Body Mass Index (BMI)

Body Mass Index (BMI) is the measure most often used to distinguish normal weight from overweight and obesity. It is the quickest check of whether a person's weight is healthy for their height, and because it needs only height and weight it is cheap, fast and used everywhere from schools to hospitals. Clinicians and researchers use it to assess "normalcy" of body weight; it shows a moderate association with body fat and disease risk - higher than estimates based on height and weight alone.

Before the formula, it helps to separate three terms the book is careful about:

• Overweight - a body weight that exceeds some average for stature (and perhaps age). It often, but not always, goes with increased body fat (male power athletes are a classic exception).
• Overfat - a condition where body fat exceeds an age- and/or sex-appropriate average by a set amount; this is the correct term when actually assessing body-fat levels.
• Obesity - the overfat condition accompanied by a cluster of comorbidities - glucose intolerance, type-2 diabetes, hypertension, and increased risk of coronary heart disease and cancer.

The formula

The single most common mistake is leaving height in centimetres; it must be converted to metres before squaring.

Worked example (from the textbook)

A male of stature 175.3 cm and body mass 97.1 kg:

• Convert height: 175.3 cm = 1.753 m
• Square it: (1.753)^2 = 3.073 m^2
• Divide: BMI = 97.1 / 3.073 = 31.6 kg/m^2

A BMI of 31.6 falls in the 30.0-34.9 band - class I obesity.

A second worked example

A student weighs 60 kg and is 150 cm tall:

1. Convert height: 150 cm = 1.5 m
2. Square it: (1.5)^2 = 2.25 m^2
3. Divide: BMI = 60 / 2.25 = 26.67 kg/m^2
4. Interpret: 26.67 lies in the 25.0-29.9 band, so the student is overweight.

BMI classification table

The textbook gives six bands for reading a computed BMI:

Limitations of BMI

BMI must be understood as a screening tool, not a direct measure of body fat: …