Growth & measurement
BMI calculator
Two numbers from height and weight: the WHO class for the assessment, and the body surface area that per-m² drug dosing actually needs.
How to read the result
Body mass index (kg/m²) falls into these published bands.
A 178 cm man weighing 86.5 kg.
BMI 27.3 kg/m² — overweight; BSA 2.07 m²
In the WHO overweight band (25–29.9), a prompt for lifestyle and cardiovascular risk discussion — and the 2.07 m² is the denominator ready for any per-m² dose.
Who this is for
For clinicians doing routine assessment, nutritional screening, or anaesthetic work-up — and for anyone prescribing drugs dosed per square metre of body surface, where the Mosteller BSA appears alongside the BMI without a second calculation. WHO adult cutoffs are drawn on a scale so the class is read, not remembered.
How it works
What it calculates
Body mass index (weight ÷ height²) with the WHO adult classification, and body surface area by the Mosteller formula — the BSA most chemotherapy dosing references.
When to use it
Routine assessment, nutritional screening, anaesthetic work-up, and whenever a drug is dosed per m² of body surface.
Interpreting the result
WHO cutoffs: under 18.5 underweight, 18.5–24.9 normal, 25–29.9 overweight, 30 and above obese. BMI misclassifies very muscular patients and is not validated in pregnancy or children — paediatric growth percentiles are the right tool there.
Built on published evidence
- WHO classification of adult BMI
Codification last reviewed 2026-08-28, and checked against published reference values in automated tests on every release — see the full evidence ledger.
Questions clinicians ask
Which BMI cutoffs does the calculator use?
The WHO adult classification: under 18.5 underweight, 18.5–24.9 normal, 25–29.9 overweight, and 30 or above obese. The result is placed on a scale with those cutoffs marked, so the class is visible rather than recalled.
Which body surface area formula is used?
Mosteller — BSA = √(height × weight / 3600) — the formula most chemotherapy dosing references use. It needs only height and weight and agrees closely with the older Du Bois formula across the adult range.
When does BMI mislead?
It misclassifies very muscular patients as overweight because it cannot separate muscle from fat, and it is not validated in pregnancy or in children — paediatric growth percentiles are the right tool there.
Does it work offline and is patient data stored?
It works fully offline once loaded, and no patient data is stored or transmitted — the values you enter stay in your browser tab only.
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One calculator is the door. The ward is the house.
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