According to WebMd, the Body Mass Index (BMI) calculator is the basis for the recommended height and weight scale. The BMI is used to calculate all weight guidelines for people over the age of 20. The CDC provides a separate BMI calculator for people younger than 19.
Measuring BMI involves dividing weight by height; this number is then divided by 703. Generally, a BMI between 18.5 and 24.9 is considered healthy. A person with a BMI under 18.5 is considered under weight. Overweight people have a BMI of 25 to 29.9. A person with a BMI greater than 30 is considered obese.
Although it is the preferred measure for determining weight guidelines, the BMI calculator does not accurately reflect the health of all people. According to WebMD, an athlete will sometimes have a higher BMI due to their larger muscle mass.