Intermittent Fasting Calculator
Plans your daily eating window, meal end time, and per-meal calorie distribution for any intermittent fasting protocol. Ideal for structuring 16:8, 18:6, or custom fasting schedules around your lifestyle.
About this calculator
Intermittent fasting (IF) restricts eating to a defined daily window, cycling between fasting and feeding phases. This calculator derives four key outputs from your inputs: Eating Window = 24 − fastingHours; Window End Time = (startTime + eatingWindow) mod 24; Calories per Meal = dailyCalories ÷ mealCount; Estimated Fasting Burn = fastingHours × 50 kcal. The eating window formula is straightforward — if you fast for 16 hours, you eat within an 8-hour window. The end-time formula uses modular arithmetic so that, for example, a window starting at 12:00 (noon) that spans 8 hours correctly ends at 20:00 (8 PM). The fasting burn estimate applies a rough 50 kcal/hour metabolic baseline active during fasting, acknowledging that true burn varies with body size. Distributing calories evenly across meals helps prevent over-eating in a compressed window.
How to use
Say you choose a 16-hour fast (fastingHours = 16), start your first meal at noon (startTime = 12), target 1,800 daily calories, and plan 2 meals. Eating window = 24 − 16 = 8 hours. Window end time = (12 + 8) mod 24 = 20:00 (8 PM). Calories per meal = 1,800 ÷ 2 = 900 calories each. Estimated fasting burn = 16 × 50 = 800 kcal. So you eat two 900-calorie meals between noon and 8 PM, fasting from 8 PM to noon the next day, burning an estimated 800 kcal during the fast.
Frequently asked questions
What is the best intermittent fasting window for weight loss?
The 16:8 protocol (16 hours fasting, 8 hours eating) is the most researched and widely used for weight loss. It is sustainable for most people because the fasting window overlaps with sleep. Studies suggest 16:8 can reduce daily calorie intake by 200–500 kcal naturally without strict counting. However, the 'best' window is the one you can consistently maintain — even 12:12 provides metabolic benefits over unrestricted eating.
How should I distribute calories across meals in an intermittent fasting eating window?
Even distribution across meals is the simplest approach and helps prevent hunger-driven overeating at one sitting. However, some research supports front-loading calories — placing larger meals earlier in the eating window aligns with circadian rhythms and may improve insulin sensitivity. Regardless of distribution, total daily calorie intake remains the primary driver of weight loss. Protein should be prioritized at every meal (25–40g) to preserve muscle mass during the fasting-induced caloric deficit.
Does intermittent fasting burn extra calories compared to regular dieting?
Fasting modestly elevates norepinephrine, which can increase metabolic rate by 3–14% in short-term fasts of up to 48 hours. However, prolonged fasting (beyond 72 hours) can suppress metabolism. For typical 16–20 hour daily fasts, the additional calorie burn is modest — roughly 50 kcal per fasting hour as estimated here — and the primary weight-loss benefit comes from reduced overall calorie intake rather than a dramatic metabolic boost.