BMI
While the Body Mass Index is inaccurate for certain individuals (such as elite athletes), it will give the average person a basic idea of where they are at with their body composition. There are numerous calculators, both web-based and standalone, available online.
“Body Mass Index (BMI) is an index used to estimate ones body fat. BMI indicates your health risks by comparing your height and weight. Males with a BMI above 27.8 and women over 27.3 are considered overweight. Males with a BMI less than 20.7 and women below 19.1 are classified as underweight.”
See also BMR.