BMI Weight Classes

Body mass index, or BMI, is a body composition measurement. A person's BMI is determined by a simple calculation relating height and body weight. BMI is used to estimate how much body fat a person has. Using this simple calculation allows you to estimate the likelihood you may be at risk for certain conditions, such as cardiovascular disease, diabetes and hypertension, according to "ACSM's Guidelines for Exercise Testing and Prescription." Children's BMI categories differ from adult BMI categories, but BMI is the same for adult men and women. BMI is just one of many factors to consider when evaluating health risks.

Underweight

If an adult's BMI is less than 18.5 she is considered underweight. Someone with a BMI in this category is considered to be at risk for malnutrition, lowered immune function and osteoporosis, according to Better Health Channel, a service run by Australia's State of Victoria.

Normal Weight

A BMI between 18.5 and 24.9 falls in the normal range, according to the Centers for Disease Control and Prevention. People with a BMI in this category are not considered to have an elevated health risk compared to the general population. There are many other factors that influence health and fitness, but a BMI in this range is not considered one of them.

Overweight and Obese

A person with a BMI between 25.0 and 29.9 is overweight. A BMI greater than 30.0 is categorized as obese. People with a BMI in either of these two categories is at a greater risk for a variety of diseases and conditions. According to the CDC, some of these diseases include hypertension, type 2 diabetes, coronary heart disease, stroke, sleep apnea and respiratory problems.