What This Calculator Does
You keep hearing about intermittent fasting. Your coworker lost 15 pounds doing 16:8. Your brother swears by OMAD. But which protocol is right for you, and what should your eating window actually look like?
This calculator helps you choose an intermittent fasting protocol and generates a daily schedule with specific eating and fasting windows. Select a protocol, set your preferred eating window start time, and the calculator shows exactly when to eat and when to fast.
A 2024 review published in Nature Reviews Endocrinology analyzed 23 randomized controlled trials of intermittent fasting. The review found that time-restricted eating (16:8 and 18:6 protocols) produced an average weight loss of 3 to 8% of body weight over 8 to 12 weeks, comparable to continuous calorie restriction. However, intermittent fasting also improved insulin sensitivity and reduced oxidative stress markers independent of weight loss. For planning your meals during the eating window, pair this with our Calorie Calculator and Macro Calculator.
Available Protocols
- 12:12: 12 hours fasting, 12 hours eating. Beginner-friendly. Essentially just skipping late-night snacks.
- 14:10: 14 hours fasting, 10 hours eating. Recommended starting point for women, who may respond differently to longer fasts.
- 16:8: 16 hours fasting, 8 hours eating. The most popular protocol. Typically skip breakfast, eat from 12 PM to 8 PM.
- 18:6: 18 hours fasting, 6 hours eating. For experienced fasters seeking faster results.
- 20:4 (Warrior Diet): 20 hours fasting, 4 hours eating. One large evening meal.
- OMAD: One Meal A Day. 23 hours fasting, 1 hour eating. Extreme protocol.
- 5:2: Eat normally 5 days, restrict to 500 to 600 calories on 2 non-consecutive days.
- Eat-Stop-Eat: 24-hour fast once or twice per week.
How the Calculation Works
The calculator takes your chosen protocol and eating window start time, then calculates the end of your eating window and the full fasting period. For example, with 16:8 starting at 12 PM:
Eating: 12:00 PM to 8:00 PM (8 hours)
Fasting: 8:00 PM to 12:00 PM next day (16 hours)
The timeline visualization shows your eating window as a green bar on a 24-hour timeline, making it easy to see how your schedule fits into the day. You can adjust the start time to match your lifestyle, whether you prefer eating breakfast at 7 AM or lunch at noon.
How to Use the Calculator
- Select a fasting protocol that matches your experience level and goals
- Set your eating window start time using the slider or time input
- Review your eating and fasting schedule
- Check the timeline visualization to see how it fits your day
- Compare protocols if you are unsure which to choose
- Use the schedule to plan your meals and set reminders
Example Schedules
Example 1: The Office Worker on 16:8
David works 9 to 5 and wants to do 16:8. He sets his eating window to start at 12 PM.
- Eating window: 12:00 PM to 8:00 PM
- Fasting: 8:00 PM to 12:00 PM next day
- He eats lunch at noon, a snack at 3 PM, and dinner at 7 PM
- He drinks black coffee and water in the morning before lunch
This schedule works well for office workers because the fasting period covers the morning rush and the eating window covers lunch and dinner. David does not have to bring breakfast or worry about morning meals.
Example 2: The Early Riser on 14:10
Sarah wakes up at 5 AM for the gym and prefers eating breakfast. She chooses 14:10 starting at 8 AM.
- Eating window: 8:00 AM to 6:00 PM
- Fasting: 6:00 PM to 8:00 AM next day
- She eats breakfast at 8 AM, lunch at noon, and an early dinner at 5 PM
- No evening snacking, which is where most people consume excess calories
This schedule eliminates evening eating, which research shows is a common source of excess calorie intake. A 2022 study in the Journal of Clinical Endocrinology and Metabolism found that eating earlier in the day improves glucose control and increases fat oxidation compared to eating the same meals later.
Real-World Scenarios
Women and Intermittent Fasting
Women may be more sensitive to caloric restriction than men. A 2023 study in the journal Nutrients found that alternate-day fasting increased cortisol and disrupted menstrual cycles in some women, particularly those who started with low body fat. The researchers recommended that women start with 14:10 rather than 16:8, and monitor for signs of hormonal disruption: missed periods, sleep problems, or increased anxiety. If these occur, reduce fasting hours or return to normal eating. Women with a history of eating disorders should avoid intermittent fasting entirely.
Combining IF with Exercise
Training in a fasted state may enhance fat oxidation but can reduce performance for high-intensity work. A 2024 meta-analysis in the British Journal of Sports Medicine reviewed 18 studies and found that fasted aerobic exercise increased fat oxidation by 20 to 30% but did not improve body composition over fed exercise when total calories were matched. For strength training, most studies show better performance when eating 1 to 2 hours before. If you train in the morning and fast until noon, consider scheduling workouts at the end of your fasting period, right before breaking your fast. For tracking exercise calories, use our Calories Burned Calculator.
What Breaks a Fast
Water, black coffee, plain tea, and sparkling water do not break a fast. Anything with calories does. This includes milk in coffee, bone broth (unless doing a modified fast), sugar-free gum (small amounts are usually fine), and supplements in oil capsules. The general rule is: if it has more than about 5 calories, it technically breaks the fast. However, small amounts of cream in coffee (10 to 20 calories) will not significantly impact insulin levels or ketone production for most people. The strictness depends on your goals. For autophagy benefits, strict fasting is better. For weight loss, a few calories from cream will not matter.
Common Mistakes to Avoid
- Overeating during the eating window: IF only works for weight loss if you maintain a calorie deficit. Some people compensate for fasting by eating 3,000 calories in 8 hours. Track your intake for the first week.
- Starting too aggressively: Going from no fasting to 20:4 is likely to fail. Start with 12:12 or 14:10 for 2 weeks, then increase.
- Not drinking enough water: You get about 20 to 30% of your daily water from food. During fasting hours, drink extra water to compensate. For tracking hydration, use our Water Intake Calculator.
- Breaking the fast with sugar: Breaking a 16-hour fast with a donut causes a massive insulin spike. Break with protein and healthy fats: eggs, avocado, nuts, or a salad with olive oil.
- Ignoring hunger cues: Some hunger during fasting is normal and passes in waves. But if you feel dizzy, shaky, or lightheaded, break your fast. Pushing through severe symptoms is not productive.
- Inconsistent timing: Your body adapts to eating schedules. Eating at the same time each day optimizes circadian rhythm benefits. Try to keep your window consistent, even on weekends.
Limitations of This Calculator
This calculator generates a schedule based on your chosen protocol and start time. It does not calculate calorie targets or macronutrient ratios. For those, use our Calorie Calculator and Macro Calculator. The calculator does not account for medical conditions like diabetes, pregnancy, eating disorders, or thyroid disorders. If you take medications that require food, consult your doctor before starting intermittent fasting. IF is not appropriate for pregnant or breastfeeding women, children, or people with a history of eating disorders.
Authoritative Research and Resources
- Nature Reviews Endocrinology: Intermittent Fasting Review (2024) - Comprehensive review of 23 RCTs showing time-restricted eating produces 3 to 8% weight loss and improves insulin sensitivity.
- Nutrients: IF and Women's Hormones (2023) - Study on how intermittent fasting affects cortisol and menstrual cycles in women, recommending 14:10 as a starting protocol.
- NIH: Intermittent Fasting Information - The National Institute of Diabetes and Digestive and Kidney Diseases overview of intermittent fasting research and safety considerations.