Intermittent Fasting Window Calculator
Calculate your fasting and eating windows based on your chosen IF protocol and preferred eating start time.
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
- The 16:8 protocol is the most studied form of time-restricted eating (TRE) and is the default selection.
- Fasting and eating windows repeat daily on a 24-hour cycle for all protocols except 5:2.
- TDEE estimate uses a simplified factor of 30 kcal/kg/day for sedentary individuals; actual TDEE varies with age, sex, and activity level.
- A 20% caloric deficit is a commonly recommended moderate target for sustainable fat loss (Hall & Kahan, 2018).
- 1 kg of body fat ≈ 7,700 kcal; 1 lb ≈ 3,500 kcal (Wishnofsky, 1958).
- 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).
- 5:2 protocol: 500 kcal/day for women and 600 kcal/day for men on fasting days is the original recommendation (Mosley & Spencer, 2013).
- Water, black coffee, and plain tea are generally considered acceptable during fasting windows and do not break the fast.
- Consult a healthcare provider before starting any fasting regimen, especially if pregnant, diabetic, or on medication.