What Is BMI?

JGI/Jamie Grill/Getty Images

BMI stands for “Body Mass Index,” and is a term that you will see often in discussions regarding obesity. BMI is measured in units of kg/m2, which means that it requires height and weight for the calculation. BMI calculators are readily available online, such as the one offered by the National Heart, Lung, and Blood Institute (NHLBI) : https://www.nhlbi.nih.gov/guidelines/obesity/BMI/bmicalc.htm.

What Are the Healthy and Unhealthy BMI Ranges?

A normal BMI is defined as falling between 18.5 and 24.9 kg/m2.

Having a BMI lower than 18.5 classifies one as underweight.  A BMI of 25.0 – 29.9 is in the overweight range, and a BMI of 30.0 or greater represents obesity.  A BMI of 40.0 or greater is often referred to as “morbid obesity,” and is recommended by national guidelines as the cutpoint for identifying patients who may be eligible for bariatric surgery.

The Centers for Disease Control and Prevention (CDC) notes that athletes who are highly muscular may have a high BMI that is due to greater muscular weight rather than to body fat.

How Is BMI Used?

BMI is used as a screening tool to identify patients who need to lose weight. In the “2013 ACCF/AHA/TOS Guideline for the Management of Overweight and Obesity in Adults,” the guideline authors noted that, according to current estimates, 69% of U.S. adults are either overweight or obese (with a BMI of 25.0 or greater), with approximately 35% falling into the obese category (a BMI of 30.0 or greater).

These authors further noted that the latest data from the National Health and Nutrition Examination Surveys (NHANES) indicated that these estimates of overweight and obesity for the period from 2009 to 2010 did not differ much from the estimates for the period from 2003 to 2008. This suggests that the prevalence rates for obesity may be plateauing.

The aforementioned Obesity guideline recommends that BMI be calculated for patients at annual visits or even more frequently. This guideline further recommends that the BMI cutpoints for overweight and obesity, as outlined above, be used to identify adults who may have a higher risk of cardiovascular disease. In addition, the guideline recommends that the BMI cutpoint for obesity (30.0 or greater) be used to identify adult patients who may have a higher risk of all-cause mortality (that is, death from any cause).

Thus, the American Heart Association (AHA), American College of Cardiology Foundation (ACCF), and The Obesity Society (TOS), through this guideline, officially highlight the connection between BMI and cardiovascular disease, as well as between BMI and mortality from all causes. The guideline further recommends, along these lines, that adults who fall in the overweight and obese BMI categories should be advised that a greater BMI leads to a greater risk of cardiovascular disease, type 2 diabetes, and all-cause mortality.

As noted above, BMI measurements are used as part of guideline-based criteria to determine which patients may be eligible for weight-loss surgery or weight-loss medications.

Another use of the BMI measurement in the new Obesity guideline cited above is to define class I, class II, and class III obesity. According to the new guideline, having a BMI from 30.0 to 34.9 places a patient in the “class I obese” category; a BMI from 35.0 to 39.9 identifies the “class II obese” category; and a BMI of 40.0 or greater identifies the “class III obese (extreme obesity)” category.

Action Step: Calculate your BMI using a BMI calculator such as the one found at https://www.nhlbi.nih.gov/guidelines/obesity/BMI/bmicalc.htm. If your BMI is outside the normal range (above or below), make it a point to discuss this with your physician.


2013 AHA/ACC/TOS Guideline for the Management of Overweight and Obesity in Adults: a report of the American College of Cardiology/American Heart Association Task Force on Practice Guidelines and The Obesity Society. Circulation. published online November 12, 2013.

Continue Reading