Intermittent Fasting Calculator
Calculate your exact eating window and fasting schedule for any IF protocol — 16:8, 18:6, 20:4, 5:2, or OMAD. Enter your wake time and preferred eating start, and get a personalized fasting plan with calorie targets.
Your Fasting Schedule
Daily Calorie Targets
| Metric | Value |
|---|
Protocol Comparison
| Protocol | Fast | Eat | Difficulty | Best For |
|---|---|---|---|---|
| 16:8 | 16h | 8h | Easy | Beginners, daily adherence |
| 18:6 | 18h | 6h | Moderate | Fat loss, metabolic health |
| 20:4 | 20h | 4h | Hard | Aggressive fat loss |
| OMAD | 23h | 1h | Very Hard | Maximum simplicity |
| 5:2 | 2d/wk | 5d normal | Moderate | Flexible schedules |
What Breaks a Fast?
| Item | Breaks Fast? | Notes |
|---|---|---|
| Water | No | Drink freely |
| Black coffee | No | No milk, sugar, or cream |
| Plain tea | No | Unsweetened only |
| Sparkling water | No | No additives |
| Milk / cream | Yes | Contains calories and protein |
| Diet soda | Maybe | Sweeteners may spike insulin |
| Bulletproof coffee | Yes | High in fat calories |
| Gum (sugar-free) | Maybe | Minimal impact for most people |
How Intermittent Fasting Works
Intermittent fasting (IF) is an eating pattern that cycles between periods of fasting and eating. Unlike traditional diets, IF focuses on when you eat rather than what you eat. During the fasting window, insulin levels drop, enabling the body to access stored fat for fuel — a state called ketosis at longer fasts.
The 16:8 Protocol
The most researched and beginner-friendly protocol. Studies show 16:8 reduces average daily calorie intake by 300–500 calories without deliberate restriction. Metabolic benefits include improved insulin sensitivity, lower fasting blood glucose, and reduced inflammatory markers.
The 5:2 Protocol
On two non-consecutive days per week, limit intake to 500–600 calories (about 25% of normal). Eat normally on the other five days. Research by Dr. Michael Mosley shows 5:2 is as effective as daily calorie restriction for weight loss with better adherence for some people.
Breaking the Fast
Break your fast with a balanced meal containing protein (30–40g), healthy fats, and complex carbohydrates. Avoid ultra-processed foods and large sugar loads immediately after fasting, as insulin sensitivity is elevated and glycemic response is amplified.
Who Should Avoid Intermittent Fasting
IF is not appropriate for everyone. Those who should avoid or get medical clearance first include: pregnant or breastfeeding women (increased nutrient demands), people with type 1 diabetes or those on insulin (risk of hypoglycemia), anyone with a history of eating disorders (may trigger restrictive behaviors), children and teenagers (still growing), and people who are underweight. Those with type 2 diabetes managing through diet may benefit from IF but should monitor blood sugar closely and consult their doctor, as medication adjustments may be necessary.
Frequently Asked Questions
During a fast, stick to zero-calorie beverages: water, plain black coffee, unsweetened tea, and sparkling water. Adding cream, milk, sugar, or flavored syrups to coffee breaks the fast. Black coffee and plain tea may even enhance fasting benefits by suppressing appetite and slightly boosting fat oxidation.
Short-term fasting (16–24 hours) is unlikely to cause significant muscle loss if protein intake is adequate during eating windows. Research suggests that IF preserves lean mass as well as traditional caloric restriction when protein intake is maintained at 0.7–1.0g per pound of body weight. Resistance training during IF further protects muscle while accelerating fat loss.
Most people notice changes within 2–4 weeks. The first 1–2 weeks involve adjustment: some experience hunger, irritability, or fatigue as the body adapts to a new eating schedule. Weight loss begins once a consistent calorie deficit is established. Metabolic improvements (insulin sensitivity, fasting blood glucose) may take 8–12 weeks of consistent practice to become measurable.
Yes, and fasted exercise may enhance fat oxidation. Light-to-moderate exercise (walking, yoga, moderate cardio) is generally well-tolerated in a fasted state. High-intensity training and strength workouts are often better scheduled near the end of the fasting window or within the eating window to ensure adequate fuel and recovery nutrition. Beginners should adapt to IF first before adding intense fasted training.