BMI stands for Body Mass Index. It is a calculation that uses a person’s height and weight to identify a weight category for that individual. BMI can be used as a simple measurement to correlate health risk and, when losing weight, can indicate improved health.

What is the BMI formula?

The BMI formula is determined by taking your weight in pounds and dividing that number by your height in inches, squared. The BMI calculation is: weight (pounds) / height (inches) x height (inches) x 703.

For example, a person weighing 200 pounds and standing 70” tall would have a BMI of 200/70x70x703 = 28.7. There are many BMI calculators available online, such as the CDC's Adult BMI Calculator.

That number correlates to one of the weight categories assigned to various ranges of BMIs. There are four main categories: underweight, normal weight, overweight, and obese. The goal is the “normal weight” category, meaning a BMI that falls in the range of 18.5 to 24.9 based on the above calculation.

Generally, BMI below 18.5 and above 30 is associated with increased health risks. As BMI increases from overweight into the higher categories of obesity, health risk increases proportionally.

What are the BMI ranges?

While there are four main categories of BMI, there are additional categories for differing BMIs in the obesity classes.

  • underweight BMI <18.5
  • normal weight BMI 18.5-24.9
  • overweight BMI 25-29.9
  • obesity class I BMI 30 – 35
  • obesity class II BMI 35-40
  • obesity class III BMI >40

Health conditions associated with higher BMI

A higher BMI generally is associated with worsening of medical conditions linked to being overweight. Those include:

Can you have a high BMI and still be "healthy"?

There are exceptions to the rule that higher BMI is associated with increased health risk. Since BMI is calculated simply by weight and height, individuals who have a higher degree of muscle mass accounting for a larger percentage than normal of their total weight may have a higher BMI that is not associated with greater health risk. 

Where fat is stored on the body also influences health. Fat cells on the outside of the body cause less associated illness than fat stored inside the body in organs such as the liver and muscle. This is partly genetic, but exercise greatly helps reduce fat stored in muscle and liver.

What happens when you lower your BMI?

Obesity is a chronic disease and when it is treated by a team of professionals, your BMI goes down, and along with it, so does your chance of developing type II diabetes, cardiovascular disease, sleep apnea, hip and knee arthritis, and certain cancers.

For more information on treating a high BMI, visit the Obesity Medicine website or call 401-793-7837.

Sheenagh Bodkin, MD

Dr. Sheenagh Bodkin is director of obesity medicine at Coastal Medical.