A calorie deficit means eating fewer calories than your body burns in a day. To calculate yours, you need two numbers: your total daily energy expenditure (TDEE) and a target reduction, typically 300 to 500 calories below that number. The math itself is straightforward once you know where to start.
Step 1: Estimate Your Basal Metabolic Rate
Your basal metabolic rate (BMR) is the number of calories your body burns just to stay alive: breathing, circulating blood, maintaining body temperature, and keeping your organs running. Even if you lay in bed all day, your body would still burn this many calories. For most people, BMR accounts for the largest share of daily calorie burn.
The most widely recommended formula is the Mifflin-St Jeor equation, endorsed by the Academy of Nutrition and Dietetics. You’ll need your weight in kilograms, height in centimeters, and age in years. To convert, divide your weight in pounds by 2.2 and multiply your height in inches by 2.54.
- Men: (10 × weight in kg) + (6.25 × height in cm) − (5 × age in years) + 5
- Women: (10 × weight in kg) + (6.25 × height in cm) − (5 × age in years) − 161
For example, a 35-year-old woman who weighs 70 kg (154 lbs) and is 165 cm (5’5″) tall would calculate: (10 × 70) + (6.25 × 165) − (5 × 35) − 161 = 700 + 1,031 − 175 − 161 = 1,395 calories per day. That’s what her body burns at complete rest.
If you know your body fat percentage, you can use the Katch-McArdle formula instead, which bases the calculation on lean body mass: 370 + (21.6 × lean mass in kg). This can be more accurate for people who carry significantly more or less muscle than average, since muscle tissue burns more calories than fat tissue at rest.
Step 2: Factor In Your Activity Level
Your BMR only covers what your body burns at rest. To get your total daily energy expenditure, multiply your BMR by an activity factor that reflects how much you move throughout the week.
- Sedentary (desk job, little or no exercise): BMR × 1.2
- Lightly active (light exercise 1 to 3 days per week): BMR × 1.375
- Moderately active (moderate exercise 3 to 5 days per week): BMR × 1.55
- Very active (hard exercise 6 to 7 days per week): BMR × 1.725
- Super active (intense training or physical job): BMR × 1.9
Using the same example, if that 35-year-old woman exercises moderately three to five days per week, her TDEE would be 1,395 × 1.55 = roughly 2,162 calories per day. This is her maintenance number, the amount she’d eat to stay at her current weight.
Be honest with yourself when choosing a category. Most people overestimate their activity level. If you work out three times a week but sit at a desk the rest of the time, “lightly active” is probably more accurate than “moderately active.”
Step 3: Subtract Your Deficit
Once you have your TDEE, subtract 300 to 500 calories. That range is effective for steady weight loss without the side effects of aggressive restriction, things like low energy, hair loss, mood swings, and constipation. For the woman in our example, a 500-calorie deficit would put her daily target at roughly 1,662 calories.
You may have heard that cutting 500 calories per day equals one pound of weight loss per week, based on the old rule that a pound of fat contains 3,500 calories. The Mayo Clinic notes this isn’t true for everyone. When you lose weight, you lose a mix of fat, lean tissue, and water. Your body also adapts to lower intake over time, which means weight loss typically slows even if your eating stays consistent. The 3,500-calorie rule is a rough starting point, not a precise predictor.
Why Your Numbers Will Need Updating
Your BMR depends on your current weight, so as you lose weight, your body burns fewer calories at rest. The same deficit that produced results in month one will produce smaller results in month three. This is one reason weight loss plateaus are so common.
Recalculate your BMR and TDEE every 10 to 15 pounds, or roughly once a month if you’re losing weight steadily. Plug your new weight back into the formula, reassess your activity level, and adjust your daily target. You may also need to decrease calories slightly further or increase activity to maintain the same rate of loss.
Calorie Floors to Keep in Mind
The Dietary Guidelines for Americans estimates that adult women need between 1,600 and 2,400 calories per day and adult men need between 2,000 and 3,000, depending on age and activity. These are maintenance estimates, not minimums, but they give you a frame of reference. If your calculated deficit puts you well below the low end of those ranges, a smaller deficit combined with more physical activity is a safer approach than cutting calories further.
The Tracking Accuracy Problem
Even a perfectly calculated deficit only works if your calorie counts are reasonably accurate, and most people’s aren’t. Research on self-reported food intake shows that even non-obese adults have an average error margin of about 20%. Individual underestimates of 50% are not uncommon. That means someone tracking 1,800 calories per day might actually be eating 2,160 to 2,700 calories.
The most common sources of error are cooking oils and condiments (which add calories without feeling like “food”), eyeballing portion sizes instead of weighing them, and forgetting to log small snacks and drinks. A kitchen scale and consistent logging close much of this gap. You don’t need to weigh food forever, but doing it for a few weeks builds a much more accurate sense of what portions actually look like.
Protecting Muscle During a Deficit
A calorie deficit doesn’t just burn fat. Without the right inputs, your body will also break down muscle tissue for energy. Losing muscle lowers your metabolic rate further, making future weight loss harder and changing your body composition in ways most people aren’t aiming for.
The two most effective tools for preserving muscle are protein intake and resistance training. Research on athletes in a calorie deficit recommends 1.6 to 2.4 grams of protein per kilogram of body weight per day. For a 70 kg (154 lb) person, that’s 112 to 168 grams of protein daily. You don’t need to be an athlete for this range to apply. Spreading protein across meals rather than loading it into one sitting helps your body use it more efficiently for muscle repair.
Resistance training signals your body to hold onto muscle even when calories are low. Two to three sessions per week is enough to make a meaningful difference in how much of your weight loss comes from fat versus lean tissue.
Putting It All Together
Here’s the full process in order: calculate your BMR using the Mifflin-St Jeor equation, multiply by your activity factor to get your TDEE, then subtract 300 to 500 calories. Track your food intake as accurately as possible for at least a few weeks. Weigh yourself at the same time each day (morning, after using the bathroom) and look at weekly averages rather than daily fluctuations. If you’re losing roughly 0.5 to 1 pound per week, your numbers are in the right range. If nothing is changing after two to three weeks of consistent tracking, your TDEE estimate is likely too high, your calorie tracking has gaps, or both. Adjust by dropping your intake by another 100 to 200 calories or increasing activity, then reassess.