Body Composition and Dosing Weight Calculators

The body mass index (BMI) calculator computes BMI for screening underweight, normal weight, overweight, and obesity categories as defined by the World Health Organization. BMI is a population-screening metric: it is quick, weight and height based, and does not by itself distinguish muscle from fat or describe where fat is carried.

The waist-to-hip ratio calculator complements BMI by measuring central (abdominal) adiposity, which the WHO expert consultation on waist circumference and waist–hip ratio addressed as a risk-stratification measure in its own right.

The body surface area (BSA) calculator uses the Mosteller formula to estimate BSA from height and weight. BSA is used to normalize physiologic parameters and to dose selected medications, and it is a distinct quantity from BMI — it grows more slowly than weight and captures both height and weight together.

Three calculators provide size-adjusted weight references for dosing. The ideal body weight calculator applies the Devine formula, a height-based reference weight originally described in the context of gentamicin therapy. The adjusted body weight calculator builds on ideal body weight by adding a fraction of the excess weight for overweight and obese patients, the approach used when a medication distributes into lean tissue and fat differently. The lean body weight calculator applies the Boer formula to estimate lean body mass, useful in dosing, anesthesia, and nutritional assessment.

Dosing labels and protocols specify which weight basis to use — actual, ideal, adjusted, or lean — and using a different basis than the one the label assumes will change the dose for the same patient. The choice is dictated by the medication and the evidence behind its label, not by which value is numerically most appealing. When in doubt, follow the basis declared by the prescribing information.