Body Mass Index (BMI) is a numerical measure calculated from a person's height and weight used to estimate body fatness and screen for weight-related health risks in adults. Calculate BMI below:
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BMI calculators take weight and height as inputs and return a value in kg/m², which it maps against WHO adult categories. These classifications include underweight for a BMI below 18.5, normal weight from 18.5 to 24.9, overweight from 25.0 to 29.9, and obesity at 30.0 or above. Obesity is subdivided into Class I (30 to 34.9), Class II (35 to 39.9), and Class III (40 or above).
What the number is not is a measure of body composition. WHO describes BMI as a surrogate marker of fatness and recommends waist circumference as a supplement.
The CDC makes the same point operationally: BMI screens, it does not diagnose, and the result should be discussed in the context of the patient's overall health. Population-level correlation between BMI and total body fat sits at roughly r = 0.80, which supports screening use while leaving real room for misclassification in any individual patient.
Making that connection between the patient and the GP, like a puzzle fitting together.
Deborah Hawthorne
Clinicians at Connect2Care
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