Understanding How many calories should I eat while intermittent fasting 18/6?

October 18, 2025 •

5 min read

Research suggests that time-restricted eating, like the 18/6 method, can naturally help reduce overall caloric intake, leading to weight loss. However, a key question remains for many: How many calories should I eat while intermittent fasting 18/6? The answer is not one-size-fits-all, but depends on your individual goals and metabolism.

Quick Summary

The ideal calorie intake during 18/6 intermittent fasting is based on personal factors and goals. Calculate your Total Daily Energy Expenditure (TDEE) and aim for a controlled calorie deficit within your 6-hour eating window to promote sustainable weight loss while prioritizing nutrient-dense foods.

Key Points

Personalize Your Intake: There is no universal number for how many calories to consume; it depends on your body, goals, and activity level.
Calculate Your Deficit: Use a TDEE calculator (BMR x Activity Factor) to determine your maintenance calories, then subtract approximately 500 calories for a sustainable weight loss deficit.
Prioritize Nutrient-Dense Foods: Focus on lean protein, healthy fats, and fiber-rich carbs within your 6-hour eating window to maximize nutrient intake and satiety.
Stay Hydrated: Drink plenty of water and calorie-free beverages like black coffee or tea during your fasting window to manage hunger and avoid dehydration.
Transition Gradually: If new to fasting, start with shorter windows (e.g., 12/12) and work your way up to 18/6 to allow your body to adapt and minimize side effects.
Break Fast Mindfully: Reintroduce food with easily digestible, nutrient-rich options in moderate portions to avoid overwhelming your digestive system after an extended fast.

Your Calorie Intake: The Individualized Approach

Determining your caloric intake during 18/6 intermittent fasting requires a personalized approach rather than a single magic number. The primary goal for most people is weight loss, which still relies on creating a calorie deficit—consuming fewer calories than your body burns. The 18/6 schedule, which involves fasting for 18 hours and eating all your daily calories within a 6-hour window, can help create this deficit naturally. By limiting the time available for eating, many people find it easier to consume fewer calories overall.

Step 1: Calculate Your Baseline Daily Needs

To understand your deficit, you first need to know your maintenance calories, or the energy your body needs to sustain its current weight. This is your Total Daily Energy Expenditure (TDEE), which is determined by your Basal Metabolic Rate (BMR) and your activity level.

Calculate your BMR: The Mifflin-St Jeor equation is a widely used and reliable formula.
- Men: $(10 \times \text{weight in kg}) + (6.25 \times \text{height in cm}) - (5 \times \text{age in years}) + 5$
- Women: $(10 \times \text{weight in kg}) + (6.25 \times \text{height in cm}) - (5 \times \text{age in years}) - 161$
Factor in your activity level: Multiply your BMR by an activity factor to get your TDEE.
- Sedentary (little to no exercise): BMR x 1.2
- Lightly active (1-3 days/week exercise): BMR x 1.375
- Moderately active (3-5 days/week exercise): BMR x 1.55
- Very active (6-7 days/week exercise): BMR x 1.725

Step 2: Establish a Healthy Calorie Deficit

For sustainable weight loss, a deficit of about 500 calories per day is often recommended, which can lead to losing roughly 1 pound (0.45 kg) per week. However, it is crucial not to drop below a certain minimum calorie threshold for essential bodily functions—generally 1,200 calories for women and 1,500 for men, unless under a doctor's supervision.

Maximizing Your 6-Hour Eating Window

What you eat within your limited eating window is just as important as when you eat it. Since you have a shorter time to get all your daily nutrients, prioritizing high-quality, nutrient-dense foods is crucial for satiety, energy, and overall health.

Here’s how to build your meals effectively:

Prioritize Lean Protein: Including adequate protein is vital for preserving muscle mass while losing weight. It also increases satiety, helping you feel full for longer and manage hunger pangs during your fasting period. Sources include poultry, fish, eggs, and legumes.
Incorporate Healthy Fats: Healthy fats from sources like avocados, nuts, seeds, and olive oil provide sustained energy and support brain function.
Choose Fiber-Rich Carbohydrates: Opt for complex carbohydrates like whole grains, vegetables, and fruits. These provide fiber, which aids digestion and promotes a feeling of fullness.
Focus on Micronutrients: Ensure you include a wide variety of fruits and vegetables to get essential vitamins and minerals. With a restricted window, nutrient density becomes a key concern.

18/6 vs 16/8 Intermittent Fasting: Which is Right for You?

Both 18/6 and 16/8 are popular forms of time-restricted eating, but they differ in intensity and potential benefits. The choice between them often comes down to individual preference, lifestyle, and goals.


Feature	18/6 Intermittent Fasting	16/8 Intermittent Fasting
Eating Window	6 hours	8 hours
Fasting Period	18 hours	16 hours
Intensity	More advanced; may lead to more rapid results due to a greater natural calorie deficit.	More beginner-friendly and flexible; easier to accommodate social eating.
Metabolic Impact	Extended fasting allows more time for the body to burn fat and promotes more significant metabolic shifts, like increased autophagy.	Offers benefits for metabolic health, such as improved insulin sensitivity, but may be less intense than 18/6.
Sustainability	May be challenging initially, but many adapt well over time as hunger hormones normalize.	Generally easier to adhere to long-term for many people due to the wider eating window.

Practical Tips for Adherence and Health

Starting any new eating pattern comes with challenges. Here are some tips to make 18/6 fasting a sustainable part of your lifestyle:

Ease In: If you are new to intermittent fasting, do not jump straight into an 18-hour fast. Start with a shorter window, like 12/12, and gradually increase your fasting time over several weeks as your body adapts.
Stay Hydrated: Thirst can often be mistaken for hunger. Drink plenty of water throughout the day, including during your fasting window. Black coffee and herbal teas without sugar are also acceptable and can help curb appetite.
Manage Hunger: Your body will adjust over time, and hunger hormones like ghrelin will stabilize. Distract yourself with work or a hobby during hunger pangs. Ensuring your final meal is rich in protein and healthy fats can also help with satiety.
Break Your Fast Mindfully: When your eating window opens, resist the urge to overeat. Start with easily digestible, nutrient-rich foods to avoid shocking your digestive system. Consider bone broth, soups, or a small, protein-rich meal first.
Consider Electrolytes: Especially during the initial phase, your body releases water and electrolytes. Replenishing these with an electrolyte supplement or a pinch of Himalayan salt in your water can prevent headaches and fatigue.

Important Considerations and Cautions

While intermittent fasting can be a powerful tool, it is not suitable for everyone. It is crucial to consult a healthcare professional before starting, particularly if you have pre-existing health conditions or are in a sensitive group.

Individuals who should be cautious or avoid intermittent fasting include:

Pregnant or breastfeeding women
Individuals with a history of eating disorders
Those with diabetes or blood sugar regulation issues
People with a BMI below 18.5
Children and teens under 18
Older adults who may be at risk of losing too much weight

Conclusion

For those asking how many calories should I eat while intermittent fasting 18/6?, the answer is a tailored one. Begin by calculating your individual TDEE and establishing a safe calorie deficit for your weight loss goals. The 18/6 method provides a natural way to reduce overall calorie intake, but success depends heavily on the nutritional quality of the food you consume within your 6-hour window. Focus on a balanced diet rich in protein, healthy fats, and fiber to maximize satiety and nutrient intake. Listen to your body, stay hydrated, and consult with a healthcare provider to ensure a safe and sustainable approach to this eating pattern. Consistency and mindful eating, not just timing, will be the true keys to achieving your health objectives. More information on calculating your dietary needs can be found from authoritative sources, such as the National Academy of Sports Medicine's calorie calculator, which uses the well-regarded Mifflin-St Jeor equation.

Frequently Asked Questions

You can calculate your needs by first finding your Basal Metabolic Rate (BMR) using an equation like Mifflin-St Jeor, then multiplying it by an activity factor to determine your Total Daily Energy Expenditure (TDEE). From your TDEE, you can create a safe calorie deficit (e.g., 500 calories less per day) based on your weight goals.

Intermittent fasting can lead to muscle breakdown if not done properly. To prevent this, ensure adequate protein intake during your 6-hour eating window and consider resistance exercises. Protein is crucial for satiety and preserving lean muscle mass.

When breaking your fast, start with small portions of easily digestible, nutrient-dense foods. Good options include bone broth, soups, cooked vegetables, and lean protein sources like fish or eggs. Avoid large, high-carb or sugary meals initially to prevent digestive distress.

Intermittent fasting is not recommended for everyone, including pregnant or breastfeeding women and those with a history of eating disorders. Older adults and those with certain health conditions like diabetes should consult a doctor before starting.

Some common side effects, especially in the beginning, include headaches, fatigue, irritability, and hunger. These typically subside as your body adapts to the new eating pattern. Ensuring proper hydration and nutrient intake can also help mitigate these effects.

The main difference is the fasting duration: 18 hours versus 16 hours, resulting in a 6-hour versus an 8-hour eating window. The 18/6 method is more restrictive and may lead to more significant metabolic benefits and a larger calorie deficit, while 16/8 offers more flexibility.

While 18/6 fasting can naturally lead to a calorie deficit, calorie counting can be necessary for more precise results, especially if weight loss stalls. The key to weight loss remains eating fewer calories than you burn, regardless of the eating pattern.