Your basal metabolic rate (BMR) tells you the minimum number of calories your body burns just to stay alive, and it’s the starting point for building a calorie deficit that leads to weight loss. BMR accounts for 60% to 70% of your total daily energy expenditure, covering basics like breathing, circulating blood, and maintaining body temperature. Once you know this number, you can estimate how many calories you actually burn each day, then eat below that threshold to lose fat at a controlled, sustainable pace.
What BMR Actually Tells You
BMR is not the number of calories you burn in a day. It’s the number you’d burn if you stayed in bed and did absolutely nothing. Your body still needs fuel to keep your heart beating, your lungs expanding, your cells dividing, and your organs functioning. That baseline energy cost is your BMR.
The reason this matters for weight loss is simple: your BMR is the floor. You should generally not eat fewer calories than your BMR, because that number represents the energy your body needs for its most basic operations. Weight loss calories come from the gap between what you eat and what you burn through daily movement and exercise, not from starving your organs of fuel.
How to Calculate Your BMR
The Mifflin-St. Jeor equation is considered the most accurate formula for estimating BMR, and it’s the one recommended by the Academy of Nutrition and Dietetics. You’ll need your weight in kilograms, height in centimeters, and age in years.
For men: (9.99 × weight) + (6.25 × height) − (4.92 × age) + 5
For women: (9.99 × weight) + (6.25 × height) − (4.92 × age) − 161
To convert, divide your weight in pounds by 2.2 to get kilograms, and multiply your height in inches by 2.54 to get centimeters. So a 35-year-old woman who weighs 160 pounds (72.7 kg) and stands 5’6″ (167.6 cm) would calculate: (9.99 × 72.7) + (6.25 × 167.6) − (4.92 × 35) − 161, which gives roughly 1,415 calories per day. That’s her body’s baseline energy cost before any movement.
Converting BMR to Total Daily Calories
Your BMR alone isn’t enough to plan a deficit. You need your total daily energy expenditure (TDEE), which adds in all the calories you burn through walking, working, exercising, and even fidgeting. To get this number, multiply your BMR by an activity factor:
- Sedentary (desk job, little exercise): BMR × 1.2
- Lightly active (light exercise 1–3 days per week): BMR × 1.375
- Moderately active (moderate exercise 3–5 days per week): BMR × 1.55
- Very active (hard exercise 6–7 days per week): BMR × 1.725
- Extremely active (intense training or physical job): BMR × 1.9
Using the example above, that 35-year-old woman with a BMR of 1,415 who exercises moderately three to five days a week would multiply by 1.55, giving her an estimated TDEE of about 2,193 calories. That’s the number she needs to eat below in order to lose weight.
Most people overestimate their activity level. If you’re unsure, start with the lower category. You can always adjust upward if you’re losing weight too quickly or feeling drained.
Setting Your Calorie Deficit
A pound of body fat stores roughly 3,500 calories. To lose about one pound per week, you need a daily deficit of 500 calories. To lose half a pound per week, aim for 250. These deficits can come from eating less, moving more, or a combination of both.
The critical rule: your daily calorie intake should not drop below 1,200 for women or 1,500 for men without medical supervision. Eating below these thresholds deprives your body of essential nutrients and can trigger your metabolism to slow down protectively, making weight loss harder over time. If your calculated deficit would push you below these floors, reduce the deficit size or add more physical activity instead of cutting more food.
For the woman in our example, a 500-calorie deficit from her TDEE of 2,193 means eating about 1,693 calories per day. That’s comfortably above the 1,200 minimum and well above her BMR of 1,415, which means her body has enough fuel for basic functions plus her daily activities.
Why Your BMR Changes Over Time
Your BMR isn’t fixed. It shifts as your body changes, and understanding these shifts helps explain why weight loss often stalls.
Age is the most predictable factor. Metabolism stays relatively stable from your 20s through your 50s, contrary to the popular belief that it tanks in middle age. Research published in Science found that the real decline begins around age 60, when both BMR and total energy expenditure drop by about 0.7% per year. By age 90, adjusted energy expenditure is roughly 26% lower than in middle-aged adults. This decline partly reflects a loss of metabolically active tissue and partly reflects reduced organ-level metabolic activity.
Body composition matters more than body weight. Muscle tissue burns more calories at rest than fat tissue does. Two people who weigh the same can have meaningfully different BMRs if one carries more muscle. This is why strength training is valuable during weight loss: it helps preserve the muscle that keeps your metabolism higher.
Weight loss itself lowers your BMR. A smaller body requires less energy to maintain. This is normal, but it means the calorie target that created your initial deficit will eventually become your maintenance level. Recalculating every 10 to 15 pounds of weight loss keeps your plan accurate.
Putting It Into Practice
Here’s the step-by-step process, from calculation to daily use:
First, calculate your BMR using the Mifflin-St. Jeor equation. Then multiply it by the activity factor that honestly reflects your lifestyle. Subtract 250 to 500 calories from that TDEE number to set your daily intake target. Check that this target stays above 1,200 (women) or 1,500 (men).
Track your actual weight changes over two to three weeks before making adjustments. The formulas are estimates, and individual variation is real. If you’re losing more than two pounds per week, eat slightly more. If the scale hasn’t moved at all, your activity multiplier may be too high, or your calorie tracking may be off. Small adjustments of 100 to 200 calories are enough to correct course.
As you lose weight, recalculate. Plug your new weight into the formula every 10 to 15 pounds, and adjust your TDEE and deficit accordingly. A person who started at 200 pounds and dropped to 175 has a meaningfully different BMR, and continuing to eat at the old deficit level will eventually stop producing results.
Physical activity is where you have the most leverage. Your BMR is largely determined by your age, sex, height, and body composition, and you can’t change most of those quickly. But you can change your activity multiplier. Moving from sedentary (1.2) to moderately active (1.55) on a BMR of 1,500 adds over 500 calories to your daily burn, creating space for a comfortable deficit without severe food restriction.