What BMI Can and Can't Tell You

Body mass index turns height and weight into one number with a label attached, and that label shows up everywhere from doctors’ offices to insurance forms. BMI was designed to describe populations, and it works well for that. Applied to one person, it’s a rough screen: it can’t tell muscle from fat, it ignores where fat sits, and the same number means different things depending on age, sex and ethnicity. Below: the formula and its history, the official categories, a worked example in both unit systems, and the measures guidelines now pair with it.

This is general information, not medical advice. BMI is a starting point for a conversation with a clinician, not a diagnosis.

A 19th-century statistic with a 1972 name

Adolphe Quetelet, a Belgian mathematician and statistician, studied human growth in the 1830s and concluded that, outside the growth spurts of infancy and puberty, “the weight increases as the square of the height”. A history of the index by Garabed Eknoyan dates the ratio to 1832. It was known as the Quetelet Index until 1972, when the physiologist Ancel Keys and colleagues renamed it body mass index in “Indices of relative weight and obesity”.

Quetelet was describing the “average man” across a population, and Keys was choosing an index for epidemiological studies. Neither was designing a diagnostic test for one patient.

The formula, in both unit systems

The CDC gives both versions:

Metric:  BMI = weight (kg) ÷ height (m)²
US:      BMI = 703 × weight (lb) ÷ height (in)²

The 703 is a unit conversion. One pound is exactly 0.45359237 kg and one inch is exactly 0.0254 m, so the exact factor is 0.45359237 ÷ 0.0254² = 703.0696. Rounding to 703 changes the result by about 0.01%.

Worked example: 5 ft 10 in, 175 lb

US formula:

703 × 175 = 123,025
70 in² = 4,900
123,025 ÷ 4,900 = 25.107…  →  25.1

Same person in metric. 70 in × 2.54 = 177.8 cm, and 175 lb × 0.45359237 = 79.379 kg:

1.778² = 3.161284
79.379 ÷ 3.161284 = 25.110…  →  25.1

The third-decimal difference comes from the rounded 703. Either way it’s 25.1, just past the healthy-weight boundary.

Run the formula backwards and at 177.8 cm, one BMI unit is 3.16 kg, or about 7.0 lb. The adult healthy range of 18.5 to 24.9 works out to 58.5–78.7 kg, or 128.9–173.5 lb. This person is about 1.5 lb above the top of it. The cut-points are hard lines, so a change of a pound or two can flip the label without any meaningful change in health.

The adult categories

For adults 20 and older, the CDC uses these categories “regardless of age, sex, or race”:

BMI (kg/m²)	Category
Below 18.5	Underweight
18.5 to less than 25	Healthy weight
25 to less than 30	Overweight
30 to less than 35	Obesity, class 1
35 to less than 40	Obesity, class 2
40 or greater	Obesity, class 3 (severe)

The WHO uses the same thresholds for adults: overweight at a BMI of 25 or more and obesity at 30 or more. Bands written as 18.5–24.9 and 25–29.9 are the same thing rounded to one decimal.

Why it’s a screening tool, not a diagnosis

The CDC is explicit: “BMI is a screening measure,” and it “should be considered with other factors when assessing an individual’s health.” It also says: “For most people, BMI is a good indicator of whether they have too much or too little body fat. However, BMI is not a direct measure of body fat.”

That second sentence is the catch. A screening test flags people for a closer look, accepting some false alarms and misses in exchange for being cheap and easy to run on everyone. It isn’t the final word on one person’s health.

What BMI misses

Muscle versus fat

Weight is weight to the formula. Muscle, bone, water and fat all count the same, and the CDC notes that BMI “cannot distinguish fat mass from lean body mass (muscle and bone).” A muscular athlete can land in the overweight or even obesity range while carrying little fat. Going the other way, someone with little muscle can read “healthy” while carrying more fat than the number suggests.

Where the fat is

BMI also says nothing about where fat sits. NICE links central adiposity, meaning fat around the middle, to type 2 diabetes, hypertension and cardiovascular disease, and notes that BMI “is not a direct measure of central adiposity.”

Sex and age

In the HERITAGE Family Study, 665 adults aged 17 to 65 had body fat measured by underwater weighing: “for the same BMI, the %fat of females was 10.4% higher than that of males,” and age shifted the relationship too. The UK’s NICE guideline tells clinicians to interpret BMI “with caution in people aged 65 and over,” and also in adults with high muscle mass.

Ethnicity

In 2004 a WHO expert consultation published in The Lancet concluded that “the proportion of Asian people with a high risk of type 2 diabetes and cardiovascular disease is substantial at BMIs lower than the existing WHO cut-off point for overweight.” Because the risk cut-off varied from 22 to 25 between Asian populations, the panel kept the standard WHO thresholds as the international classification and identified extra “public health action points” at 23.0, 27.5, 32.5 and 37.5 kg/m².

NICE applies the lower thresholds to a wider group. For people of South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean background, it uses overweight at 23 to 27.4 and obesity at 27.5 or above, with obesity classes 2 and 3 lowered by 2.5.

Scale	Healthy	Overweight / increased risk	Obesity / high risk
Standard WHO/CDC	18.5–24.9	25–29.9	30+
WHO 2004 action points / NICE lower thresholds	18.5–22.9	23–27.4	27.5+

Our 5 ft 10 in, 175 lb example reads “overweight” on the standard scale at 25.1, but it sits well inside the 23–27.4 band on the lower scale. On that scale the healthy range for the same height tops out at about 159.6 lb.

Children and teens: percentiles, not cut-points

None of the adult numbers apply to anyone under 20. Body fat changes as children grow and differs between boys and girls, so the CDC classifies ages 2 to 19 by BMI-for-age percentile on sex-specific growth charts. In the CDC’s words, using percentiles “is important because they are still growing.” The CDC’s child and teen categories are:

Underweight: less than the 5th percentile
Healthy weight: 5th to less than the 85th percentile
Overweight: 85th to less than the 95th percentile
Obesity: 95th percentile or greater
Severe obesity: 120% of the 95th percentile or greater, or 35 kg/m² or greater

The arithmetic is the same, but the interpretation isn’t: the same BMI can fall at a different percentile for a child of a different age or sex.

What to measure alongside it

Waist circumference

The NHLBI says a waist circumference of “more than 35 inches for women or more than 40 inches for men” increases the risk of heart disease and type 2 diabetes.

Waist-to-height ratio

NICE goes a step further. For adults with a BMI below 35, it recommends measuring waist-to-height ratio as well as BMI, and it classifies the result as follows:

Waist ÷ height	NICE classification
0.4 to 0.49	Healthy central adiposity
0.5 to 0.59	Increased central adiposity, increased health risks
0.6 or more	High central adiposity, further increased risks

NICE says these classifications work for both sexes and all ethnicities, “including adults with high muscle mass,” and puts the message simply: keep your waist to less than half your height. For our 70-inch example, that means a waist under 35 inches (88.9 cm). NICE says to measure midway between the bottom of the ribs and the top of the hips, after breathing out normally.

The 2025 Lancet Commission

The biggest recent push came from the Lancet Diabetes & Endocrinology Commission on clinical obesity (58 experts, endorsed by 76 organizations). It says BMI-based measures “can both underestimate and overestimate adiposity,” and recommends that BMI be used “only as a surrogate measure of health risk at a population level, for epidemiological studies, or for screening purposes, rather than as an individual measure of health.”

The Commission recommends confirming excess fat with a direct body-fat measurement where available, or with at least one other body measurement (waist circumference, waist-to-hip ratio or waist-to-height ratio) in addition to BMI. The one exception is a BMI above 40, where it says excess fat can be assumed. It then distinguishes “clinical obesity”, where excess fat affects organ function or daily activities, from “preclinical obesity”, where it doesn’t yet. BMI alone can’t separate the two.

How our calculator handles all this

The BMI calculator takes metric (cm, kg) or imperial (ft/in, lb) input. It converts imperial with the exact pound and inch factors and assigns the category from the one-decimal value it displays, so a shown 25.0 is always labeled overweight. It also shows your healthy weight range, the distance to it, and BMI prime (BMI ÷ 25). For the example above it shows 25.1, Overweight, a range of 128.9–173.5 lb and a BMI prime of 1.00.

An optional toggle applies the WHO Asian cut-offs (healthy below 23, high risk from 27.5; it doesn’t break out 32.5 and 37.5). The child/teen mode for ages 2 to 19 shows the raw BMI with no category. It doesn’t ask for age or sex, so it can’t compute a percentile, and instead points you to the CDC growth chart or a pediatrician. Adult mode is the default, so switch it for anyone under 20. The tool doesn’t handle waist size or body fat.

For daily energy needs, the calorie calculator uses the Mifflin-St Jeor equation, with an optional body-fat input for Katch-McArdle. It warns that the equations were built for adults if you enter an age under 18.

Try it

Open the BMI calculator, enter your height and weight, then divide your waist by your height. Two numbers tell you more than one, and neither replaces a clinician. Everything runs in your browser; nothing you type is sent anywhere.