Intermittent Fasting Calculator: Your Optimal Eating Window
On a 16:8 intermittent fasting schedule starting your eating window at 12:00 PM, you eat between 12:00 PM - 8:00 PM and fast from 8:00 PM - 12:00 PM. During the 8-hour window, eat your normal calorie target across 2-3 meals. 16:8 is the most popular and sustainable IF protocol for beginners.
Intermittent Fasting Schedule
Find your optimal eating window.
Intermittent Fasting Protocols
| Protocol | Fasting | Eating | Meals | Best For | ||||
|---|---|---|---|---|---|---|---|---|
| 16:8 | 16 hours | 8 hours | 2-3 meals | Beginners, sustainable daily use | ||||
| 18:6 | 18 hours | 6 hours | 2 meals | Intermediate, faster results | ||||
| 20:4 (Warrior) | 20 hours | 4 hours | 1-2 meals | Advanced, experienced fasters | ||||
| OMAD | 23 hours | 1 hour | 1 meal | Advanced, significant deficit | ||||
| 5:2 | 2 days/week at 500-600 cal | 5 normal days | Normal | Those who prefer weekly vs daily structure | ||||
| ADF | Alternating full-day fasts | Alternating eat days | Normal | Research-backed, difficult to sustain | What Breaks a Fast? | Item | Breaks Fast? | Notes |
| Water | No | Drink freely | ||||||
| Black coffee | No | Under 5 calories | ||||||
| Green/black tea | No | No sugar or milk | ||||||
| Diet soda | Technically no | Artificial sweeteners may trigger insulin in some people | ||||||
| Cream in coffee | Yes | Adds calories and fat | ||||||
| BCAA/protein | Yes | Triggers protein synthesis | ||||||
| Bone broth | Yes (minimal) | Contains protein and calories | ||||||
| Any food | Yes | Any caloric intake breaks a fast |
Calorie Distribution Across Your Eating Window
For a 16:8 protocol with 2,000 calories:
- Meal 1 (12 PM): 700-800 cal — protein-rich to break fast (eggs, Greek yogurt, lean meat)
- Meal 2 (4 PM): 500-600 cal — balanced meal or snack
- Meal 3 (7:30 PM): 700-800 cal — largest meal, complete protein + vegetables
Frequently Asked Questions
Does intermittent fasting work for weight loss?
Yes, primarily by making it easier to eat fewer calories. A 2020 systematic review in the Annual Review of Nutrition found IF produces equivalent weight loss to traditional calorie restriction when calories are matched. The advantage: many people find it simpler to skip a meal than to eat small portions at every meal. IF does not have a unique metabolic advantage for fat loss beyond the calorie deficit it creates.
Is 16:8 intermittent fasting safe?
For most healthy adults, yes. 16:8 fasting is essentially skipping breakfast, which many people do naturally. It is the most studied and sustainable IF protocol. However, IF is not appropriate for pregnant/breastfeeding women, children and teens, people with a history of eating disorders, individuals with type 1 diabetes, or those taking medications that require food.
Can I exercise while fasting?
Yes. Most people can exercise safely during a fast, especially for moderate-intensity activities like walking, jogging, or light lifting. For intense training (heavy weights, HIIT, sprints), some people perform better with pre-workout nutrition. If you train fasted and feel dizzy, weak, or nauseous, consider shifting your eating window to include a pre-workout meal.
Will fasting slow my metabolism?
Short-term fasting (up to 72 hours) does not decrease metabolism — in fact, studies show a slight increase in metabolic rate during the first 24-48 hours of fasting, likely due to increased norepinephrine. Metabolic slowdown occurs with chronic calorie restriction over weeks/months, not from daily time-restricted eating. The key factor is total calorie intake, not meal timing.
How long does it take to see results from IF?
Most people notice reduced bloating and improved energy within 1-2 weeks. Measurable weight loss typically appears after 2-4 weeks if a calorie deficit is maintained. Body composition changes (visible fat loss) usually require 4-8 weeks. IF is most effective when combined with adequate protein intake and resistance training.