Body Mass Index

Body mass index (BMI) is a measure that can help you determine if you are underweight, overweight, or about normal for your height. It's also the quickest way to know whether your weight is healthy for your height.

Your BMI is useful for predicting your health risks, but it's not perfect. If you're muscular, for example, you could end up with a high BMI, even though no one would call you overweight. For a full picture of your health, it's a good idea to consider other factors along with BMI, including total body fat, waist circumference, and waist-to-hip ratio.

How is BMI calculated?

The calculation for BMI is Weight in pounds / (Height in inches) x 703

You don't need to do the math yourself -- it's easy to find a BMI calculator online. The National Institutes of Health has one: https://www.nhlbi.nih.gov/health/educational/lose_wt/BMI/bmicalc.htm So does the Centers for Disease Control and Prevention (CDC) (https://www.cdc.gov/healthyweight/assessing/bmi/adult_bmi/english_bmi_calculator/bmi_calculator.html), but both are based on the same formulas.

What does my BMI mean?

Here are some basic guidelines for interpreting your BMI, according to CDC guidelines::

  • Underweight: Underweight: Less than 18.5
  • Normal weight: 18.5 to 24.9
  • Marginally overweight: 25.8 to 27.3 for women, 26.4 to 27.8 for men
  • Overweight: 25.0 to 29.9
  • Obese: 30 and above

What are the limitations of BMI?

Using BMI to predict overweight still has its drawbacks. It doesn't take frame size into account, so people with stockier builds may be considered overweight even if they don't have a lot of body fat.

Tests that physically measure body fat and distribution are better than BMI at telling if you are overweight. A skinfold measure, for example, uses an instrument called a caliper to measure the thickness of the fat layer on your arm or stomach.

A bioelectrical impedance test shows how easily electricity travels through your body (fatty tissue slows down the current) to estimate what percentage of the body is fat. These tests involve a physical measurement by a trained technician at a gym or doctor's office.

Also, BMI is not a good predictor of overweight for these groups of people:

  • Children and teens, because BMI ranges are based on adult heights
  • Competitive athletes and bodybuilders, because heavier muscle weight may skew the results
  • Pregnant or nursing women, because they need more fat reserves than usual
  • People over 65, because even BMI values of 29 do not appear to be unhealthy at this age, and may even be a useful energy reserve in case of illness

References

Clinical Guidelines on the Identification, Evaluation, and Treatment of Overweight and Obesity in Adults. http://www.nhlbi.nih.gov/guidelines/obesity/ob_home.htm

Centers for Disease Control. Adult BMI Calculator. https://www.cdc.gov/healthyweight/assessing/bmi/adult_bmi/english_bmi_calculator/bmi_calculator.html

National Institutes of Health. Calculate Your Body Mass Index. https://www.nhlbi.nih.gov/health/educational/lose_wt/BMI/bmicalc.htm

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