The number of calories you should eat during 18/6 intermittent fasting depends on your goal, not the fasting schedule itself. The 18-hour fast and 6-hour eating window don’t change your fundamental calorie needs. Whether you want to lose fat, maintain weight, or build muscle, the target is the same as it would be on any other eating pattern. The fasting window simply compresses when you eat those calories.
Start With Your Baseline Calorie Needs
Your total daily energy expenditure (TDEE) is the number of calories your body burns in a full day, including everything from breathing to exercise. To find it, you first calculate your basal metabolic rate (BMR), the energy your body uses at complete rest, then multiply by an activity factor.
The Mifflin-St Jeor equation is the most widely recommended formula because it doesn’t require knowing your body fat percentage:
- Men: BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age in years) + 5
- Women: BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age in years) − 161
Once you have your BMR, multiply it by an activity factor: 1.2 for a mostly sedentary lifestyle, 1.375 if you exercise lightly a few times a week, 1.55 for moderate exercise three to five days a week, and 1.725 if you train hard most days. The result is your TDEE, the number of calories you’d eat to maintain your current weight. For many adults, this lands somewhere between 1,800 and 2,800 calories per day.
Adjusting Calories for Fat Loss
If you’re using 18/6 fasting to lose weight, you need a calorie deficit. The NIH recommends a deficit of 500 to 1,000 calories per day, which translates to roughly 0.5 to 1 kilogram (about 1 to 2 pounds) of fat loss per week. A pound of fat stores approximately 3,500 calories, so the math is straightforward: cut 500 calories a day, lose about a pound a week.
In practice, most people don’t need to calculate this precisely. Research on time-restricted eating shows that simply compressing your eating window to six hours tends to create a mild calorie deficit on its own, even without tracking food. One study in adults with obesity found that participants in a 6-hour eating window lost weight without being asked to count calories or change what they ate. The restricted window naturally limits how much you can consume. That said, it’s entirely possible to eat at maintenance or even a surplus within six hours, especially with calorie-dense foods. If the scale isn’t moving, you’re likely eating close to your TDEE despite the shorter window.
A good starting point for most people: subtract 500 calories from your TDEE. If your TDEE is 2,200, aim for about 1,700 calories during your 6-hour eating window. If your TDEE is 2,800, target around 2,300.
Minimum Calorie Floors to Respect
Harvard Health recommends that women not drop below 1,200 calories per day and men not drop below 1,500 calories per day without medical supervision. Eating below these thresholds makes it very difficult to get adequate vitamins, minerals, and protein, and can trigger metabolic adaptations that work against you.
This matters for 18/6 fasting because the compressed window can make it tempting to undereat. If you’re only fitting in two meals, it’s easy to accidentally land well below your floor, particularly if those meals are heavy on vegetables and light on calorie-dense foods. Chronic undereating on an 18/6 schedule can lower your resting energy expenditure. One study of healthy adults following an 18/6 pattern found that resting metabolic rate dropped by about 6.5% over the fasting period, with women experiencing a larger decline (8.1%) than men (4.6%). Eating enough calories within your window helps minimize this effect.
Calories for Muscle Gain on 18/6
If your goal is building muscle while following an 18/6 schedule, you’ll need a calorie surplus. Research on time-restricted eating combined with resistance training used a 10% surplus above TDEE, which typically works out to an extra 350 to 500 calories per day. This modest overshoot supports muscle growth without excessive fat gain.
That said, fitting a surplus into a 6-hour window is genuinely challenging. You’re eating more food in less time, which can feel uncomfortable. Sports nutrition researchers have noted that intermittent fasting schedules lend themselves better to fat loss than to muscle-building diets for exactly this reason. If gaining size is your primary goal, you may find a wider eating window more practical.
How to Split Calories in a 6-Hour Window
Two to three meals spaced across the six hours works best for most people. Research suggests that cramming all your daily calories into a single meal can impair glucose tolerance and raise LDL cholesterol, even if the total calorie count is appropriate. Spreading food across at least two sittings avoids this problem.
A common structure: a larger meal when you break your fast, a smaller meal or snack two to three hours later, and a final meal near the end of your window. If your target is 1,800 calories, that might look like a 700-calorie first meal, a 300-calorie snack, and a 800-calorie dinner. The exact split matters less than hitting your daily target consistently.
Protein Deserves Extra Attention
Protein is the one macronutrient worth tracking more carefully on an 18/6 schedule. Because you have fewer meals to distribute protein across, each meal needs to carry more of the load. Research on muscle preservation during time-restricted eating recommends at least 1.6 grams of protein per kilogram of body weight per day, with meals spaced three to five hours apart within the eating window.
For a 75 kg (165 lb) person, that’s roughly 120 grams of protein daily. Split across two meals, that’s 60 grams per meal, equivalent to about 8 ounces of chicken breast or a large serving of Greek yogurt paired with eggs. This level of protein intake helps protect muscle mass during fat loss and supports growth during a surplus. If you find yourself consistently falling short, a protein-rich snack between your two main meals can bridge the gap.
A Quick Calorie Reference by Goal
To put this together practically, here’s what the math looks like for someone with a TDEE of 2,200 calories:
- Fat loss: 1,700 calories per day (500-calorie deficit), all consumed within the 6-hour window
- Maintenance: 2,200 calories per day, useful if you’re fasting for metabolic benefits rather than weight loss
- Muscle gain: 2,420 to 2,700 calories per day (10% surplus, or 350 to 500 extra calories)
Your numbers will differ based on your size, age, sex, and activity level. The 18/6 schedule doesn’t change the calorie target. It only changes how quickly you need to eat them. Calculate your TDEE, adjust it for your goal, and then focus on fitting that amount of food, with enough protein, into your six-hour window.