: 3px solid var(--ac, #3d5a80); border-radius: 0 6px 6px 0; display: none; } .calc-result.visible { display: block; } .calc-result-label { font-size: 0.78rem; text-transform: uppercase; letter-spacing: 0.06em; color: #666; margin-bottom: 0.25rem; } .calc-result-value { font-size: 1.6rem; font-weight: 700; color: var(--ac, #3d5a80); } .calc-result-detail { font-size: 0.85rem; color: #555; margin-top: 0.5rem; line-height: 1.5; } .calc-note { margin-top: 1rem; font-size: 0.8rem; color: #888; font-style: italic; } .calc-grid { display: grid; grid-template-columns: 1fr 1fr; gap: 0.75rem; margin-top: 0.75rem; } .calc-grid-item { padding: 0.6rem 0.8rem; background: #f8f9fa; border-radius: 4px; border: 1px solid #eee; } .calc-grid-item .label { font-size: 0.75rem; color: #888; text-transform: uppercase; letter-spacing: 0.04em; } .calc-grid-item .value { font-size: 1.1rem; font-weight: 600; color: #1a1a1a; } @media (max-width: 720px) { .calc-row { flex-direction: column; align-items: flex-start; gap: 0.3rem; } .calc-row label { min-width: auto; } .calc-row input[type="number"], .calc-row select { max-width: 100%; width: 100%; } .calc-grid { grid-template-columns: 1fr; } } .calc-chart { margin: 1rem 0; text-align: center; } .calc-chart svg { max-width: 100%; height: auto; } .calc-chart-legend { display: flex; flex-wrap: wrap; justify-content: center; gap: 0.6rem 1.2rem; margin-top: 0.6rem; font-size: 0.8rem; color: #555; } .calc-chart-legend span { display: inline-flex; align-items: center; gap: 0.3rem; } .calc-chart-legend i { display: inline-block; width: 10px; height: 10px; border-radius: 2px; font-style: normal; } .calc-related { max-width: 640px; margin: 2rem 0 1rem; padding: 1.25rem 1.5rem; background: #f8f9fa; border: 1px solid #e8e8e8; border-radius: 8px; } .calc-related h3 { font-family: Georgia, serif; font-size: 1rem; color: #1a1a1a; margin: 0 0 0.75rem; padding-bottom: 0.4rem; border-bottom: 2px solid var(--ac, #3d5a80); } .calc-related-list { list-style: none; padding: 0; margin: 0 0 0.75rem; display: grid; grid-template-columns: 1fr 1fr; gap: 0.4rem 1.5rem; } .calc-related-list li a { font-size: 0.88rem; color: var(--ac, #3d5a80); text-decoration: none; } .calc-related-list li a:hover { text-decoration: underline; } .calc-browse-all { margin: 0.5rem 0 0; font-size: 0.9rem; font-weight: 600; } .calc-browse-all a { color: var(--ac, #3d5a80); text-decoration: none; } .calc-browse-all a:hover { text-decoration: underline; } @media (max-width: 720px) { .calc-related-list { grid-template-columns: 1fr; } }
Intermittent Fasting Window Calculator
Calculate your fasting and eating windows based on your chosen IF protocol and preferred eating start time.
Fasting Protocol
16:8 – Fast 16 hrs, Eat 8 hrs (most popular) 18:6 – Fast 18 hrs, Eat 6 hrs 20:4 – Fast 20 hrs, Eat 4 hrs (Warrior Diet) 14:10 – Fast 14 hrs, Eat 10 hrs (beginner-friendly) 12:12 – Fast 12 hrs, Eat 12 hrs (entry level) 5:2 – Eat normally 5 days, restrict 2 days Custom – Enter your own fasting hours
Custom Fasting Hours (1–23)
Eating Window Start Time
Last Meal Time (Day 1, for 5:2)
Calorie Intake on Fasting Days (kcal)
Body Weight (optional, for calorie guidance)
kg lbs
Calculate
function intUpdateProtocol() { var proto = document.getElementById('int-protocol').value; document.getElementById('int-custom-group').style.display = (proto === 'custom') ? 'block' : 'none'; document.getElementById('int-eating-start-group').style.display = (proto === '5:2') ? 'none' : 'block'; document.getElementById('int-last-meal-group').style.display = (proto === '5:2') ? 'block' : 'none'; document.getElementById('int-restrict-cal-group').style.display = (proto === '5:2') ? 'block' : 'none'; }
function intParseTime(timeStr) { // Returns total minutes from midnight var parts = timeStr.split(':'); return parseInt(parts[0]) * 60 + parseInt(parts[1]); }
function intFormatTime(totalMinutes) { // Wraps around 24 hours totalMinutes = ((totalMinutes % 1440) + 1440) % 1440; var h = Math.floor(totalMinutes / 60); var m = totalMinutes % 60; var ampm = h >= 12 ? 'PM' : 'AM'; var h12 = h % 12 === 0 ? 12 : h % 12; return (h12 1000) { resultDiv.style.display = 'block'; resultDiv.innerHTML = 'Calorie intake on fasting days must be between 0 and 1000 kcal.
'; return; }
var lastMealMin = intParseTime(lastMealStr); // On fasting days eat only one small meal; fast resumes after that meal // Typical 5:2: 2 non-consecutive days per week with ~500 kcal (women) / ~600 kcal (men) var fastingDaysPerWeek = 2; var normalDaysPerWeek = 5;
var weightKg = null; var weightNote = ''; if (weightVal !== '') { weightKg = parseFloat(weightVal); if (isNaN(weightKg) || weightKg 300) { resultDiv.style.display = 'block'; resultDiv.innerHTML = 'Weight must be between 30 and 300 ' + weightUnit + '.
'; return; } if (weightUnit === 'lbs') weightKg = weightKg * 0.453592; // Rough TDEE estimate: 30 kcal/kg/day (sedentary baseline) var tdee = Math.round(weightKg * 30); var weeklyDeficit = fastingDaysPerWeek * (tdee - restrictCal); var weeklyDeficitLbs = (weeklyDeficit / 7700 * 2.205).toFixed(2); // 7700 kcal ≈ 1 kg fat weightNote = 'Est. TDEE (sedentary)' + tdee + ' kcal/day' + 'Weekly calorie deficit' + weeklyDeficit.toLocaleString() + ' kcal' + 'Est. weekly fat loss~' + weeklyDeficitLbs + ' lbs / ' + (weeklyDeficit/7700).toFixed(2) + ' kg'; }
resultDiv.style.display = 'block'; resultDiv.innerHTML = '### Your 5:2 Intermittent Fasting Schedule ' + '' + 'Protocol5:2 (Two-Day Modified Fast)' + 'Normal eating days / week' + normalDaysPerWeek + ' days' + 'Fasting / restricted days / week' + fastingDaysPerWeek + ' days (non-consecutive)' + 'Calorie limit on fasting days' + restrictCal + ' kcal' + 'Last meal on Day 1' + intFormatTime(lastMealMin) + '' + 'Suggested fasting day meal timeAround ' + intFormatTime(lastMealMin) + ' (one sitting)' + weightNote + '' + 'Tip: Choose Monday & Thursday or Tuesday & Friday as your fasting days to avoid consecutive restriction.
'; return; }
// ── Daily time-restricted eating branch ───────────────────────────────────── var fastingHours; if (proto === 'custom') { fastingHours = parseFloat(document.getElementById('int-custom-hours').value); if (isNaN(fastingHours) || fastingHours 23) { resultDiv.style.display = 'block'; resultDiv.innerHTML = 'Custom fasting hours must be between 1 and 23.
'; return; } } else { fastingHours = parseFloat(proto.split(':')[0]); }
var eatingHours = 24 - fastingHours; var fastingMins = fastingHours * 60; var eatingMins = eatingHours * 60;
var eatStartStr = document.getElementById('int-eating-start').value; if (!eatStartStr) { resultDiv.style.display = 'block'; resultDiv.innerHTML = 'Please enter your eating window start time.
'; return; }
var eatStartMin = intParseTime(eatStartStr); var eatEndMin = eatStartMin + eatingMins; // may exceed 1440 → wraps var fastStartMin = eatEndMin; // fast begins when eating ends var fastEndMin = fastStartMin + fastingMins; // = eatStartMin + 1440
// Optimal eating window: research suggests earlier windows (e.g. 8 AM–4 PM) align // better with circadian rhythms (Sutton et al., 2018; Wilkinson et al., 2020) var eatStartHour = Math.floor(eatStartMin / 60); var circadianNote = ''; if (eatStartHour >= 6 && eatStartHour 10 && eatStartHour 300) { resultDiv.style.display = 'block'; resultDiv.innerHTML = 'Weight must be between 30 and 300 ' + weightUnit + '.
'; return; } if (weightUnit === 'lbs') weightKg = weightKg * 0.453592; var tdee = Math.round(weightKg * 30); // sedentary baseline var lossTarget = Math.round(tdee * 0.80); // 20% deficit for fat loss var maintTarget = tdee; weightRows = 'Est. TDEE (sedentary)' + tdee + ' kcal/day' + 'Maintenance calories' + maintTarget + ' kcal/day' + 'Fat-loss target (−20%)' + lossTarget + ' kcal/day'; }
// Autophagy note: typically begins ~16–18 hrs into a fast (Alirezaei et al., 2010) var autophagyNote = ''; if (fastingHours >= 16) { autophagyNote = 'Autophagy onset (est.)~' + intFormatTime(fastStartMin + 16*60) + ' (after ~16 hrs fasting)'; }
resultDiv.style.display = 'block'; resultDiv.innerHTML = '### Your Intermittent Fasting Schedule ' + '' + 'Protocol' + fastingHours + ':' + eatingHours + '' + 'Fasting window' + intFormatDuration(fastingMins) + '' + 'Eating window' + intFormatDuration(eatingMins) + '' + '🍽️ Eating window' + intFormatTime(eatStartMin) + ' → ' + intFormatTime(eatEndMin) + '' + '🚫 Fasting window' + intFormatTime(fastStartMin) + ' → ' + intFormatTime(fastEndMin) + '' + 'First meal (break fast)' + intFormatTime(eatStartMin) + '' + 'Last meal (cutoff)' + intFormatTime(eatEndMin) + '' + autophagyNote + weightRows + '' + '' + circadianNote + '
'; }
#### Formulas Used
Eating window end time: Eating End = Eating Start + Eating Hours
Fasting window: Fasting Start = Eating End Fasting End = Eating Start (next day) Eating Hours = 24 − Fasting Hours
Estimated TDEE (sedentary baseline): TDEE ≈ Body Weight (kg) × 30 kcal/kg/day
Fat-loss calorie target: Target = TDEE × 0.80 (20% deficit)
Autophagy onset (time-restricted eating): Autophagy Start ≈ Fasting Start + 16 hours
5:2 weekly calorie deficit: Weekly Deficit = 2 × (TDEE − Fasting Day Calories) Est. Fat Loss (kg/week) = Weekly Deficit ÷ 7,700
#### Assumptions & References
- Autophagy (cellular self-cleaning) is estimated to begin after ~16 hours of fasting (Alirezaei et al., 2010, Autophagy).
- Early time-restricted eating (eTRE, e.g., 8 AM–4 PM) shows superior metabolic outcomes vs. late windows (Sutton et al., 2018, Cell Metabolism; Wilkinson et al., 2020, Cell Metabolism).
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