Your basal metabolic rate (BMR) tells you how many calories your body burns just to stay alive, but it’s not the number you subtract from directly. To calculate a calorie deficit, you first need to convert your BMR into your total daily energy expenditure (TDEE), then eat below that number. The process takes about five minutes with a calculator, and the result gives you a personalized daily calorie target for weight loss.
Why BMR Alone Isn’t Enough
BMR represents the energy your body uses for basic survival functions: keeping your heart beating, your brain running, your liver processing, and your lungs breathing. It’s the calorie cost of simply being alive with zero physical activity. For most people, BMR accounts for 60 to 70% of all the calories burned in a day.
But you don’t lie perfectly still all day. You walk, cook, exercise, fidget, and digest food. All of that burns additional calories on top of your BMR. Your total daily energy expenditure (TDEE) captures everything: your BMR plus all the calories burned through movement and digestion. TDEE is your maintenance number. If you eat roughly that amount, your weight stays stable. To lose weight, you eat less than your TDEE.
Think of BMR as the foundation and TDEE as the complete picture. The calculation works in two steps: estimate your BMR, then multiply it to get your TDEE.
Step 1: Estimate Your BMR
The most widely recommended formula is the Mifflin-St Jeor equation, which uses your weight, height, age, and sex. You’ll need your weight in kilograms and height in centimeters. To convert, divide your weight in pounds by 2.2 and multiply your height in inches by 2.54.
For men: (9.99 × weight in kg) + (6.25 × height in cm) − (4.92 × age in years) + 5
For women: (9.99 × weight in kg) + (6.25 × height in cm) − (4.92 × age in years) − 161
As a quick example, a 35-year-old woman who weighs 70 kg (154 lbs) and stands 165 cm (5’5″) would calculate: (9.99 × 70) + (6.25 × 165) − (4.92 × 35) − 161 = 699.3 + 1031.25 − 172.2 − 161 = roughly 1,397 calories per day. That’s her BMR, the bare minimum her body needs at complete rest.
If You Know Your Body Fat Percentage
People with unusually high or low amounts of muscle sometimes get more accurate results from the Katch-McArdle formula, which factors in lean body mass instead of total weight. The equation is simpler: 370 + (21.6 × lean body mass in kg). To find your lean mass, multiply your weight in kg by your body fat percentage, then subtract that from your total weight. This formula works the same regardless of sex, since it’s based on lean tissue rather than a sex-based correction.
Step 2: Multiply by Your Activity Level
To convert BMR to TDEE, multiply it by a number that reflects how active you are on a typical day. These multipliers, called physical activity levels, are classified into four broad categories:
- Sedentary (1.2): Desk job, little to no exercise. Most of your day is spent sitting.
- Lightly active (1.375): Some walking throughout the day or light exercise one to three days per week.
- Moderately active (1.55): Regular exercise three to five days per week, or a job that keeps you on your feet.
- Very active (1.725): Intense exercise six to seven days per week, or a physically demanding job combined with regular workouts.
Using the example above, if that 35-year-old woman exercises moderately, her TDEE would be 1,397 × 1.55 = roughly 2,165 calories per day. That’s her estimated maintenance intake.
Be honest with yourself here. Most people overestimate their activity level. If you work out three times a week but sit at a desk the other 12 waking hours, “lightly active” is probably more accurate than “moderately active.” Starting with a conservative estimate and adjusting based on real results is more reliable than picking a generous multiplier.
Step 3: Subtract to Create Your Deficit
A safe and sustainable rate of weight loss is one to two pounds per week. To hit that range, subtract 500 to 1,000 calories from your TDEE. A 500-calorie daily deficit targets about one pound per week, and a 1,000-calorie deficit targets about two pounds.
For our example, a TDEE of 2,165 minus 500 gives a daily target of roughly 1,665 calories for steady, moderate weight loss. If that same person subtracted 1,000, she’d land at 1,165, which is getting close to uncomfortable territory and may not be sustainable.
A percentage-based approach can be more flexible. Subtracting 15 to 25% of your TDEE keeps the deficit proportional to your size. Someone with a TDEE of 2,800 can handle a larger absolute deficit than someone at 1,800, even if both are cutting the same percentage. For most people, a 20% reduction strikes a practical balance between noticeable progress and not feeling miserable.
Why the “3,500 Calories Per Pound” Rule Falls Short
You’ve probably heard that cutting 3,500 calories equals one pound of fat lost. That number has been repeated in textbooks and government health sites for decades, but it consistently overestimates actual weight loss. In one analysis, subjects lost an average of 20 pounds when the 3,500-calorie rule predicted they should have lost about 28, a gap of nearly eight pounds.
The reason is that weight loss isn’t linear. Your body adapts as you get lighter, and the math changes over time. Dynamic models that account for these shifts predict a curving pattern of weight loss that levels off into a plateau around 1.4 years. The practical takeaway: expect faster results in the first few weeks and slower progress later, even if your deficit stays the same.
Your BMR Changes as You Lose Weight
One of the most important things to understand about this process is that the number you calculate today won’t stay accurate forever. As you lose weight, your BMR drops. Research published in the International Journal of Obesity found that participants who lost about 16 pounds saw their resting metabolic rate drop by roughly 100 calories per day.
About 60% of that drop comes from simply having less tissue for your body to maintain, particularly less fat tissue. But the other 40% comes from metabolic adaptation, your body actively becoming more efficient with energy in response to eating less. Hormones that regulate hunger, metabolism, and fat storage all shift downward during a deficit, contributing to this slowdown.
This is why weight loss stalls. The deficit you calculated at the start gradually shrinks as your body adjusts. Recalculating your BMR and TDEE every 10 to 15 pounds, or roughly every couple of months, keeps your numbers current and your expectations realistic.
Putting It All Together
Here’s the complete sequence, condensed into five steps:
- Calculate your BMR using the Mifflin-St Jeor formula (or Katch-McArdle if you know your body fat percentage).
- Pick your activity multiplier honestly, erring toward the conservative side.
- Multiply BMR × activity level to get your TDEE.
- Subtract 500 to 1,000 calories (or 15 to 25%) from your TDEE to set your daily intake target.
- Recalculate every 10 to 15 pounds to account for your lower body weight and metabolic changes.
Track your actual weight over two to three weeks before making adjustments. Daily weight fluctuates due to water retention, sodium intake, and digestive timing. A weekly average gives you a much clearer signal. If you’re losing about one pound per week, your numbers are dialed in. If you’re losing faster than two pounds per week or not losing at all, adjust your calorie target by 100 to 200 calories in either direction and reassess after another two weeks.