What Is BMR and How to Calculate It

If you have ever tried to figure out how many calories you should eat in a day, you have probably run into the term BMR. It shows up on fitness apps, in diet articles, and on the labels of various supplements. But what actually is BMR, where does the number come from, and does it matter for your specific goals? This guide breaks it all down with actual numbers and examples.

What BMR actually measures

BMR stands for Basal Metabolic Rate. It is the number of calories your body burns at complete rest to keep you alive. We are talking about the energy required for breathing, pumping blood, maintaining body temperature, producing cells, and keeping your organs functioning. Imagine lying in bed for 24 hours without moving a muscle, without eating, without even sitting up. The calories burned during that time are your BMR.

For context, BMR accounts for roughly 60-75% of your total daily calorie burn. Physical activity accounts for 15-30%, and the thermic effect of food (energy used to digest what you eat) takes up the remaining 5-10%. So your BMR is by far the largest component of your metabolism, which is why understanding it matters.

The average adult male has a BMR of about 1,600-2,000 kcal per day, and the average adult female has a BMR of about 1,200-1,500 kcal per day. But these are broad averages. Your actual BMR depends on your specific body composition, height, weight, age, and sex.

The Mifflin-St Jeor formula

There are several formulas for estimating BMR, but the one used most often by health professionals is the Mifflin-St Jeor equation, published in 1990. It was developed by Dr. Mifflin and Dr. St Jeor using data from indirect calorimetry measurements, and it has held up well in research as the most accurate formula for most people.

Here is the formula:

• Males: BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age) + 5
• Females: BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age) − 161

Let us walk through two worked examples.

Example 1: Male, 180 cm tall, 80 kg, 30 years old

Example 2: Female, 165 cm tall, 60 kg, 28 years old

These numbers tell us the calories each person burns at complete rest. But nobody stays in bed all day, so we need to account for movement and activity to get a useful calorie target.

BMR vs TDEE: why the distinction matters

TDEE stands for Total Daily Energy Expenditure. It is your BMR multiplied by an activity factor that accounts for everything you do beyond lying still. Here are the standard multipliers:

• Sedentary (desk job, no exercise): BMR × 1.2
• Lightly active (light exercise 1-3 days/week): BMR × 1.375
• Moderately active (exercise 3-5 days/week): BMR × 1.55
• Very active (hard exercise 6-7 days/week): BMR × 1.725
• Extra active (very hard exercise, physical job): BMR × 1.9

For our 1,780 BMR male who exercises four times per week (moderately active), TDEE = 1,780 × 1.55 = 2,759 kcal/day. That is the number of calories he needs to maintain his current weight. To lose weight, he would eat below that number. To gain weight, he would eat above it.

The most common mistake people make is confusing BMR with TDEE and eating at their BMR level thinking it is the right deficit. Eating at your BMR means you are creating a larger deficit than intended (since your BMR is 20-40% below your TDEE depending on activity). This can lead to fatigue, muscle loss, and metabolic adaptation over time.

What affects your BMR

BMR is not a fixed number. Several factors push it up or down:

• Muscle mass. This is the single biggest factor you can control. A pound of muscle burns roughly 6-10 calories per day at rest, compared to about 2 calories per day for a pound of fat. Two people who weigh the same but have different body compositions can have noticeably different BMRs. Someone who strength trains regularly might have a BMR that is 100-200 calories higher than a sedentary person of the same weight.
• Age. BMR declines by about 1-2% per decade, mostly because of muscle loss. A 50-year-old who does not strength train might have a BMR that is 5-10% lower than when they were 30, even at the same weight. This decline is not inevitable, though. Research shows that older adults who maintain muscle mass through resistance training preserve their metabolic rate much better.
• Body size.Taller and heavier people burn more calories at rest simply because there is more tissue to sustain. A 6'2" person will always have a higher BMR than a 5'4" person at the same body fat percentage.
• Thyroid function. Thyroid hormones (T3 and T4) are the primary regulators of metabolic rate. Hypothyroidism can lower BMR by 15-30%, while hyperthyroidism can raise it by a similar amount. If you suspect a thyroid issue, a simple blood test can check your levels.
• Temperature and environment. Your body burns extra calories to maintain its core temperature in extreme cold or heat. Living in a very cold climate can raise BMR by 5-10%.

How to use your BMR for weight goals

Once you know your BMR and TDEE, you can set a calorie target based on your goal:

• Weight loss: Eat 300-500 kcal below your TDEE. This creates a moderate deficit that produces sustainable fat loss of roughly 0.5-1 lb per week. Avoid eating below your BMR unless under medical supervision.
• Muscle gain: Eat 200-400 kcal above your TDEE while following a structured strength training program. This surplus supports muscle growth without excessive fat gain.
• Maintenance: Eat at your TDEE. Track your weight for 2-3 weeks and adjust up or down by 100-200 kcal if your weight drifts. Calculated TDEE is an estimate; real-world data is more accurate.

One thing that trips people up: as you lose weight, your BMR and TDEE both decrease. Someone who loses 20 pounds might see their TDEE drop by 150-250 calories per day. This is normal and means you need to recalculate periodically and adjust your intake. It does not mean your metabolism is "broken."

Frequently Asked Questions

How accurate is the Mifflin-St Jeor equation?

It is accurate to within about 10% for most adults when compared to laboratory measurements. The Academy of Nutrition and Dietetics considers it the most accurate of the widely used BMR formulas. It tends to be slightly less accurate for very overweight or underweight individuals. For those populations, the Katch-McArdle formula (which uses lean body mass) can be more precise, but it requires knowing your body fat percentage.

Should I eat below my BMR to lose weight?

Generally no. Your BMR represents the calories needed for basic survival functions. Eating below this level for extended periods can lead to muscle loss, fatigue, nutrient deficiencies, and a slowed metabolism. A better approach is to eat between your BMR and your TDEE. For example, if your BMR is 1,500 and your TDEE is 2,200, eating 1,700-1,900 calories creates a useful deficit while still fueling your body properly.

Does BMR decrease as I get older?

Yes, by roughly 1-2% per decade after age 20. This is primarily caused by loss of muscle mass (sarcopenia) and hormonal changes, not by aging itself. A 60-year-old who strength trains regularly can maintain a BMR that is close to what they had at 40. The decline is largely preventable with resistance training and adequate protein intake.

Can I increase my BMR?

The most effective strategy is building muscle. Each pound of muscle adds about 6-10 calories per day to your BMR. Gaining 10 pounds of muscle raises your resting burn by 50-80 calories per day. Over a year, that adds up to about 5-8 pounds of fat equivalent. It is not a huge number on its own, but combined with the calories you burn during the workouts themselves, it becomes significant. High-intensity interval training and adequate protein intake (0.7-1.0g per pound of body weight) also support a healthy metabolic rate.

Is BMR the same as RMR?

They are related but not identical. BMR is measured under strict conditions: 12 hours of fasting, after a full night of sleep, in a dark, temperature-controlled room, at complete rest. RMR (Resting Metabolic Rate) is measured under less restrictive conditions and typically runs 10-20% higher than BMR because it includes the thermic effect of a recent meal and minimal daily movement. In practice, most online calculators (including ours) estimate BMR, but the distinction is small enough that the terms are often used interchangeably.

Related Calculators

• BMR Calculator — Calculate your BMR and TDEE with the Mifflin-St Jeor formula
• Calorie Calculator — Plan your daily calorie intake for weight loss or gain
• Body Fat Calculator — Estimate your body fat percentage with the US Navy method

Ready to find your number? Try our BMR calculator — it runs the Mifflin-St Jeor equation instantly and shows your TDEE across five activity levels so you can pick the right calorie target for your goals.