BMI Calculator

Background.

This BMI calculator computes your body mass index using the standard formula adopted by the World Health Organization and the U.S. Centers for Disease Control and Prevention, then maps the result to the four canonical adult categories — underweight, normal weight, overweight, and obese — so you can see at a glance where your number falls relative to population-level thresholds. Enter your weight and height in either metric (kilograms and centimetres) or imperial (pounds and inches) units and the calculator updates instantly; there is no submit button, no account requirement, and no data is transmitted off your device. The output is a single decimal number expressed in kilograms per square metre, accompanied by a band label drawn from the WHO classification that has been in continuous public-health use since the late 1990s.

Body mass index, as a concept, has a longer history than most people realise. The Belgian polymath Adolphe Quetelet introduced what he called the Quetelet index in the mid-nineteenth century while developing statistical descriptions of the 'average man'. Quetelet was a statistician, not a physician, and his index was intended to characterise populations rather than diagnose individuals. The label 'body mass index' itself was coined by the American physiologist Ancel Keys in a 1972 Journal of Chronic Diseases paper that compared several height-weight ratios against skinfold measurements of body fat and concluded that weight divided by height squared correlated best across the samples studied. From there the index migrated into actuarial tables, then into NIH guidelines in 1998, and finally into the WHO global classification that this calculator mirrors.

Understanding that lineage matters because BMI was never designed as a diagnostic test. It is a screening tool — a cheap, fast, reproducible number that lets clinicians, researchers, insurers, and public-health agencies compare body sizes at scale without measuring body composition directly. Its strength is precisely that it requires only a tape measure and a scale; its weakness is the same: it cannot distinguish between muscle, fat, bone, organ tissue, and water. A 95-kilogram rugby prop forward and a 95-kilogram sedentary office worker of the same height will return identical BMI numbers despite having radically different body compositions, fat distributions, and cardiometabolic risk profiles. The calculator on this page reports the number honestly and applies the WHO thresholds exactly — 18.5, 25, and 30 — but it does not pretend the number tells you everything about your health.

The math is straightforward. In metric units BMI equals weight in kilograms divided by height in metres squared. A person who weighs 70 kg and stands 1.75 metres tall has a BMI of 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 ≈ 22.86 kg/m², which lands in the normal range. In imperial units the same arithmetic gives a different-looking but mathematically equivalent expression: BMI equals 703 times weight in pounds divided by height in inches squared, where the 703 is simply the conversion factor that lets you skip converting pounds to kilograms and inches to metres by hand. Our calculator does the conversion internally using the exact definitions — 0.45359237 kilograms per pound and 0.0254 metres per inch — so metric and imperial entries for the same person return BMI values that agree to two decimal places.

The WHO categories applied to that number are stable across most adult populations and are the ones you will see on charts in clinics, gyms, and insurance forms worldwide. Underweight is anything below 18.5 kg/m². Normal weight runs from 18.5 up to but not including 25. Overweight covers 25 up to but not including 30. Obese begins at 30 and is sometimes subdivided into Class I (30 to <35), Class II (35 to <40), and Class III, also called severe or 'morbid' obesity, at 40 and above. The CDC presents the same thresholds for adults aged 20 and over and notes explicitly that BMI is one potential indicator that should be considered together with other measures when assessing an individual's health.

We agree, and we want to be direct about the limits before you read your result. First, BMI ignores body composition entirely. Athletes with substantial lean muscle mass — bodybuilders, sprinters, rowers, rugby players, many martial artists — routinely score in the overweight or obese ranges despite having low body-fat percentages, because muscle is denser than fat. Second, BMI does not capture fat distribution. Two people with identical BMIs of 28 can carry their excess weight in very different patterns; visceral fat around the abdomen is associated with substantially higher cardiometabolic risk than peripheral fat on the hips and thighs, and BMI alone cannot tell them apart. This is why many clinical guidelines now pair BMI with a waist circumference measurement or waist-to-height ratio.

Third, the WHO cut-points were derived primarily from European-ancestry populations and do not generalise cleanly to all ethnic groups. The WHO Expert Consultation in 2004 reviewed Asian-specific data and recommended supplementary 'public health action points' at BMI 23 and 27.5 for many Asian populations because cardiovascular and diabetes risk rises at lower BMI in those groups than in Europeans. South Asian, East Asian, and Pacific Islander populations all show this pattern to varying degrees. Some health systems in Singapore, Hong Kong, and India have formally adopted lower overweight and obesity thresholds; ours uses the WHO global cut-points by default.

Fourth, BMI behaves unreliably at the extremes of stature. Very short adults tend to be classified as heavier than they 'should' be on appearance, and very tall adults often appear lighter, because the height-squared denominator does not scale perfectly with body geometry. Adolphe Quetelet himself noted this when proposing the index.

Fifth, BMI does not apply to children and adolescents under 20 the way it applies to adults — children require age- and sex-specific percentile charts because body composition changes substantially through growth. Sixth, BMI loses interpretive power in pregnancy, in older adults (where sarcopenia masks fat gain), in people with substantial oedema, in people with limb amputations, and in people whose spinal curvature makes standing height unreliable.

None of these caveats mean the number is useless — they mean the number is one data point, best read alongside waist circumference, blood pressure, lipid panel, fasting glucose or HbA1c, family history, and an honest assessment of fitness and diet. Used that way, BMI is genuinely informative. The Prospective Studies Collaboration meta-analysis published in the Lancet in 2009, pooling 894,576 adults across 57 studies, found that mortality was lowest in the 22.5 to 25 BMI range and rose in roughly J-shaped fashion both above and below that band, with each 5-unit increase above 25 associated with roughly a 30% increase in all-cause mortality across the studied populations. Subsequent and ongoing work has refined that picture and emphasised that the 'optimal' BMI varies by age, ethnicity, and the specific outcome being measured, but the basic shape of the relationship is robust.

So treat the number this calculator gives you as a starting point. If it lands in the normal range, that is reassuring but not a clean bill of health — body composition, fitness, and metabolic markers still matter. If it lands in the underweight, overweight, or obese ranges, that is information worth bringing to a conversation with a clinician who can place it in the context of your medical history, family history, lifestyle, and lab results. The calculator below is fast, exact, and uses the same WHO and CDC thresholds your doctor uses. What you do with the result is the part that actually changes outcomes.

What is bmi calculator?

Body mass index (BMI) is a population-level screening metric defined as a person's weight in kilograms divided by the square of their height in metres, expressed in units of kg/m². The metric was developed in the nineteenth century by the Belgian statistician Adolphe Quetelet as part of his work on the 'average man' and was renamed body mass index in a 1972 paper by physiologist Ancel Keys, who showed that it correlated more closely with directly measured body fat than other height-weight ratios then in use. The World Health Organization formalised the now-standard adult thresholds — underweight below 18.5, normal weight 18.5 to under 25, overweight 25 to under 30, and obese at 30 and above — in its 1995 and 2000 expert reports, and the CDC applies the identical cut-points to adults aged 20 and over. BMI is intentionally simple to compute and reproduce: it requires only a scale and a tape measure, which is why it became the default body-size descriptor in epidemiology, insurance underwriting, and primary care. It is important to understand what BMI is not. It is not a measurement of body fat. It cannot distinguish muscle from fat, and it cannot tell you where on your body fat is stored. It was not designed for children, where age- and sex-adjusted percentile charts are used instead. It generalises imperfectly across ethnic groups; the WHO has acknowledged that several Asian populations show elevated cardiometabolic risk at BMI values below the standard overweight threshold and has published supplementary 23 and 27.5 action points for those populations. It also behaves less reliably at the extremes of height, in older adults with sarcopenia, in pregnant women, in highly trained athletes, and in people with conditions that affect body water or limb length. In modern clinical practice BMI is typically read alongside waist circumference, blood pressure, fasting glucose or HbA1c, lipid panel, and a family history to build a richer picture of cardiometabolic risk than the index can provide alone.

How to use this calculator.

1. 01Choose your unit system from the Units dropdown — metric (kilograms and centimetres) or imperial (pounds and inches). The calculator automatically reads only the matching weight and height fields.
2. 02Enter your weight. In metric mode enter kilograms (for example 70). In imperial mode enter pounds (for example 154). Decimals are accepted.
3. 03Enter your height. In metric mode enter centimetres (for example 175). In imperial mode enter total inches — a person who is 5'9" is 69 inches.
4. 04Read the BMI number in the results panel. It is rounded to two decimal places and expressed in kg/m².
5. 05Read the category label beneath the number — Underweight, Normal, Overweight, or Obese — based on the WHO and CDC adult thresholds of 18.5, 25, and 30.
6. 06Interpret the result in context. If you are an athlete with high muscle mass, if you are of Asian ancestry, if you are over 65, or if you are pregnant, the standard thresholds may not apply cleanly to you. Use the number as a starting point for a conversation with a clinician rather than a verdict on your health.

The formula.

The BMI formula in metric units is BMI = weight (kg) ÷ height (m)². Height must be converted from centimetres to metres before squaring — divide by 100. In imperial units the equivalent expression is BMI = 703 × weight (lb) ÷ height (in)², where 703 is the conversion constant that bundles the pound-to-kilogram and inch-to-metre conversions into one factor. This calculator does not use the 703 shortcut; instead it converts imperial inputs to metric using the exact conversions defined by the international yard and pound agreement of 1959 — 1 pound = 0.45359237 kilograms and 1 inch = 0.0254 metres — and then applies the metric formula directly. That means metric and imperial entries for the same person return BMI values that agree to two decimal places without rounding drift. The category assignment is a simple band lookup: BMI below 18.5 is Underweight (code 0), 18.5 up to but not including 25 is Normal (code 1), 25 up to but not including 30 is Overweight (code 2), and 30 or above is Obese (code 3). These cut-points are the WHO global adult thresholds, identical to those used by the CDC for adults aged 20 and over. The output BMI is rounded to two decimal places before the category lookup runs, which means values right at the boundary (for example 24.995 rounding to 25.00) are placed in the higher band — this matches the convention used by most published clinical calculators.

A worked example.

Frequently asked questions.

References& sources.

1. [1]World Health Organization — Obesity and overweight fact sheet, which formalises the global adult BMI cut-points of 18.5, 25, and 30 kg/m².
2. [2]Centers for Disease Control and Prevention — Adult BMI Categories, listing the underweight, healthy weight, overweight, and obesity (Class 1, 2, 3) ranges used by the CDC for adults aged 20 and over.
3. [3]National Heart, Lung, and Blood Institute (NIH) — Clinical Guidelines on the Identification, Evaluation, and Treatment of Overweight and Obesity in Adults (1998), the U.S. clinical foundation for current BMI thresholds.
4. [4]WHO Expert Consultation — Appropriate body-mass index for Asian populations and its implications for policy and intervention strategies. Lancet 363:157–163 (2004). Establishes supplementary public-health action points at BMI 23 and 27.5 for relevant Asian populations.
5. [5]Prospective Studies Collaboration — Body-mass index and cause-specific mortality in 900,000 adults: collaborative analyses of 57 prospective studies. Lancet 373:1083–1096 (2009). The benchmark meta-analysis on BMI and mortality risk across the full adult range.
6. [6]Keys A, Fidanza F, Karvonen MJ, Kimura N, Taylor HL — Indices of relative weight and obesity. Journal of Chronic Diseases 25(6):329–343 (1972). The paper that introduced the term 'body mass index' and validated weight/height² against skinfold-derived body-fat estimates.
7. [7]Nuttall FQ — Body Mass Index: Obesity, BMI, and Health: A Critical Review. Nutrition Today 50(3):117–128 (2015). Peer-reviewed review of the well-known limitations of BMI as an individual-level health measure, including its insensitivity to body composition and fat distribution.