MathBench

BMI Calculator

Work out your body mass index, see the healthy weight range for your height, and check it against waist-to-height ratio — the companion measure that captures the fat distribution BMI is blind to.

Estimates, not medical advice. These calculators apply published equations to the numbers you enter. They describe averages across large groups, and any individual can differ from the average by a wide margin — metabolism, body composition, medication, and medical conditions all shift the result. Nothing here diagnoses, treats, or replaces advice from a doctor or registered dietitian, and no target produced by a formula should override how you actually feel. If you are managing a health condition, are pregnant, or have a history of disordered eating, talk to a clinician before acting on any of it.

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Your result

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The category scale

A general-wellness estimate, not a diagnosis. It cannot tell you whether you are healthy — only where a measurement falls on a published scale. Everything is calculated in your browser: nothing you type is sent anywhere or stored. If counting is making things worse rather than better, the National Eating Disorders Association keeps a current list of support options.

How this calculator works

Body mass index divides your weight by the square of your height. In metric that is kilograms over metres squared and needs no adjustment; in imperial the same ratio is multiplied by 703, a factor that exists purely to convert pounds and inches into the metric quantities the index is defined in. The calculator shows the result on a scale marked with the standard categories, and converts each category back into a weight range at your own height, which is usually more useful than the index itself — knowing that the healthy band for you runs from 60 to 81 kilograms says more than knowing your BMI is 24.7.

Because BMI is silent about where weight sits on the body, the calculator also asks for a waist measurement. Waist-to-height ratio divides one by the other and compares the result against a single boundary of 0.5 — keep your waist under half your height. That measure picks up central fat, which carries substantially more cardiometabolic risk than the same weight distributed elsewhere, and it does so without needing separate tables for each sex or population.

Two threshold sets are offered. The familiar WHO international cutoffs place overweight at 25 and obesity at 30. WHO also publishes lower public-health action points for Asian populations, at 23 and 27.5, because risk of type 2 diabetes and cardiovascular disease rises at lower BMI values in those groups. Switching between the two shows how much of a "category" is a decision about where to draw a line on a continuous scale.

The formula

Metric      BMI = kg ÷ m²
Imperial    BMI = 703 × lb ÷ in²

    where 703 = 0.45359237 kg/lb ÷ (0.0254 m/in)² = 703.0696…

Waist-to-height ratio = waist ÷ height        (any consistent unit)
    healthy below 0.5 — keep your waist under half your height

Weight range for a target BMI at your height:
    kg = BMI × m²

The index was formulated by Adolphe Quetelet in the 1830s as a population statistic and renamed body mass index by Ancel Keys in 1972, who found it the best of the simple weight-for-height ratios for describing groups while cautioning that it was not suitable for individual diagnosis. The category thresholds are those of the World Health Organization; the alternative Asian action points come from the WHO Expert Consultation published in The Lancet 363 (2004) 157–163. Waist circumference thresholds of 94 and 102 centimetres for men and 80 and 88 centimetres for women follow WHO and the US National Institutes of Health.

The waist-to-height boundary of 0.5 is supported by a substantial meta-analytic literature finding it at least as good as BMI, and often better, at predicting cardiometabolic outcomes. The UK's National Institute for Health and Care Excellence recommends it alongside BMI in guideline NG246 — but only for adults with a BMI below 35, so this calculator still reports the ratio above that threshold while withholding the risk band, which is what the guideline actually says. The waist circumference thresholds are WHO's, from Technical Report 894; WHO titles that table as applying to Caucasians and notes explicitly that risk by waist circumference is population-specific.

Worked example

Someone 180 cm tall weighing 80 kg with an 86 cm waist— the calculator's default:

  1. Height in metres: 180 ÷ 100 = 1.8, and 1.8² = 3.24 m²
  2. BMI: 80 ÷ 3.24 = 24.7, which is inside the healthy band of 18.5 to 25
  3. The same person in imperial — which is what the calculator shows by default: 176.37 lb and 70.87 in, so 703 × 176.37 ÷ 70.87² = 24.7 — the two forms agree, as they must
  4. Healthy weight range at this height: 18.5 × 3.24 = 59.9 kg up to 25 × 3.24 = 81.0 kg
  5. Waist-to-height: 86 ÷ 180 = 0.48, just inside the 0.5 boundary
  6. Half of 180 cm is 90 cm, so this waist has 4 cm of margin

Switch the threshold set to the WHO Asian action points and the same BMI of 24.7 moves from healthy to overweight, because the boundary drops from 25 to 23. Nothing about the person changed — only the line drawn through a continuous scale, which is worth remembering whenever a single number is treated as a verdict.

Assumptions & tips

  • Measure the waist properly. Stand relaxed, breathe out normally, and lay the tape at the narrowest point between ribs and hips without pulling it tight. Sucking in or tightening the tape moves the number by several centimetres, which is more than the difference between categories.
  • Watch the trend, ignore the boundary. A BMI of 24.9 and one of 25.1 are the same body; only the label changes. What matters is which direction your numbers are moving over months, and whether your waist is growing while your weight stays flat — a sign of muscle being replaced by fat.
  • BMI overstates fatness in muscular people. If you train seriously with weights, expect BMI to place you higher than your body fat justifies. The waist-to-height ratio on this page, or a proper body composition measurement, will describe you far better.
  • It understates risk in older adults. Muscle and bone are lost gradually with age while fat is gained, so BMI can stay flat while body composition worsens. Grip strength, waist measurement and how easily you climb stairs are all more informative.
  • Never apply adult BMI to a child. Children and teenagers are assessed against age-and-sex percentile charts, because normal body composition changes throughout growth. Adult categories applied to a 12-year-old are meaningless and can be actively harmful.
  • One number is not a diagnosis. Blood pressure, blood glucose, lipid panel, family history and fitness all carry more information about your health than any weight-for-height ratio. Use this to decide whether to raise the subject with a doctor, not to conclude anything on your own.

Frequently asked questions

What does BMI actually measure?

Weight relative to height, and nothing else. It divides your mass by the square of your height, which produces a number that correlates with body fatness across large populations. It does not measure body fat, does not distinguish muscle from fat, and does not know where on your body the weight sits. The Belgian statistician Adolphe Quetelet devised it in the 1830s to describe populations, not to assess individuals — the name "body mass index" was attached over a century later. It remains useful precisely because it is cheap and reproducible, not because it is precise.

Why do people say BMI is misleading?

Because for particular bodies it is. A muscular athlete carries dense tissue that pushes BMI into the overweight or obese range while carrying very little fat. Older adults lose muscle and bone with age, so an unchanged BMI can hide a rising fat percentage. Shorter and taller people are not treated identically, since the height-squared denominator is an approximation. And BMI is silent about fat distribution, which matters: visceral fat around the abdomen carries far more cardiometabolic risk than the same mass on the hips and thighs. That last gap is why this calculator asks for a waist measurement as well.

Is waist-to-height ratio better than BMI?

For predicting cardiometabolic risk, the evidence increasingly says yes, or at least that it adds information BMI misses. The rule is memorable — keep your waist under half your height — and the 0.5 boundary works across sexes and most ethnic groups without separate tables. The UK's National Institute for Health and Care Excellence now recommends using waist-to-height ratio alongside BMI for people with a BMI under 35. Neither measure diagnoses anything on its own; both are screening signals that suggest whether a conversation with a clinician is worthwhile.

Do the BMI thresholds apply to everyone?

No, and the World Health Organization says so explicitly. Risk begins to rise at lower BMI values in many Asian populations, so WHO published alternative public-health action points: overweight from 23 and obesity from 27.5, rather than 25 and 30. The calculator shows both sets. Thresholds also do not transfer to children and teenagers, who are assessed against age-and-sex percentile charts rather than fixed numbers — this calculator asks your age and withholds the adult category below 20 rather than printing a label that does not apply. They also do not apply during pregnancy, or to people with substantial muscle mass or limb loss.

What should I do with the result?

Treat it as one data point among several, and pay more attention to trend than to category. A BMI a point over a threshold is not a diagnosis, and the thresholds themselves are round numbers chosen for convenience rather than sharp biological boundaries. If your BMI and waist-to-height ratio both sit in an elevated range, that is worth raising with a doctor, who can look at blood pressure, blood glucose, lipids and family history — the things that actually determine risk. A number from a web page cannot.

Sources

  1. Obesity: preventing and managing the global epidemic — report of a WHO consultation — World Health Organization, WHO Technical Report Series 894, 2000. iris.who.intThe international BMI classification used on the default scale (underweight below 18.50, overweight from 25.00, obesity classes at 30.00, 35.00 and 40.00) from Table 2.1, and the waist circumference thresholds of 94 and 102 cm for men and 80 and 88 cm for women from Table 2.2 — the table WHO titles "in Caucasians" and footnotes as an example only, which is why this page repeats that caveat.
  2. Appropriate body-mass index for Asian populations and its implications for policy and intervention strategies — WHO Expert Consultation, The Lancet 363(9403), 157–163, 2004. doi.orgThe alternative public-health action points behind the second threshold set: overweight from 23 and obesity from 27.5.
  3. Overweight and obesity management, NICE guideline NG246 — National Institute for Health and Care Excellence, published 14 January 2025, last updated January 2026. nice.org.ukThe waist-to-height ratio bands this page reports — healthy 0.4 to 0.49, increased 0.5 to 0.59, high 0.6 or more — and recommendation 1.9.8, which applies them only to adults with a BMI below 35, which is why the band label is withheld above that.
  4. CDC Growth Charts — Centers for Disease Control and Prevention, National Center for Health Statistics. cdc.govThe BMI-for-age percentile charts that replace the adult categories under the age of 20, which is why this calculator prints the index but withholds the label for anyone younger.
  5. Indices of relative weight and obesity — A. Keys, F. Fidanza, M. J. Karvonen and colleagues, Journal of Chronic Diseases 25(6–7), 329–343, 1972. doi.orgThe paper that named the body mass index and compared it against the other simple weight-for-height ratios, finding it the best for describing populations rather than individuals.
  6. Clinical Guidelines on the Identification, Evaluation, and Treatment of Overweight and Obesity in Adults: The Evidence Report — National Heart, Lung, and Blood Institute, NIH Publication No. 98-4083, September 1998. nhlbi.nih.govThe US National Institutes of Health adoption of the same BMI cutoffs and of the 102 cm and 88 cm waist thresholds for men and women that this page reports alongside WHO's.

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