The BMI is a convenient rule of thumb used to broadly categorize a person as based on tissue mass (muscle, fat, and bone) and height.

[2] Adolphe Quetelet, a Belgian astronomer, mathematician, statistician, and sociologist, devised the basis of the BMI between 1830 and 1850 as he developed what he called "social physics".

[4] According to Lars Grue and Arvid Heiberg in the Scandinavian Journal of Disability Research, Quetelet's idealization of the average man would be elaborated upon by Francis Galton a decade later in the development of Eugenics.

[6][7][8] The interest in an index that measures body fat came with observed increasing obesity in prosperous Western societies.

BMI provides a simple numeric measure of a person's thickness or thinness, allowing health professionals to discuss weight problems more objectively with their patients.

[10] A common use of the BMI is to assess how far an individual's body weight departs from what is normal for a person's height.

The weight excess or deficiency may, in part, be accounted for by body fat (adipose tissue) although other factors such as muscularity also affect BMI significantly (see discussion below and overweight).

Instead of comparison against fixed thresholds for underweight and overweight, the BMI is compared against the percentiles for children of the same sex and age.

[23] In the UK, NICE guidance recommends prevention of type 2 diabetes should start at a BMI of 30 in White and 27.5 in Black African, African-Caribbean, South Asian, and Chinese populations.

The duality of the BMI is that, while it is easy to use as a general calculation, it is limited as to how accurate and pertinent the data obtained from it can be.

Generally, the index is suitable for recognizing trends within sedentary or overweight individuals because there is a smaller margin of error.

This general correlation is particularly useful for consensus data regarding obesity or various other conditions because it can be used to build a semi-accurate representation from which a solution can be stipulated, or the RDA for a group can be calculated.

Smaller effects are seen in prospective cohort studies which lend to support active mobility as a means to prevent a further increase in BMI.

[39] In France, Italy, and Spain, legislation has been introduced banning the usage of fashion show models having a BMI below 18.

[46] One study found that BMI had a good general correlation with body fat percentage, and noted that obesity has overtaken smoking as the world's number one cause of death.

[47] A 2010 study that followed 11,000 subjects for up to eight years concluded that BMI is not the most appropriate measure for the risk of heart attack, stroke or death.

Part of the statistical limitations of the BMI scale is the result of Quetelet's original sampling methods.

[52] As noted in his primary work, A Treatise on Man and the Development of His Faculties, the data from which Quetelet derived his formula was taken mostly from Scottish Highland soldiers and French Gendarmerie.

As noted by sociologist Sabrina Strings, the BMI is largely inaccurate for black people especially, disproportionately labelling them as overweight even for healthy individuals.

[52][verification needed] A 2012 study of BMI in an ethnically diverse population showed that "adult overweight and obesity were associated with an increased risk of mortality ... across the five racial/ethnic groups".

Since mass increases to the third power of linear dimensions, taller individuals with exactly the same body shape and relative composition have a larger BMI.

This results in taller people having a reported BMI that is uncharacteristically high, compared to their actual body fat levels.

Conversely, large framed (or tall) individuals may be quite healthy, with a fairly low body fat percentage, but be classified as overweight by BMI.

In the reverse, the man with a larger frame and more solid build should increase by 10%, to roughly 75 kg or 165 lb (BMI 23.7).

However, falling into one's ideal weight range for height and build is still not as accurate in determining health risk factors as waist-to-height ratio and actual body fat percentage.

One study found that the vast majority of people labelled 'overweight' and 'obese' according to current definitions do not in fact face any meaningful increased risk for early death.

In a quantitative analysis of several studies, involving more than 600,000 men and women, the lowest mortality rates were found for people with BMIs between 23 and 29; most of the 25–30 range considered 'overweight' was not associated with higher risk.

The corpulence index yields valid results even for very short and very tall people,[65] which is a problem with BMI.

A 2013 study identified critical threshold values for waist-to-height ratio according to age, with consequent significant reduction in life expectancy if exceeded.

Within some medical contexts, such as familial amyloid polyneuropathy, serum albumin is factored in to produce a modified body mass index (mBMI).