As of 2015, WebMD, fitnessmagazine.com and the website of the National Heart, Lung, and Blood Institute all offer BMI calculators. BMI, or body mass index, is used to determine the amount of fat on a person's body.
Adults 20 years and older all use the same BMI chart. Anyone under the age of 20 uses charts based on age and gender. It is important to use the correct chart in order to get an accurate reading.
Fitness and medical specialists use BMI numbers to help determine a person's possible future health. A high BMI indicates a high amount of body fat, which may lead to heart problems and other health conditions.