What is BMI?
The body mass index (BMI) is a measure that uses your height and weight to work out if your weight is healthy. For example, BMI = 25 means 25 kg per square meter (m2).

How is BMI used?
BMI is not accurate enough to be used as a diagnostic tool. However, it is used as a screening tool to identify potential weight problems in adults. Overweight and obese adults are at increased risk for many diseases.

BMI Table for Adults

Category BMI range - kg/m2
Severe Thinness < 16
Moderate Thinness 16 - 17
Mild Thinness 17 - 18.5
Normal 18.5 - 25
Overweight 25 - 30
Obese Class I 30 - 35
Obese Class II 35 - 40
Obese Class III > 40

BMI for Men vs BMI for Women
There is no such thing as a BMI calculator for men, but men and women have different weight distributions. Women accumulate weight in the hips and buttocks, whereas Men tend to accumulate mass at the waist. Women carry more body fat than men.

Is BMI interpreted the same way for children and teens as it is for adults?
BMI is interpreted differently for children and teens, even though it is calculated using the same formula as adult BMI. Children and teen's BMI need to be age and sex-specific because the amount of body fat changes with age and the amount of body fat differs between girls and boys.

Muscle vs Fat
Muscle is much denser than fat, so very muscular people, such as heavyweight boxers, weight trainers and athletes, may be a healthy weight even though their BMI is classed as obese.

Muscle is about 18% denser than fat. This means that if you heroically exercised so much that you converted 10% of your body volume from fat to muscle (wow!), your BMI reading would go up just 1.8%.

How good is BMI as an indicator of body fatness?
The correlation between the BMI and body fatness is fairly strong, but even if 2 people have the same BMI, their level of body fatness may differ.

Quick Facts:

  • Women tend to have more body fat than men
  • Blacks have less body fat than do Whites, and Asians have more body fat than do Whites
  • Older people, on average, tend to have more body fat than younger adults
  • Athletes have less body fat than do non-athletes

BMI is not Applicable For:

  • Children and teenagers who have not reached physical maturity and are still growing
  • Lactating or pregnant women
Body Mass Index Formula
The official formula for BMI was was devised in the 1830s by Belgian mathematician Lambert Adolphe Jacques Quetelet (1796-1874).

Metric BMI = ( weight (kg) / ( height (m)2 ) )
Imperial BMI = ( weight (lb) / ( height (in)2) ) x 703

Nick Trefethen's unofficial BMI formula:

Metric BMI = (1.3 x weight (kg) ) / (height (m)2.5)
Imperial BMI = (5734 x weight (lb) ) / (height (in)2.5)

In its 5 January 2013 issue the Economist published this letter:

SIR - The body-mass index that you (and the National Health Service) count on to assess obesity is a bizarre measure. We live in a three-dimensional world, yet the BMI is defined as weight divided by height squared. It was invented in the 1840s, before calculators, when a formula had to be very simple to be usable. As a consequence of this ill-founded definition, millions of short people think they are thinner than they are, and millions of tall people think they are fatter.

Nick Trefethen
Professor of numerical analysis
University of Oxford