BMI vs. BMR: What's the Difference?

The medical industry is full of acronyms, from MD (medical doctor) and CDC (the Centers for Disease Control and Prevention) to various health conditions like CAD (coronary artery disease) and STD — you probably know that one. Two in particular are often mentioned when discussing weight management: BMI and BMR.

Understanding BMI versus BMR can be confusing at first, mainly because they share the same two first letters.

So, what is BMI and BMR, exactly? BMI is short for body mass index, and BMR stands for basal metabolic rate.

But what do these words mean, and what’s the difference between BMI and BMR? We’ll answer these questions below and go over what information they provide, how to calculate them and how they’re related.

BMI is body mass index. It’s a measurement used to assess a person’s body weight (in kilograms) in relation to how tall they are (the square of their height in meters).

If you want to calculate it using imperial (U.S.) units, however, you can divide your weight (in pounds) by your height squared (in inches) and multiply that number by 703. (We know — that sounds like a random number, but it works.)

Bad at math? We got you. Check out the Hers BMI Calculator.

This simple calculation — albeit not perfect — is a numerical representation of your body composition in terms of what’s considered “normal” and “healthy.” (We use quotes here because it can be subjective and doesn’t paint a complete picture of health.)

BMI is a common, standard screening tool. Many healthcare providers use it to evaluate patients’ weight-related health risks and figure out what treatments or lifestyle changes might be appropriate.

BMI Categories

The calculated number places you in one of these BMI categories:

• Severely underweight: <16

• Underweight: 16.0 to 18.4

• Normal weight: 18.5 to 24.9

• Overweight: 25.0 to 29.9

• Moderate obesity: 30.0 to 34

• Severe obesity: 35.0 to 39.9

• Extreme obesity: ≥40.0

Medical providers might use this information (and, ideally, other details about your overall health and lifestyle) to offer recommendations for achieving a healthier weight and reducing disease risk.

Being significantly below or above what’s considered “normal” weight is a risk factor for certain health complications, such as malnutrition, osteoporosis, impaired immunity, high blood pressure and diseases like type 2 diabetes, cancer and heart disease.

Of course, these conditions can also occur among people who fall in the normal weight category — the point being, body composition isn’t the only thing at play.

BMI Has Limitations

While it provides useful insight for population studies and general health assessments, BMI leaves out plenty of details about you as an individual.

It doesn’t take into account factors like muscle mass, bone density, body fat percentage and distribution or overall body composition.

For instance, an older adult who leads a more sedentary lifestyle (with minimal physical activity) and a bodybuilder could theoretically have the same BMI but very different body compositions. (And yes, we realize that your Grandma Berta could be an 80-year-old bodybuilder — this is just an example.)

BMR is basal metabolic rate. It tells you the minimum number of calories necessary for basic functions for survival at rest, like breathing, blood circulation, digestion and cell production. Inversely, it’s how many calories your body burns each day doing these basic functions (like if you stayed in bed all day and hardly lifted a muscle).

Believe it or not, many people burn more calories doing nothing (aka the resting state) than anything else they do all day.

For example, your BMR could be 1,300 calories, and you may only burn another 700 through physical activity, whether that’s walking around your home or going to the gym).

BMR is influenced by some factors you can’t control, like age, gender and genetics. Women, on average, have a lower BMR than men, but resistance training may help increase BMR in healthy women.

Having a rough idea of your BMR can be helpful when creating a fitness and nutrition plan to reach certain weight or body composition goals. However, no single BMR value is appropriate for all adults.

BMR vs. RMR

As you scour the internet for information on calculating BMR, you’re bound to come across RMR (resting metabolic rate). RMR is sometimes also called resting energy expenditure (REE — another acronym, sorry).

Are BMR and RMR related, and if so, what’s the difference?

The main distinction between basal metabolic rate and resting metabolic rate is that RMR estimates the number of calories your body actually burns while at rest.

Studies consistently show differences in RMR based on height, sex, age and body composition. It also changes throughout seasons of life, particularly in adolescence.

RMR is usually measured in the morning, with a few prerequisites:

• You’ve fasted overnight and haven’t eaten breakfast

• You haven’t exercised for at least 24 hours

• You’re not dealing with significant emotional stress

• You’re completely rested

One study looked at whether a lower RMR was an independent predictor of weight gain. After evaluating BMR and body composition data from studies between 1995 and 2012, the authors concluded that other factors are much more effective at predicting individual weight gain than RMR — like exercise and nutrition.

Finally, while BMR and RMR calculations differ slightly, there’s typically only around a 10 percent variance in their results.

In other words, there’s no need to obsess over using all the calculations in hopes of determining your exact needs. These two are pretty close and provide similar insight.

How to Determine BMR

BMR calculations are different for men and women. You can use an online BMR calculator or the Harris-Benedict equation.

Women can plug their weight, height and age into this equation:

BMR = 655 + (4.35 x weight in pounds) + (4.7 x height in inches) – (4.7 x age in years)

Men can input their weight, height and age into this variation:

BMR = 66 + (6.23 x weight in pounds) + (12.7 x height in inches) – (6.8 x age in years)

BMR Activity Factors

Your BMR can help you estimate your basic calorie needs. Once you have this number, you can multiply it by an activity factor to figure out your needs for weight maintenance, weight loss or weight gain.

Activity factors are based on how active you are with your typical lifestyle. Think about where you fall within the categories below and multiply your BMR by the corresponding activity factor.

• Sedentary lifestyle: 1.2 (minimal to no exercise)

• Lightly active lifestyle: 1.375 (light exercise one to three days a week)

• Moderately active lifestyle: 1.55 (moderate-intensity exercise three to five days a week)

• Very active lifestyle: 1.725 (hard exercise six or seven days a week)

• Extra-active lifestyle: 1.9 (very hard exercise six or seven days a week or a physically taxing job)

For example, if your BMR is 1,400 calories but you’re a moderately active walking machine, you’d multiply 1,400 by 1.55. This gives you a total daily calorie goal of 2,170 to maintain your current weight at your current activity level.

If your goal is weight loss, you might subtract 500 calories from this number. If your goal is weight gain, you might add 500 calories to it. (This would, theoretically, result in one pound of weight loss or a week.)

BMI and BMR serve two distinct purposes. BMI evaluates body weight relative to height, offering insight into potential weight-related health risks like obesity or underweight. BMR measures the minimum amount of energy your body needs at rest to maintain basic functions.

Though they don’t directly impact one another, BMI can indirectly influence BMR. How so? Your body composition is a factor in how much energy you burn in a day.

But if your BMI is higher due to having more lean muscle mass — a pound of muscle weighs more than a pound of fat — your BMR might be higher too. (It takes more energy to build and maintain muscle than it does to maintain fat).

Overall, BMI and BMR aren’t directly associated, but each can provide insight into your health while helping you evaluate your habits and goals.

And if anyone’s wondering, basal metabolic index calculation isn’t a real thing — it’s just a jumble of both terms.

Understanding BMI versus BMR can give you an idea of where you stand in terms of body composition and metabolism.

BMI and BMR formulas are simple calculations that provide basic information about your general health. However, they’re only two pieces of a much larger puzzle.

When calculating your BMI and BMR and considering how to use them, keep these things in mind:

• They provide different metrics. While BMI can be used to assess weight-related health risks, BMR measures the minimum amount of energy your body burns to stay alive at rest.

• They have distinct purposes. BMI is a screening tool for evaluating potential weight-related health risks, whereas BMR can serve as a baseline for figuring out your daily calorie needs, depending on your goals.

• They have an indirect relationship. BMI and BMR are related through factors like body composition and serve complementary roles in assessing your overall health. However, you don’t need one to calculate the other.

If you’re starting a weight management journey and want to connect with a licensed healthcare provider or explore weight loss medication options, Hers has you covered. Start by taking our free assessment.