Introduction
Intermittent fasting is one of the most popular ways to manage weight and improve overall health. Instead of focusing on what you eat, it focuses on when you eat by splitting your day into eating and fasting windows. But figuring out the right schedule, calorie targets, and meal timing on your own can be confusing.
This Fasting Calculator handles the calculations for you. Enter your age, weight, height, and activity level, then pick a fasting protocol (like 16:8, OMAD, 5:2, or a custom plan), and the calculator builds a complete, personalized fasting schedule. It estimates your basal metabolic rate (BMR) and total daily energy expenditure (TDEE), shows you exactly when to eat and when to fast, breaks down your recommended macros for protein, carbs, and fat, and even suggests how to split your calories across meals. This tool gives you a clear plan you can follow right away.
How to Use Our Fasting Calculator
Enter your body stats, activity level, and preferred fasting method below. The calculator will build a personalized fasting plan with your daily calorie needs, eating and fasting windows, macro breakdown, and meal timing suggestions.
Age: Type in your current age. The calculator uses this to figure out how many calories your body burns at rest.
Biological Sex: Choose male or female. Men and women burn calories at different rates, so this helps the calculator give you an accurate result.
Weight: Enter your current weight in either pounds or kilograms. Use the toggle to switch between units. Your weight plays a big role in determining your daily calorie needs.
Height: Enter your height in feet and inches or centimeters. Use the toggle to pick your preferred unit. Height is used along with weight and age to calculate your basal metabolic rate (BMR).
Activity Level: Pick the option that best matches your weekly exercise habits. Choose from Sedentary, Light Activity, Moderate Activity, or Very Active. This adjusts your total daily energy expenditure (TDEE) so your calorie targets are realistic.
Fasting Protocol: Select the intermittent fasting schedule you want to follow. Options include 16:8 (great for beginners), 18:6, 20:4 (Warrior Diet), OMAD (one meal a day), 5:2 (two low-calorie days per week), Alternate-Day Fasting, or a Custom plan where you set your own fasting and eating hours.
First Meal Time: Set the time you want your eating window to start. Use the plus and minus buttons to adjust by 15-minute steps, or click one of the preset time buttons for a quick selection. Your full daily schedule is built around this time.
Calculate Fasting Plan: Click this button to generate your results. You will see your BMR, TDEE, eating window calories, a visual daily schedule, recommended macros for protein, carbs, and fat, meal timing suggestions, and a calorie distribution chart. Click Reset at any time to start over with the default settings.
Intermittent fasting is an eating pattern where you cycle between periods of eating and not eating. Unlike most diets, it does not tell you what to eat. It tells you when to eat. By limiting the hours you eat each day, or by reducing calories on certain days of the week, intermittent fasting can help your body burn stored fat, manage weight, and improve overall metabolic health.
How Intermittent Fasting Works
When you eat, your body spends several hours processing and burning that food for energy. During this time, it is harder for your body to tap into fat stores because insulin levels are high. When you fast, insulin drops, and your body begins using stored fat as fuel instead. The longer the fasting window, the more time your body spends in this fat-burning state.
Common Fasting Protocols
There are several popular intermittent fasting schedules, each with different levels of difficulty:
- 16:8 (Leangains): You fast for 16 hours and eat during an 8-hour window. This is the most common method and a great starting point for beginners. A typical example is eating between 12:00 PM and 8:00 PM.
- 18:6: A tighter version of 16:8, with 18 hours of fasting and a 6-hour eating window. This works well for people who have already adjusted to the 16:8 schedule.
- 20:4 (Warrior Diet): You fast for 20 hours and eat during a 4-hour window. Most people eat one large meal and a small snack during this time.
- OMAD (One Meal a Day): You eat all your daily calories in a single meal within a 1-hour window. This is one of the most advanced protocols and is not recommended for beginners.
- 5:2 (Fast Diet): You eat normally for 5 days of the week and restrict calories to about 500–600 on the other 2 non-consecutive days.
- Alternate-Day Fasting (ADF): You alternate between normal eating days and fasting days where you eat only about 500–600 calories.
Key Terms: BMR and TDEE
Your Basal Metabolic Rate (BMR) is the number of calories your body burns at rest just to keep you alive: breathing, pumping blood, and maintaining organs. This calculator estimates it with the Mifflin-St Jeor equation, derived in 1990 from 498 healthy adults aged 19 to 78.1 A systematic review found that equation the most reliable of the common ones.3 If you enter a body fat percentage, the calculator switches to the equation usually called Katch-McArdle, 370 + 21.6 × lean body mass in kilograms, which was published by Cunningham in 1991.2 Lean body mass is your weight minus the fat fraction you entered. Your Total Daily Energy Expenditure (TDEE) takes your BMR and multiplies it by an activity factor based on how much you move and exercise throughout the day. TDEE represents the total number of calories you burn in a full day and is the number you should aim to eat during your eating window to maintain your current weight.
What to Eat and Drink While Fasting
During fasting hours, you should only consume zero-calorie drinks like water, black coffee, and plain tea. Be mindful of your caffeine intake if you rely on coffee to get through fasting periods. Adding sugar, cream, or milk breaks your fast. During your eating window, focus on nutrient-dense whole foods: lean proteins, vegetables, fruits, whole grains, and healthy fats. Avoid filling your eating window with processed foods, sugary snacks, or fast food, as this can cancel out the benefits of fasting. Make sure you're also drinking enough water throughout the day; our Water Intake Calculator can help you set a daily hydration target.
Benefits of Intermittent Fasting
Intermittent fasting is commonly promoted for benefits such as:
- Weight loss and fat loss by naturally reducing calorie intake and increasing fat burning
- Improved insulin sensitivity, which can lower the risk of type 2 diabetes
- Better mental clarity and focus during fasting periods
- Simpler meal planning, since you prepare and eat fewer meals each day
- Reduced inflammation, which is linked to many chronic diseases
Who Should Be Careful
Intermittent fasting is not right for everyone. Pregnant or breastfeeding women, children and teenagers, people with a history of eating disorders, and anyone with a chronic medical condition should talk to a doctor before starting any fasting plan. If you feel dizzy, extremely fatigued, or unwell while fasting, stop and consult a healthcare professional.
Tips for Getting Started
If you are new to intermittent fasting, start with the 16:8 method. Most people find it easiest to skip breakfast and eat their first meal around noon. Stay well hydrated during fasting hours, keep busy to avoid thinking about food, and ease into longer fasting windows over time rather than jumping straight to advanced protocols. Consistency matters more than perfection. Even fasting a few days per week can produce meaningful results.
To get the most out of your fasting plan, pair it with the right nutrition strategy: dial in your protein, carb, and fat targets, and make sure you're getting enough protein to preserve muscle during fasting. And don't overlook recovery, since rest is essential for the metabolic benefits of fasting.