Intermittent Fasting Calculator
Calculate your eating and fasting windows for 16:8, 18:6, 20:4, and OMAD protocols.
Quick Answer
16:8 = fast 16 hours, eat within 8 hours. Most people eat noon–8pm. Skip breakfast to easily complete the fast overnight.
Calculate Your Fasting Window
Frequently Asked Questions
What Is Intermittent Fasting?
Intermittent fasting (IF) is an eating pattern, not a diet. It does not tell you which foods to eat — it tells you when to eat them. You cycle between periods of eating and fasting throughout the day or week.
The most common method is 16:8: you fast for 16 hours and eat all your meals within an 8-hour window. Because the fasting period includes overnight sleep, most people find it manageable — skip breakfast, eat from noon to 8pm, and you are done.
IF = timing, not restriction
The fasting window controls when you eat. What you eat during the window still determines your results.
Fasting Methods Compared: 12:12 to OMAD to 5:2
Every IF method trades eating time for fasting time. Here are the major protocols, with example schedules and who each suits:
| Method | Fast | Eat | Example Schedule | Best For |
|---|---|---|---|---|
| 12:12 | 12h | 12h | Eat 8am–8pm, fast overnight | Beginners, gentle entry |
| 14:10 | 14h | 10h | Eat 10am–8pm | Beginners ready for more |
| 16:8 | 16h | 8h | Eat 12pm–8pm (most popular) | Most people, sustainable |
| 18:6 | 18h | 6h | Eat 1pm–7pm | Experienced fasters |
| 20:4 | 20h | 4h | Eat 4pm–8pm (Warrior Diet) | Advanced, short-term |
| OMAD | 23h | 1h | One meal, e.g. 6pm–7pm | Advanced, not for beginners |
| 5:2 | 2 days/wk | 5 days normal | Eat 500–600 cal on 2 non-consecutive days | Those who dislike daily fasting |
16:8 is the most popular and sustainable for most people. OMAD and 5:2 are the most intense and are not recommended for beginners.
How the Schedule Builder Works
The calculator above takes two inputs and builds your daily schedule:
1. Protocol
Choose your fasting ratio (16:8, 18:6, 20:4, or OMAD). This sets the number of fasting and eating hours.
2. Eating window start time
Pick when your eating window begins (e.g., 12:00 PM). The calculator adds the eating hours to find the end, and the fasting window fills the rest of the 24-hour day.
Example: 16:8 starting at 12:00 PM
Eating: 12:00 PM – 8:00 PM (8 hours)
Fasting: 8:00 PM – 12:00 PM next day (16 hours)
What Breaks a Fast: An Honest Guide
The simple rule: anything with calories breaks a fast. Here is an honest guide to common items:
| Item | Breaks Fast? | Why |
|---|---|---|
| Water | No | 0 calories — drink freely |
| Black coffee | No | ~2 cal — does not break a fast |
| Plain tea (green/black/herbal) | No | ~0–2 cal — fine |
| Sparkling water | No | 0 calories |
| Milk or cream | Yes | Contains calories and protein |
| Sugar or honey | Yes | Calories and insulin response |
| Juice or smoothies | Yes | Calories and sugar |
| Bone broth (with protein) | Yes | Protein triggers insulin |
| MCT oil or coconut oil | Maybe | Debated — may end a "clean" fast but not a "fat fast" |
| Chewing gum (sugared) | Yes | Sugar and sweeteners |
| Chewing gum (sugar-free) | Maybe | Sweeteners may trigger insulin in some |
A good rule of thumb: if it has more than about 5 calories, it breaks a fast. For autophagy and insulin goals, even sweeteners may be enough to end a "clean" fast.
What to Eat in Your Window: Protein First
IF controls when you eat, not what. To lose fat and preserve muscle, prioritize protein during your eating window:
Protein target
1.6 to 2.2 g per kg of body weight per day (0.8 to 1.0 g per lb)
Example: 80 kg person → 128 to 176 g protein/day, split across 2 to 3 meals in the window
- Protein first: Lean meat, fish, eggs, Greek yogurt, tofu, legumes. This preserves muscle and keeps you full.
- Vegetables second: Fill half your plate with non-starchy vegetables for fiber and micronutrients.
- Whole carbs and fats last: Oats, brown rice, olive oil, avocado, nuts. Do not fear carbs — just avoid ultra-processed foods.
- Do not binge: Eating all your calories in one meal can cause bloating and poor absorption. Spread across 2 meals if possible.
For a full macro breakdown tailored to your goals, use the TDEE Calculator and the Calorie Deficit Calculator.
Who Should NOT Fast
Intermittent fasting is safe for most healthy adults, but it is not safe for everyone. Do not fast if any of the following apply to you:
- Pregnant or breastfeeding: Your body needs steady nutrition for you and your baby.
- History of eating disorders: Fasting can trigger restrictive eating patterns.
- Type 1 diabetes or on insulin: Fasting risks dangerous low blood sugar (hypoglycemia).
- Type 2 diabetes on medications: Talk to your doctor — medications may need adjustment.
- Underweight (BMI under 18.5): Fasting can worsen undernutrition.
- Under 18: Growing bodies need regular meals.
- History of low blood sugar or fainting: Fasting can trigger these episodes.
This is not medical advice — talk to your doctor before starting intermittent fasting, especially if you have any medical condition or take any medication.
Tips for Your First 2 Weeks: A Gradual Ramp Plan
Do not jump straight to 16:8 or OMAD. Ease in over two weeks to reduce hunger, headaches, and the chance of quitting:
Days 1–3: 12:12
Eat 8am–8pm. This is just a normal overnight fast — almost no hunger. Get used to the idea.
Days 4–7: 14:10
Push breakfast to 10am. You may feel mild hunger in the morning — drink black coffee or water.
Days 8–14: 16:8
Push breakfast to 12pm. This is the standard protocol. Eat 2 to 3 protein-rich meals between noon and 8pm.
- Stay hydrated: Drink water, black coffee, and plain tea during the fast. Dehydration causes most "fasting headaches."
- Eat enough protein: Under-eating during the window leads to hunger the next day. Aim for 30 g protein per meal.
- Do not over-exercise early: Light walks are fine; save intense training for after you have adapted (week 2+).
- Expect hunger to peak then fade: Hunger comes in waves. It usually peaks around hour 14 to 16, then subsides.
- Be patient: Adaptation takes 1 to 2 weeks. Energy and focus often improve after that.
Intermittent Fasting Calculator FAQs
Disclaimer: This intermittent fasting calculator provides estimates for educational purposes only and is not medical advice. Fasting is not safe for everyone, especially people who are pregnant, breastfeeding, have diabetes, take insulin, have a history of eating disorders, or are under 18. This is not medical advice — talk to your doctor before starting any fasting plan, especially if you have a medical condition or take medication.