Tools - BMI

Tools - BMI

Tools - BMI - Body Mass Index

The body mass index (BMI), or Quetelet index, is a heuristic measure of body weight based on a person''s weight and height. It is important to realise that it does not actually measure the percentage of body fat, it is used to estimate a healthy body weight based on a person''s height, assuming an average/typical body composition. Consequently, for certain people, such as top bodybuilders, it will not be entirely accurate. Due to its ease of measurement and its calculation, it has become one of the worlds most widely used diagnostic tool to identify weight problems within a population, usually whether individuals are underweight, overweight or obese. It was invented between 1830 and 1850 by the Belgian polymath Adolphe Quetelet during the course of developing "social physics. Body mass index is defined as the individual''s body weight divided by the square of his or her height. The formulae universally used in medicine produce a unit of measure of kg/m2. BMI can also be determined using a BMI chart which displays BMI as a function of weight (horizontal axis) and height (vertical axis) using contour lines for different values of BMI or colours for different BMI categories.

BMI Prime, a straightforward modification of the BMI system, is the ratio of actual BMI to upper limit BMI (currently defined at BMI 25). This means that a BMI prime of 1 is the border between normal and underweight, with a value greater than one indicating an overweight individual. As defined, BMI Prime is also the ratio of body weight to upper body weight limit, calculated at BMI 25. Since it is the ratio of two separate BMI values, BMI Prime is a dimensionless number, without associated units. Individuals with BMI Prime < 0.74 are underweight; those between 0.74 and 0.99 have optimal weight; and those at 1.00 or greater are overweight. BMI Prime is useful clinically because individuals can tell, at a glance, by what percentage they deviate from their upper weight limits. For instance, a person with BMI 34 has a BMI Prime of 34/25 = 1.36, and is 36% over his or her upper mass limit.


BMI range - kg/m2

BMI Prime

Mass (weight) of a 1.8 metres (5 ft 11 in) person with this BMI

Severely underweight

less than 16.5

less than 0.66

less than 53.5 kilograms (8.42 st; 118 lb)


from 16.5 to 18.4

from 0.66 to 0.73

between 53.5 and 59.9 kilograms (8.42 and 9.43 st; 118 and 132 lb)


from 18.5 to 24.9

from 0.74 to 0.99

between 60 and 80.9 kilograms (9.4 and 12.74 st; 130 and 178 lb)


from 25 to 29.9

from 1.0 to 1.19

between 81 and 96.9 kilograms (12.8 and 15.26 st; 180 and 214 lb)

Obese Class I

from 30 to 34.9

from 1.2 to 1.39

between 97 and 112.9 kilograms (15.3 and 17.78 st; 210 and 249 lb)

Obese Class II

from 35 to 39.9

from 1.4 to 1.59

between 113 and 129.9 kilograms (17.8 and 20.46 st; 250 and 286 lb)

Obese Class III

over 40

over 1.6

from 130 kilograms (20 st; 290 lb)

Calculate your BMI here with our online tool:

Sex (Male or Female) Male
Height Inches
Weight Pounds

Population Average