Intermittent Fasting Calculator
Calculates your eating window end time and per-meal calorie distribution based on your fasting protocol, first meal time, and daily calorie goal. Use it when starting 16:8, 18:6, or any time-restricted eating plan.
About this calculator
Intermittent fasting (IF) protocols restrict eating to a defined window within each 24-hour cycle. The eating window duration equals 24 minus the fasting hours — for example, a 16:8 protocol has an 8-hour eating window. The last meal time (window close) is calculated as: lastMealTime = (firstMealTime + eatingWindowHours) mod 24. This ensures the time wraps correctly past midnight if needed. Once the window is defined, daily calories are evenly distributed across the number of meals in that window: caloriesPerMeal = dailyCalories ÷ mealCount. Popular protocols include 16:8 (fast 16 hours, eat for 8), 18:6, and 20:4. IF works for weight loss primarily through caloric restriction and, in some people, improved insulin sensitivity.
How to use
Example: 16:8 protocol, first meal at 12:00 (hour 12), 2,000 daily calories, 3 meals. Step 1 — Eating window: 24 − 16 = 8 hours. Step 2 — Last meal time: 12 + 8 = 20, i.e., 8:00 PM. Step 3 — Calories per meal: 2,000 ÷ 3 ≈ 667 calories each. Your eating window is 12:00 PM–8:00 PM, with meals at roughly 12 PM, 4 PM, and 8 PM, each around 667 calories.
Frequently asked questions
How do I calculate my eating window for a 16:8 intermittent fasting schedule?
Simply subtract your fasting hours from 24 to get your eating window length — for 16:8, that is 8 hours. Add that window length to your first meal time to get your last allowed meal time. For example, if you eat your first meal at 10 AM, your eating window closes at 6 PM. Consistency in your window timing each day helps regulate circadian rhythm and hunger hormones like ghrelin, which makes adherence easier over time.
What is the best intermittent fasting protocol for weight loss beginners?
The 16:8 protocol is widely recommended for beginners because the 16-hour fast is largely covered by sleep — typically 7–8 hours — requiring only a few additional fasting hours in the morning. It allows two to three satisfying meals within the window, making it easier to hit protein targets. More aggressive protocols like 20:4 or OMAD (one meal a day) can be effective but increase the risk of nutrient deficiencies and muscle loss if protein intake is not carefully managed. Starting with 16:8 and adjusting based on hunger and energy levels is a sensible approach.
Can intermittent fasting help preserve muscle while losing fat?
Intermittent fasting is not inherently muscle-sparing, but it can be made so with the right strategy. Consuming adequate protein — at least 0.7–1.0 g per pound of body weight — within the eating window is the primary driver of muscle preservation. Resistance training, ideally scheduled near the eating window so you can consume protein post-workout, further signals the body to retain lean mass. Short-term fasting periods (under 24 hours) do not significantly elevate muscle protein breakdown in healthy individuals, especially those adapted to the protocol.