The abbreviations BMI and BMR sound similar, but they stand for two separate things. Your BMI, or body mass index, is a number calculated from your height and weight which is then used to assess your body composition. Your BMR, or basal metabolic rate, is the number of calories you burn when your body is at rest. Although the two terms are independent of each other, your BMI may indirectly affect your BMR.
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Body Mass Index
Your BMI is an indirect measure of your body composition -- or how much body fat you have. Although BMI doesn’t measure body fat directly, it uses your weight and height to determine whether you’re classified as underweight, normal weight, overweight or obese. This measurement correlates moderately well with other measurements of body fat such as skinfold measurements and underwater weighing, according to the Centers for Disease Control and Prevention.
BMI is measured by dividing your weight in pounds by the square of your height in inches, then multiplying by 703. The equation looks like this: BMI = (weight / height x height) x 703.
If you're a woman who is 125 pounds and 5 feet 4 inches, your BMI = (125 / 64 x 64) x 703 = 21.4. This BMI puts you in the normal weight range.
A BMI below 18.5 indicates that you’re underweight; a BMI between 18.5 and 24.9 classifies you as a normal weight; a BMI between 25 and 29.9 puts you in the overweight category; a BMI of 30 or above classifies you as obese.
Limitations of BMI
Because it's not a direct measurement, BMI is only used as a screening tool and is not considered a diagnostic test. There are also some limitations to this measurement. Because BMI uses only height and weight, it doesn’t account for people who may be of below-average height but above-average muscle mass, like bodybuilders. If a man has a lot of muscle, which is denser than fat, his BMI may categorize him as overweight, when her weight is actually healthy.
BMI also doesn’t adjust for age or gender. Women naturally have more body fat than men, and older people tend to have more body fat than younger people. So a sedentary older woman with low muscle mass may be overly fat, even though she has a normal BMI.
Basal Metabolic Rate
Your BMR is the number of calories your body burns at rest to do basic functions like breathing, digesting, keeping your heart beating and all the other physiological tasks that keep you alive. Your BMR is partly determined by genetics, but other factors, like your body composition and activity level, may also have an effect on your BMR.
There are equations used to determine your BMR, and they differ based on whether you're a man or a woman. A woman can determine her BMR by plugging her stats into the Harris-Benedict equation: 655 + (4.35 x weight in pounds) + (4.7 x height in inches) – (4.7 x age in years). The equation for a man to calculate his BMR is as follows: 66 + (6.23 x weight in pounds) + (12.7 x height in inches) – (6.8 x age in years).
How BMI and BMR are Connected
Several factors affect your basal metabolic rate -- and body fat composition is one of them. Those with more muscle mass tend to burn more calories at rest because muscle tissue requires more calories to maintain than fat tissue. Although BMI and BMR are not directly related, if you have a high BMI because of a high body fat percentage, your BMR may be lower. If you have a high BMI because of a large amount of muscle mass, your BMR may be increased. Additionally, if you are overweight, but very active, that doesn't mean you'll necessarily have a low BMR.
BMI and BMR are guidelines that allow nutrition and medical professionals to make educated determinations about your body composition and calorie burn, but every person should still be viewed individually. Check with your doctor or a dietitian to if you have questions about your own BMI and BMR.