Intermittent Fasting Calculator

The Intermittent Fasting Calculator helps you plan your daily eating and fasting windows based on popular IF protocols: 16:8, 18:6, 20:4, or custom splits. Intermittent fasting cycles between periods of eating and voluntary fasting to reduce overall calorie intake, improve insulin sensitivity, and support metabolic health. Enter your preferred eating window start time and the calculator maps out exactly when to eat and when to fast.

How to Use This Calculator

1. Select your IF protocol — 16:8, 18:6, 20:4, or enter a custom fasting duration.
2. Enter the time you prefer to start eating (your eating window open time).
3. Optionally enter your goal — weight loss, metabolic health, or autophagy.
4. Click Calculate to see your full daily schedule with eating window start/end and fast start/end times.

What This Calculator Measures

This calculator outputs a daily fasting and eating schedule tailored to your chosen protocol.

• Eating window: The number of hours per day during which you consume all calories.
• Fasting window: The remaining hours during which you consume only water, black coffee, or plain tea.
• 16:8 protocol: Fast for 16 hours, eat within an 8-hour window — the most popular method.
• Autophagy window: Fasts of 18+ hours begin to stimulate cellular autophagy (self-cleaning) more actively.

Formula or Logic

Eating window end = Eating window start + eating hours. Fasting window start = Eating window end. Fasting window end = Eating window start the next day. The calculator outputs these four times along with suggested meal timings within the eating window to distribute calories and protein optimally.

Example Calculations

Example 1: 16:8 protocol starting at 12:00 PM. Eating window: 12:00 PM – 8:00 PM. Fasting window: 8:00 PM – 12:00 PM next day (16 hours).

Example 2: 18:6 protocol starting at 1:00 PM. Eating window: 1:00 PM – 7:00 PM. Fasting window: 7:00 PM – 1:00 PM next day (18 hours).

Understanding Your Results

Consistency with your eating window timing is more important than the exact hours chosen. Aligning your eating window with daylight hours (earlier eating) tends to produce better metabolic outcomes. The longer the fast, the more pronounced the insulin-lowering effect.

Common Mistakes to Avoid

• Breaking the fast with high-sugar, high-calorie foods that spike insulin immediately.
• Using intermittent fasting as license to overeat within the eating window.
• Starting with a strict 20:4 protocol without building up gradually from 12:12 or 14:10.
• Fasting while pregnant, breastfeeding, or managing an eating disorder without medical supervision.