Height and weight get used two different ways in medicine, and mixing them up causes real errors. BMI screens for weight-related risk; body surface area sizes drug doses and physiological measurements. Choose the tool that matches your question.
Enter height and weight in metric or imperial units. Get your body mass index with the WHO weight category, from underweight through obesity class III, and a plain-language explanation of what the number means.
Open BMI calculatorEnter height and weight to get body surface area in square metres by five standard formulas: DuBois, Mosteller, Haycock, Gehan and George, and Boyd, compared side by side.
Open BSA calculatorThe two numbers answer different questions. BMI asks: is this person's weight healthy for their height? It is a screening instrument, designed to flag people who merit a closer look at metabolic risk. Guidelines use it as the entry gate to weight-management pathways, epidemiologists use it to track populations, and clinics record it because it costs nothing and the whole world uses the same cutoffs. What it never does is set a drug dose.
BSA asks: how big is this patient for the purposes of dosing and measurement? Chemotherapy protocols specify milligrams per square metre, cardiac output is divided by BSA to give cardiac index, and kidney function is reported per 1.73 m². These are precision uses where the number flows directly into milligrams drawn up or litres per minute interpreted. Using BMI where BSA belongs, or the reverse, is a category error: a screening index cannot size a cytotoxic dose, and a surface-area estimate cannot tell you someone's weight category.
| Question | Use | Calculator |
|---|---|---|
| Is weight healthy for height? | BMI (kg/m²) | BMI calculator |
| What chemotherapy dose? | BSA (m²) | BSA calculator |
| How is the heart performing for this body size? | Cardiac index (CO/BSA) | BSA calculator |
| Should weight management be discussed? | BMI category | BMI calculator |
BMI has one formula: weight in kilograms divided by height in metres squared, or 703 times weight in pounds divided by height in inches squared. The WHO cutoffs are fixed: 18.5, 25, 30, 35 and 40. There is nothing to choose, which is part of its appeal.
BSA has five established formulas because each generation of researchers fitted a better curve to measured data. The DuBois formula from 1916 (0.007184 x W^0.425 x H^0.725) remains the reference standard. Mosteller's 1987 simplification (sqrt(H x W / 3600)) is the bedside favourite because it needs no fractional exponents. Haycock (1978) suits paediatric work, Gehan and George (1970) was fitted for chemotherapy dosing, and Boyd (1935) comes from growth studies. In average-sized adults they agree within a few percent; the full comparison, with history and guidance on choosing, is on the BSA calculator page.
Both tools are estimators, not measurements. BMI cannot distinguish muscle from fat, so it misclassifies muscular athletes upward and older adults with muscle loss downward; it is not valid in pregnancy or childhood, where different references apply. BSA formulas are empirical fits that lose accuracy at extremes of body size, and a BSA-derived chemotherapy dose still passes through clinical judgement about organ function and protocol rules. Treat both calculators as careful starting points. The final decision always belongs to a qualified clinician with the patient in front of them.
Sources: World Health Organization. Obesity and overweight fact sheet. Body Surface Area. StatPearls. NIH/NCBI Bookshelf.