BMI Calculator

Body Mass Index is a measurement of a person's leanness or corpulence based on their height and weight, intended to quantify tissue mass at a population level. It is not a body-fat percentage. Instead, it is a proxy — a single number that rises when weight increases relative to height, and falls when weight decreases relative to height. The WHO and most national health agencies use it as a first-pass screening indicator for whether a person's body weight sits in a range associated with elevated disease risk.

BMI was proposed by Belgian statistician Adolphe Quetelet in the 1830s (originally called the Quetelet Index) and re-popularised by Ancel Keys in 1972 under the modern name. Keys' large multi-country study concluded that the weight-over-height-squared formula was the best simple metric available for grouping adults into risk bands. Every widely used classification since — WHO 1998, CDC 2000, NIH NHLBI — rests on Keys' framing.

Because BMI is a ratio, the answer does not change when you switch between metric and imperial units, provided the conversions are done correctly. Metric users divide weight in kilograms by the square of height in metres. Imperial users multiply weight in pounds by 703 and divide by the square of height in inches. Both routes produce the same number, and both are supported by the calculator above.

The following classification is the WHO adult standard, drawn from Technical Report Series 894 (2000). It applies to men and women aged 20 or older, and is calibrated against European reference populations. Health authorities in some regions use adjusted thresholds; see the "Ethnicity-adjusted thresholds" section below.

The adult classification does not apply to children or teenagers. In childhood, healthy weight-for-height changes rapidly with age and sex, and both the numerator (weight) and the denominator (height²) shift in ways that break the adult thresholds. Instead, the CDC recommends interpreting a child's BMI as a percentile against same-age, same-sex peers, using the CDC 2000 growth charts.

A percentile answers a different question than a raw BMI value. A BMI of 20 in a 10-year-old is in a completely different band than a BMI of 20 in a 40-year-old. If you are calculating BMI for someone under 20, use our BMI Calculator for Children — the top calculator on this page will show a notice pointing you there.

The WHO's standard cutoffs (18.5, 25, 30) were derived largely from European populations. A 2004 WHO Expert Consultation reviewed the evidence for Asian populations and concluded that Asian adults, on average, develop cardiometabolic disease at lower BMI values than the standard thresholds would suggest. The consultation report did not replace the standard cutoffs, but it recommended additional public-health "action points" for Asian populations at BMI 23 (increased risk) and 27.5 (high risk).

Several countries have since adopted these lower thresholds in national screening guidelines, including India, Singapore, Japan, and China. If you are of South Asian, East Asian, or Pacific Islander background, a BMI reading in the standard "normal" band above 23 may still warrant a conversation with a clinician about metabolic risk factors such as waist circumference, fasting glucose, and blood pressure.

Sustained BMI in the overweight or obese range is associated with elevated risk for a number of serious health conditions. The CDC and NIH catalogue these risks based on longitudinal studies covering millions of person-years. Being overweight does not cause any of these conditions on its own — it is a marker of elevated risk, not a diagnosis. But the epidemiological association is strong enough that most national health bodies treat a BMI above 25 as a signal to look more closely at the underlying metabolic picture.

Underweight status carries its own risks, and low BMI is not benign. Health problems associated with sustained BMI below 18.5 include:

The BMI formula is deliberately simple. The SI (metric) version divides mass in kilograms by height in metres squared. The USC (imperial) version multiplies mass in pounds by 703 and divides by height in inches squared. The constant 703 is a unit-conversion factor derived from (2.20462 lb/kg) ÷ (0.0254 m/in)² and reconciles the two systems to within 0.01 BMI units at typical adult heights.

BMI Prime is the ratio of a person's measured BMI to the upper limit of what the WHO considers "normal" — 25 kg/m². It was proposed by physician-researcher Ma-Li Wong as a way to express a BMI reading in dimensionless form, so that comparisons can be made across populations that use different upper cutoffs (the WHO's standard 25 vs. the WHO Expert Consultation's Asian action-point of 23).

A BMI Prime of 1.00 is exactly at the WHO overweight threshold. Values above 1.00 mean the person is above the normal band by that fractional amount — a BMI Prime of 1.20, for example, corresponds to a BMI of 30 and represents the WHO obesity threshold. Values below 1.00 mean the person is inside the normal band or below it.

The Ponderal Index (PI), also called the Rohrer Index after Fritz Rohrer who proposed it in 1921, measures leanness or corpulence using height cubed rather than squared. In theory, cubing height should give a better weight-neutral comparison, because human body mass scales more closely with the cube of height than the square across the full range of adult heights.

In practice, BMI is more widely used because it holds up well across the middle of the adult height distribution and has decades of large-cohort research behind it. Ponderal Index becomes more reliable than BMI at the extremes — for very tall or very short adults — where BMI systematically under- or over-estimates body-fat percentage. A healthy Ponderal Index for adults is roughly 11 to 14 kg/m³.

BMI is an imperfect measure. It was designed to work at a population level, and it does not reliably distinguish an athlete's dense muscle from an inactive person's excess fat. A muscular rugby player might read as "obese" on BMI while carrying less body fat than a sedentary office worker who reads as "normal". The higher a person's muscle mass, the more BMI overestimates their body-fat percentage.

The reverse is also true. Older adults typically lose muscle mass and gain fat with age, even at a stable body weight. Two people with an identical BMI of 24 — one aged 25, one aged 75 — may have very different body-fat percentages. The CDC notes that older adults tend to carry more body fat than younger adults at the same BMI, and that women tend to carry more body fat than men at the same BMI.

BMI also ignores fat distribution. Visceral (abdominal) fat is more strongly associated with cardiovascular and metabolic risk than fat carried on hips and thighs. Two people with the same BMI can face very different risk profiles depending on where that weight sits. Waist-hip ratio, waist circumference, and body-fat percentage give complementary information that BMI cannot.

Finally, BMI is not validated for pregnant or breastfeeding women, for adults with significant edema or limb amputation, for competitive athletes, or for populations at the extremes of the height distribution. In those cases, other measures (skinfold thickness, DEXA scan, hydrostatic weighing) are preferred where available.

BMI is enough when you want a quick, no-cost, no-equipment first-pass check that your weight is in a reasonable range for your height. It is what most general practitioners use as a screening step at annual physicals, and it correlates well with body fat for roughly 90–95% of the general adult population.

BMI is not enough when precision matters — for tracking body-recomposition during training, monitoring health during a medical treatment, or ruling in or out a clinical concern. A waist-hip ratio, a body-fat percentage estimate, and (when accessible) a lipid panel and fasting glucose reading give a far more complete picture. Treat your BMI as the first sentence of a longer conversation with a clinician, not the whole story.