Body Mass Index, Mosteller/DuBois BSA, Devine IBW and Adjusted Body Weight.

What it is and when to use it

This BMI, body surface area and ideal body weight calculator gathers the anthropometric values that are often needed together when preparing a treatment or setting up a ventilator. From height, weight and sex it calculates body mass index (BMI), body surface area (BSA) by Mosteller and by DuBois, ideal body weight by the Devine formula, and adjusted body weight. They are not interchangeable: BMI describes weight status; BSA underlies doses given in mg/m²; ideal body weight, the weight expected for a given height and sex, is the reference for drugs that distribute poorly into fat and for tidal volume in mechanical ventilation; and adjusted body weight is used with some drugs in patients with obesity.

How to interpret it

The main result is BMI (kg/m²) to one decimal place, and the recommendation shown depends on BMI alone: underweight below 18.5, normal weight up to 24.9, overweight from 25.0 to 29.9, and class I, II or III obesity with cut-offs at 30, 35 and 40. Mosteller BSA is √(cm × kg / 3600) and DuBois BSA is 0.007184 × height (cm) raised to 0.725 × weight (kg) raised to 0.425. Example, a man of 170 cm and 90 kg: BMI 31.1 (class I obesity), BSA 2.06 m² (Mosteller) or 2.01 m² (DuBois), ideal body weight 65.9 kg and adjusted body weight 75.5 kg.

Limitations and when not to use it

Below 152.4 cm the Devine formula subtracts nothing and the calculator returns a flat 50 kg or 45.5 kg, so ideal body weight is meaningless in children and very short adults. If actual weight does not exceed ideal weight, adjusted weight equals actual weight. Which weight to use for each drug is set by the summary of product characteristics and the local protocol; in the Cockcroft-Gault equation, for example, that choice changes estimated clearance considerably in obesity. Height should be measured: a self-reported height shifts BMI, BSA and ideal weight at once.

Frequently asked questions

How is ideal body weight calculated with the Devine formula?

Start at 50 kg for men or 45.5 kg for women and add 2.3 kg for every inch (2.54 cm) of height above 152.4 cm. At 170 cm this gives 65.9 kg for a man and 61.4 kg for a woman.

What is adjusted body weight?

It is a weight between ideal and actual: ideal weight + 0.4 × (actual weight − ideal weight). For a woman of 160 cm and 110 kg (ideal weight 52.4 kg) the adjusted weight is 75.4 kg.

Which weight is used to set tidal volume in mechanical ventilation?

Ideal or predicted body weight, derived from height and sex, rather than actual weight: lung size depends on height and does not increase with obesity, so using actual weight would give an obese patient excessive volumes.

Why do Mosteller and DuBois body surface area values differ?

They are different equations: Mosteller takes the square root of height × weight, while DuBois raises each variable to its own exponent. At 170 cm and 70 kg they barely differ (1.82 and 1.81 m²), but at 160 cm and 110 kg they give 2.21 and 2.10 m².