Body mass index (BMI)

Calculate body mass index from height and weight and classify it against the WHO categories.

What is this for?

BMI is a screening index of weight relative to height. In pharmacy it functions mainly as a gate: several dosing-weight rules and drug eligibility criteria — for example whether to use adjusted body weight for an aminoglycoside, or whether a patient qualifies for weight-management pharmacotherapy — are keyed to a BMI threshold.

How to use it

Enter the weight and select kg or lb from the unit dropdown.
Enter the height and select cm or in.
Read the BMI and its WHO category. The result panel also flags whether the BMI ≥ 30 threshold for aminoglycoside adjusted-body-weight dosing is met.

Worked example

A patient weighs 95 kg and is 170 cm tall. Calculate BMI and classify.

Answer: 1.70 m² = 2.89; 95 ÷ 2.89 = 32.9 kg/m² — Obesity class I. Since BMI ≥ 30, aminoglycoside dosing would use adjusted body weight.

Clinical pearls & pitfalls

BMI does not distinguish fat mass from lean mass. A muscular athlete and a sarcopenic older adult can share a BMI while having entirely different body composition and drug distribution.
The WHO cut-points were derived in populations of European ancestry. WHO and several national bodies recommend lower action points for South and East Asian populations, where cardiometabolic risk rises at a lower BMI.
BMI is a screening tool, not a dosing weight. Do not dose a drug on BMI — use TBW, IBW, or AdjBW as the drug requires.
In pregnancy, ascites, or substantial oedema, BMI is unreliable because the weight is not adipose tissue.

Assumptions & limitations

Validated for adults. Paediatric assessment uses age- and sex-specific BMI percentiles, not the adult cut-points shown here.
Does not account for age, sex, ethnicity, muscularity, or fat distribution, all of which modify the risk associated with a given BMI.
Not valid in pregnancy, in amputees without correction, or where large fluid shifts are present.

References

World Health Organization. Obesity: preventing and managing the global epidemic. WHO Technical Report Series 894. Geneva: WHO; 2000.
WHO Expert Consultation. Appropriate body-mass index for Asian populations and its implications for policy and intervention strategies. Lancet. 2004;363(9403):157-163.
Nuttall FQ. Body mass index: obesity, BMI, and health — a critical review. Nutr Today. 2015;50(3):117-128.