Fasting Calculator
Calculate your fasting window, eating window, and next fast end time for any intermittent fasting protocol. Supports 16:8, 18:6, 20:4, OMAD, 5:2, and custom schedules.
How to Use This Calculator
Choose Your Fasting Protocol
Select your intermittent fasting schedule. The 16:8 protocol (16 hours fasting, 8 hours eating) is the most popular for beginners. OMAD (One Meal A Day, 23:1) is for experienced fasters. Start with 12:12 or 14:10 if new to fasting and gradually extend your window.
Enter Your Last Meal Time
Enter the hour (0-23 format) and minute of your last meal or the last time you ate anything caloric. Coffee, tea, and water do not break a fast. Anything with calories — including cream in coffee — begins the eating window.
Check Your Fast End Time
The calculator shows exactly when your fast ends and your eating window opens. Plan your first meal, errands, and activities around this time. Many people use the fasting period for work or sleep to make it easier.
Track Your Progress
Enter your body weight to see an estimate of calories burned during your fast based on your resting metabolic rate. Use this alongside your fasting app or journal to track adherence and results over time.
How We Calculate
The fasting calculator computes fasting and eating windows based on the user-selected intermittent fasting protocol, which is simply the number of consecutive hours without caloric intake. The fast end time is calculated by adding the fasting duration to the last meal time, cycling through 24 hours. For example, a 16:8 fast starting at 8:00 PM ends at 12:00 PM the next day, creating an eating window from noon to 8:00 PM. This is the fundamental arithmetic behind all time-restricted eating protocols, as described in the research of Dr. Mark Mattson (NIH) and Dr. Satchin Panda (Salk Institute).
The calorie burn estimate during fasting uses a simplified version of the Mifflin-St Jeor resting metabolic rate equation: BMR per hour = (body weight in lbs x 11) / 24. This rough estimate represents basal caloric expenditure at rest. Actual calorie burn during fasting varies based on activity level, lean body mass, hormonal state, and metabolic adaptation. The purpose of this estimate is directional only — to give users a sense of energy expenditure during the fast — not a precise measurement.
Intermittent fasting has been studied extensively for weight management, metabolic health, and longevity. The 16:8 protocol studied in clinical trials (including the work published in Cell Metabolism by Lowe et al., 2020) showed meaningful improvements in body weight and metabolic markers compared to unrestricted eating. However, individual responses vary, and fasting is not appropriate for everyone — including pregnant women, people with eating disorders, those with type 1 diabetes, and individuals on certain medications. This calculator is an informational tool; consult a healthcare provider before starting any fasting regimen.
Sources & References
- Lowe DA et al. — Effects of Time-Restricted Eating on Weight Loss (Cell Metabolism, 2020)
- Panda S — The Circadian Code: Salk Institute Research on Time-Restricted Eating (salk.edu)
- Mattson MP et al. — Intermittent Fasting: Health Benefits and Mechanisms (New England Journal of Medicine, 2019)
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Frequently Asked Questions
Intermittent fasting (IF) is an eating pattern that cycles between periods of fasting and eating. The most common protocols are 16:8 (16 hours fasting, 8 hours eating window) and 18:6. This fasting calculator and fast calculator determines when your fast ends by adding your chosen fasting duration to your last meal time, displaying the exact break-fast time and your available eating window.
Plain black coffee (without milk, cream, or sweeteners) and plain tea do not break a fast in most intermittent fasting protocols. They contain essentially no calories and do not significantly raise insulin levels. However, adding any caloric ingredient — cream, milk, oat milk, sweeteners, butter in bulletproof coffee — technically begins your eating window. Many people maintain strict fasting rules (water, black coffee, tea only) for maximum metabolic benefits.
Beginners should start with 12:12 (12 hours fasting, 12 hours eating) or 14:10 to adapt their body to time-restricted eating. Once comfortable, progress to 16:8, which is the most studied and widely practiced protocol. Going directly to 18:6 or OMAD as a beginner often leads to hunger, fatigue, and abandonment. Gradually extending the fast by 30-60 minutes each week is more sustainable than jumping to an aggressive protocol.
Yes — many people exercise in a fasted state, particularly in the morning before breaking their fast. Fasted exercise may increase fat oxidation during the workout. However, high-intensity workouts or strength training in a prolonged fast (18+ hours) can be challenging and may require carbohydrate intake for optimal performance. Light to moderate cardio is well-tolerated during shorter fasts. Listen to your body and adjust based on energy levels.
The 5:2 method involves eating normally five days per week and restricting calories to 500-600 on two non-consecutive days. Unlike time-restricted eating (16:8, 18:6), the 5:2 approach focuses on weekly caloric restriction rather than daily eating windows. This calculator focuses on daily time-restricted eating protocols, but you can use it to plan your eating windows on non-fasting 5:2 days.
Most people notice reduced hunger and improved energy within 1-2 weeks as their body adapts to time-restricted eating. Weight loss results typically appear within 2-4 weeks of consistent practice, assuming a caloric deficit is maintained. Metabolic improvements (blood sugar, insulin sensitivity) may take 4-8 weeks to become measurable. Results vary significantly based on diet quality during the eating window, activity level, starting metabolic health, and adherence consistency.
Short-term fasting (12-24 hours) does not significantly slow metabolism — in fact, a short fast can temporarily increase norepinephrine and metabolic rate by 3-14%. Prolonged caloric restriction over weeks and months, however, can reduce metabolic rate as the body adapts. Intermittent fasting with adequate protein intake and resistance training is one of the strategies studied for preserving metabolic rate while losing weight, compared to continuous caloric restriction alone.
Intermittent fasting is not appropriate for everyone. People who should avoid or consult a doctor before fasting include: pregnant or breastfeeding women, children and teenagers, people with type 1 diabetes or hypoglycemia, individuals with a history of eating disorders, those taking insulin or blood sugar medications, and people who are underweight or malnourished. Always consult a healthcare provider before starting any fasting protocol if you have existing health conditions.
ToolSite Team
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