BMI Calculator

BMI Calculator for Nutrition. Standard WHO/NIH adult bands are: underweight below 18.5, normal weight 18.5-24.9, overweight 25.0-29.9, obesity Class I 30.0-34.9, Class II 35.0-39.9, and Class III (severe/extreme) 40.0 and above. Risk of type 2 diabetes, cardiovascular disease, and all-cause mortality rises across the overweight and obesity bands, and Class III is a threshold for considering bariatric surgery. For adults of Asian descent, the WHO expert consultation recommends lower public-health action points, with increased risk from 23.0 and high risk from 27.5, reflecting greater adiposity and cardiometabolic risk at any given BMI.

How this calculator works

Body mass index divides measured weight in kilograms by the square of height in metres (kg/m squared); in US units the formula is 703 times weight in pounds divided by height in inches squared. It is a height-normalised proxy for total adiposity, not a direct measure of fat mass, and returns a single continuous number that is then mapped onto categorical weight bands. The index is mathematically identical to the historical Quetelet index popularised for population screening by Ancel Keys.

When to use this calculator

Use BMI for adults aged 20 and over as a first-pass screen for underweight, overweight, and obesity at population and primary-care level, and to trigger further assessment (waist circumference, metabolic labs, comorbidity review). Do not use the standard adult cut-offs for children or adolescents, who require age- and sex-specific BMI-for-age percentiles, nor for pregnant women. Interpret with caution in highly muscular individuals, the elderly with sarcopenia, and those with fluid overload or amputations, where BMI misclassifies body composition.

Inputs used

  • Weight
  • Height

Clinical interpretation

Standard WHO/NIH adult bands are: underweight below 18.5, normal weight 18.5-24.9, overweight 25.0-29.9, obesity Class I 30.0-34.9, Class II 35.0-39.9, and Class III (severe/extreme) 40.0 and above. Risk of type 2 diabetes, cardiovascular disease, and all-cause mortality rises across the overweight and obesity bands, and Class III is a threshold for considering bariatric surgery. For adults of Asian descent, the WHO expert consultation recommends lower public-health action points, with increased risk from 23.0 and high risk from 27.5, reflecting greater adiposity and cardiometabolic risk at any given BMI.

Worked example

A 30-year-old man weighs 92 kg and is 1.78 m tall. BMI = 92 / (1.78 x 1.78) = 92 / 3.168 = 29.0 kg/m squared. This falls in the 25.0-29.9 band, classifying him as overweight and just below the Class I obesity threshold, prompting waist circumference measurement and lifestyle counselling with follow-up. Had he been of South Asian ancestry, 29.0 would already sit within the WHO Asian action point for obesity (27.5 and above).

Limitations and safety notes

BMI cannot distinguish fat from lean mass or reflect fat distribution, so it overestimates adiposity in athletes and underestimates it in older adults with low muscle mass; visceral fat, the more cardiometabolically dangerous depot, is invisible to it. It systematically misclassifies risk across ethnic groups when a single cut-off is applied, and normal-weight individuals with central adiposity ("metabolically obese normal weight") are missed entirely. It is not valid for children, pregnancy, or people with significant oedema or limb loss.

Frequently asked questions

Why does BMI use height squared rather than cubed?

Squaring height (the Quetelet index) empirically gives the weakest correlation with height and the strongest with independent measures of body fatness across adult populations, which is why Keys and colleagues favoured it over other weight-height ratios. A cube would over-correct and penalise taller people.

Should I use different cut-offs for Asian patients?

Yes for public-health and screening purposes. The 2004 WHO expert consultation retained the international cut-offs for global comparison but added lower action points for Asian populations: 23.0 kg/m squared marks increased risk and 27.5 marks high risk, because diabetes and cardiovascular risk appear at lower BMI in these groups.

Can a very muscular person be flagged as obese?

Yes. Because BMI counts all mass equally, a lean, heavily muscled athlete can score 30 or above and be labelled obese despite low body fat. In such cases pair BMI with body-fat estimation or waist circumference before acting on the number.

What BMI qualifies for bariatric surgery?

Conventional criteria consider surgery at a BMI of 40.0 and above (Class III), or 35.0 and above with an obesity-related comorbidity such as type 2 diabetes or obstructive sleep apnoea; some guidelines now lower these thresholds by about 2.5 units for Asian populations.

Is BMI valid for children?

No. Children and adolescents must be assessed with sex- and age-specific BMI-for-age percentile charts (for example CDC or WHO growth references), not the fixed adult bands, because normal adiposity changes markedly with growth.

References

  • WHO Expert Consultation. Appropriate body-mass index for Asian populations and its implications for policy and intervention strategies. Lancet. 2004;363(9403):157-163. PMID: 14726171.
  • Keys A, Fidanza F, Karvonen MJ, Kimura N, Taylor HL. Indices of relative weight and obesity. J Chronic Dis. 1972;25(6):329-343. PMID: 4650929.
  • National Heart, Lung, and Blood Institute (NHLBI) Expert Panel. Clinical Guidelines on the Identification, Evaluation, and Treatment of Overweight and Obesity in Adults. National Institutes of Health / NHLBI. 1998.

Editorial review and citation methodology

Reviewed by the Quick Medical Calculator Editorial Team. Last reviewed: March 2, 2026. The review checks calculator inputs, intended population, interpretation, limitations, and source alignment.

  • Prefer original validation studies for scoring systems and prediction tools.
  • Use current specialty society guidance, transplant allocation policy, public health guidance, or regulator resources when they govern clinical use.
  • Include limitations and safety notes when a calculator is population-specific, context-dependent, or unsuitable as a standalone decision tool.

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