Obesity (from the Latin word obesus i.e. fat) is, according to Dorland's medical dictionary, defined as an increase in body weight beyond the limitation of skeletal and physical requirement, as the result of excessive accumulation of the fat in the body.

Accurate measurement of body fat is difficult and very costly and many methods are only suitable for experimental conditions. Therefore, a simple calculation of the so-called Body Mass Index (BMI) is used in common practice. This index gives the weight / height relationship according to the simple formula Weight (kg) / Height (m)² . A number of studies have confirmed that the BMI index in children is a fairly accurate indirect indicator of the amount of body fat. BMI correlates well with other fat measurement methods such as skinfold measurements, bioelectric impedance or X-ray absorptiometry (DXA).

The American Academy of Pediatrics (AAP) recommends the use of BMI as a screening method for being overweight or obese for children from 2 years of age.

BMI values that define being overweight and obese are expressed in percentiles and their values vary depending on what data sets were calculated. There are a number of national as well as international (eg WHO) growth charts that have different limits for being overweight or obese.

Our BMI calculator uses publicly available CDC data - i.e. from a major scientific organization that defines overweight at or above the 85th percentile, and obesity at or above the 95th percentile.