Clinically reviewed by Associate Professor Christopher O’Donnell, Nurse Practitioner, FACN · Founder and Clinical Director · AHPRA NMW0001453715 · Last reviewed
BMI is a useful population screening tool but a poor individual measure. It cannot distinguish muscle from fat, or say where fat is stored, which is what most affects health risk. Waist circumference and waist to height ratio predict metabolic risk better than BMI alone.
Where BMI came from
BMI was devised in the nineteenth century by a Belgian mathematician studying population averages. It was never designed as a clinical tool for individuals, and its creator said as much.
It survived because it is cheap and easy. Height and weight are simple to measure at scale, which makes BMI genuinely useful for tracking populations. The problem is applying a population statistic to one person in front of you.
What BMI cannot see
Four limitations matter clinically.
- It cannot distinguish muscle from fat. A muscular person can register as overweight with low body fat
- It ignores fat distribution. Two people at identical BMI can have very different metabolic risk depending on where fat is stored
- It does not account for ethnicity. People of South Asian, Chinese and some other backgrounds develop metabolic risk at lower BMI thresholds, which is why adjusted cut points are recommended
- It does not account for age or sex, despite body composition differing considerably across both
Why waist measurement adds so much
Fat stored around the abdomen and around the organs behaves differently from fat stored under the skin on hips and thighs. Visceral fat is metabolically active, releasing inflammatory signals and free fatty acids directly into the portal circulation to the liver.
This is why waist circumference predicts metabolic risk better than BMI. Someone with a BMI in the normal range but a large waist can carry higher risk than someone with a higher BMI and a smaller waist.
How to measure it properly
Consistency matters more than precision, so use the same method every time.
- Measure directly against skin, at the midpoint between the lowest rib and the top of the hip bone
- Stand relaxed and measure at the end of a normal breath out, without pulling the tape tight
- Australian guidance flags increased risk above 94cm for men and 80cm for women, with substantially increased risk above 102cm and 88cm respectively
- Waist to height ratio is a useful alternative. Keeping waist below half your height is a reasonable general target across body sizes
What we use in assessment
A full assessment looks at the whole picture: weight and BMI for context, waist circumference, blood pressure, blood glucose, lipids and liver function, alongside your history, symptoms and what you want to change.
Someone with a BMI of 32, a normal waist for their frame and entirely normal metabolic markers is in a different position from someone at BMI 27 with central fat, raised fasting glucose and high blood pressure. Treating those two identically because of a single number would be poor practice. Speak with our team.
Frequently asked questions
Is BMI accurate for individuals?
Not particularly. BMI is a population screening tool that cannot distinguish muscle from fat or account for fat distribution, ethnicity, age or sex.
What waist measurement indicates increased risk?
Australian guidance flags increased risk above 94cm for men and 80cm for women, with substantially increased risk above 102cm and 88cm.
Why is belly fat more of a concern?
Visceral fat around the organs is metabolically active, releasing inflammatory signals and fatty acids directly to the liver, which drives insulin resistance more than fat stored under the skin.
Does BMI apply equally to all ethnic groups?
No. People of South Asian, Chinese and some other backgrounds develop metabolic risk at lower BMI thresholds, so adjusted cut points are recommended.
What is waist to height ratio?
Waist circumference divided by height. Keeping waist below half your height is a practical target that works across different body sizes.