Intermittent Fasting Window Calculator
Calculate your eating and fasting windows based on popular intermittent fasting methods and your preferred first meal time.
Results
Visualization
How It Works
The Intermittent Fasting Window Calculator determines your exact eating and fasting periods based on four popular methods: 16:8, 18:6, 20:4, and OMAD (one meal a day). You choose your preferred first meal time and the calculator shows when to stop eating and when your fasting window begins and ends. Intermittent fasting may support weight management, insulin sensitivity, and cellular repair processes like autophagy.
The Formula
Variables
- Method — The fasting protocol: 16:8 (16 hours fasting, 8 eating), 18:6, 20:4, or OMAD (23:1)
- First Meal Hour — The hour of the day (0-23) when you break your fast
- Eating Hours — Duration of the eating window (8, 6, 4, or 1 hour depending on method)
- Fasting Hours — Duration of the fasting window (16, 18, 20, or 23 hours)
Worked Example
Using the 16:8 method with a first meal at 12:00 PM: Eating window = 12:00 PM to 8:00 PM (8 hours). Fasting window = 8:00 PM to 12:00 PM the next day (16 hours). Your last meal must be finished by 8:00 PM.
Practical Tips
- Start with 16:8 if you are new to intermittent fasting. It is the most sustainable method and essentially just means skipping breakfast for most people.
- Black coffee, plain tea, and water are generally considered acceptable during the fasting window as they have negligible calories.
- During your eating window, focus on nutrient-dense whole foods. Fasting does not give you a free pass to eat junk food in unlimited quantities.
- If you exercise, try to schedule workouts near the start of your eating window so you can refuel afterward for better recovery.
- Listen to your body. If you feel dizzy, extremely fatigued, or irritable, consider shortening your fasting window or consulting a healthcare provider.
Frequently Asked Questions
Does intermittent fasting help with weight loss?
Intermittent fasting can help with weight loss primarily by reducing total calorie intake through a shorter eating window. Some research also suggests metabolic benefits like improved insulin sensitivity. However, it is not magic; you still need to eat at or below your calorie needs during the eating window to lose weight.
Can I drink anything during the fasting window?
Water, black coffee, and unsweetened tea are generally fine during fasting as they contain zero or negligible calories. Avoid adding cream, sugar, or sweeteners, as these can break the fast by triggering an insulin response. Bone broth is a gray area; purists avoid it, but some protocols allow it.
Is OMAD (one meal a day) safe?
OMAD is an advanced fasting protocol that can be difficult to sustain and may lead to nutrient deficiencies if the single meal is not well-planned. It can be safe for some healthy adults in the short term, but it is not recommended for beginners, pregnant women, those with eating disorder histories, or people on certain medications.
Will I lose muscle during fasting?
Short fasting windows (16-20 hours) are unlikely to cause significant muscle loss, especially if you eat adequate protein during your eating window and maintain resistance training. Studies show that time-restricted feeding preserves lean mass when combined with strength training and sufficient protein intake of 1.6-2.2 g per kg body weight.
Who should NOT do intermittent fasting?
Intermittent fasting is not recommended for pregnant or breastfeeding women, children and teenagers, people with a history of eating disorders, those with type 1 diabetes or on insulin, and individuals who are underweight. Anyone on medication should consult their doctor, as fasting can affect drug absorption and blood sugar levels.