BMI & Body Surface Area Calculator
Calculates your Body Mass Index and Body Surface Area using your height, weight, and age. Use it to quickly screen for weight category or to estimate BSA for clinical dosing applications.
About this calculator
Body Mass Index (BMI) is a widely used screening metric defined as: BMI = weight (kg) / height (m)². The WHO classifies results as underweight (<18.5), normal weight (18.5–24.9), overweight (25–29.9), or obese (≥30). Body Surface Area (BSA) estimates the total skin area of the body, used clinically to scale chemotherapy doses, cardiac output, and burns assessment. Common BSA formulas include Mosteller: BSA = √(height × weight / 3600), DuBois: BSA = 0.007184 × height⁰·⁷²⁵ × weight⁰·⁴²⁵, and Haycock: BSA = 0.024265 × height⁰·³³⁶⁴ × weight⁰·⁵³⁷⁸, all yielding results in m². BMI is quick but does not distinguish fat from muscle; BSA adds clinical precision for dosing contexts.
How to use
Example: height = 175 cm (1.75 m), weight = 75 kg. BMI = 75 / (1.75)² = 75 / 3.0625 = 24.5 — normal weight category. For BSA using the Mosteller formula: BSA = √(175 × 75 / 3600) = √(13125 / 3600) = √3.6458 ≈ 1.91 m². This figure would be used by a clinician to calculate a body-surface-area-adjusted drug dose, for example a chemotherapy agent prescribed at 50 mg/m² would require 50 × 1.91 = 95.5 mg.
Frequently asked questions
What is the difference between BMI and Body Surface Area and when should I use each?
BMI is a population-level screening tool that flags weight categories relative to height; it is best used for broad public health assessments and initial clinical screening. Body Surface Area is a more anatomically precise measurement used when drug doses, fluid requirements, or physiological parameters need to be scaled to body size rather than weight alone. In oncology, for example, BSA-based dosing reduces the variability in drug exposure between patients of very different body sizes compared to weight-based dosing.
Why is BMI considered an inaccurate measure of body fat for athletes?
BMI divides total body mass by height squared without distinguishing between muscle and fat tissue. A muscular athlete can have a BMI in the overweight or obese range while carrying very low body fat, because dense lean muscle mass raises total weight. Conversely, an sedentary individual with low muscle mass and high fat mass may register a 'normal' BMI while being metabolically unhealthy. For athletes, direct body composition measures such as DEXA, skinfolds, or circumference-based methods provide far more meaningful data.
Which BSA formula is most accurate for clinical drug dosing in adults?
The Mosteller formula (BSA = √(height × weight / 3600)) is the most widely adopted in clinical practice due to its simplicity and strong agreement with more complex equations. The DuBois & DuBois formula was the historical gold standard and remains common in older literature. The Haycock formula is preferred for paediatric populations. In practice, the formulas agree within 2–3% for most adult patients of average build, and the choice of formula is often dictated by institutional or protocol convention rather than meaningful differences in accuracy.