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What is BMI and Why Does It Matter?

“I’m not overweight. I’m under-height!”

That old joke gets right to the heart of the much-debated Body Mass Index (BMI). While it’s not a perfect science, BMI is a standard measurement for determining if a person is overweight or obese. The formula is simple: the ratio of weight to height squared. (You can find an online calculator here.) A very low ratio is considered to be in the underweight range while a very high ratio can indicate obesity.


  • Less than 18.5 = Underweight
  • 18.5–24.9 = Healthy weight
  • 25–29.9 = Overweight
  • More than 30 = Obese

View the whole BMI table here.

A person’s weight can affect their health in significant ways.

Somebody who is obese is particularly at risk for:

  • High blood pressure
  • Type 2 diabetes
  • Heart disease
  • Stroke
  • Osteoarthritis
  • Sleep apnea and breathing problems
  • Cancer

Somebody who is severely underweight is at risk for:

  • Osteoporosis
  • Anemia
  • Irregular periods
  • Premature birth
  • Slow or impaired growth

Though BMI has been widely adopted as a reliable indicator of body fat, many have argued its weaknesses outweigh its usefulness.

BMI limitations and uses

The original calculation, created in the 19th century by a mathematician, was meant to be a quick way to measure obesity in the general population, not in individuals. Since that time, we have a better understanding of the human body, and better tools for measuring body fat, including body scans and air or water displacement technology.

Major health organizations, such as the Centers for Disease Control and the World Health Organization, use BMI as a standard measurement for healthy weight, as do many physicians. Research shows that BMI tends to align with other more exact methods for determining body fat, and its simplicity is a plus for clinicians screening people to determine whether a person’s weight is impacting their health. It’s also easy for anyone to use an online calculator to assess whether they are at a healthy weight, and the results will be accurate for most.

However, muscle and bone weigh more than fat. Since the calculation doesn’t include a way to determine how much weight comes from fat and how much comes from muscle, those with athletic or muscular builds will get skewed results. It also does not take the distribution of fat into account, though numerous studies show a connection between health problems and belly fat, even in those who are not obese.

Measuring belly fat

Studies have shown that people with high levels of belly fat are more prone to heart issues. Fat that is stored in the abdomen is called visceral fat, or deep fat, and its proximity to major organs like the liver and pancreas can lead to serious health issues, including diabetes and heart disease.

The two most common methods for determining whether belly fat poses a health risk are:

Waist circumference

A person is considered obese if their waist measures:

  • More than 35 inches for women, or
  • More than 40 inches for men.

Waist-to-hip ratio

A person is considered obese if their waist measurement divided by their hip measurement is:

  • More than 0.85 for women, or
  • More than 0.9 for men.

BMI in children

One of the major health concerns in the country right now is childhood obesity. A child who is obese is at greater risk for heart disease before age 30, type 2 diabetes and major health problems over their lifetime.

80% of children who are overweight at ages 10-15 grow up to be obese adults.

BMI is calculated differently for children and adolescents to take into account their growing and changing bodies. Though it starts with the same base ratio, it takes age and gender into consideration. The BMI for children and adolescents is determined as a percentile, in relation to children in the U.S. of the same age and gender.

A child with a BMI at the 95th percentile (greater than the BMI of 95% of children of the same age and gender in the U.S.) falls into the obese category.

BMI weight categories, ages 2-19 years old

  • Less than 5th percentile = Underweight
  • 5th to less than 85th percentile = Healthy weight
  • 85th to less than 95th percentile = Overweight
  • 95th percentile and above = Obese

Though the methodology is different, BMI for children is also designed to reliably indicate whether a child is at a healthy weight. Regular monitoring of growth and BMI can help a physician identify children at risk for weight problems and address them early.

What to do with BMI?

Being obese or overweight causes real health issues for people of all ages. Though BMI is not a perfect measurement, it can be a quick and usually reliable starting point to discuss weight issues. Talk to your doctor at your annual wellness check-up and make changes where necessary.

Nancy Henderson

Nancy Henderson, a writer and editor originally from New York, moved to Nashville more than 25 years ago and considers herself more Tennessean than Yankee these days. She has written about health care and wellness for a variety of publications.

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Get more information about specific health terms, topics and conditions to better manage your health on BlueCross BlueShield of Tennessee members can access wellness-related discounts on fitness products, gym memberships, healthy eating and more through Blue365®. BCBST members can also find tools and resources to help improve health and well-being by logging into BlueAccess and going to the Managing Your Health tab.

Filed under: Mind & Body


Nancy Henderson, a writer and editor originally from New York, moved to Nashville more than 25 years ago and considers herself more Tennessean than Yankee these days. She has written about health care and wellness for a variety of publications.


WellTuned provides inspiration and practical advice for healthy living.
WellTuned does not offer medical advice. Any personal health questions should be addressed to your doctor.

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