Body Mass Index is weight divided by height squared. It was designed in the 1830s to describe populations, not to assess individuals — which is exactly how it should still be used. This page explains what the number means, and the four situations where it reliably misleads.
The calculation
BMI = weight (kg) ÷ height (m)2
In imperial units: BMI = 703 × weight (lb) ÷ height (in)2.
A person 1.75 m tall weighing 72 kg: 72 ÷ (1.75 × 1.75) = 72 ÷ 3.0625 = 23.5.
The standard categories
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Healthy weight range |
| 25.0 – 29.9 | Overweight |
| 30.0 – 34.9 | Obesity class I |
| 35.0 – 39.9 | Obesity class II |
| 40.0 and above | Obesity class III |
These thresholds were derived largely from populations of European descent. Several health authorities use lower cut-offs for people of South Asian, Chinese and some other Asian ancestries — often treating 23 as the upper end of the healthy range and 27.5 as the obesity threshold — because cardiometabolic risk appears at lower BMI values in these groups.
Where BMI is unreliable
BMI measures mass, and it cannot tell you what that mass is made of.
- Muscular people. Athletes and regular strength trainers frequently register as overweight or obese while carrying low body fat. Muscle is denser than fat, and BMI counts both identically.
- Older adults. Muscle mass declines with age while fat increases. A stable BMI over decades can conceal a substantial change in body composition.
- Very tall or very short people. Squaring height systematically overestimates BMI in tall people and underestimates it in short people, because bodies do not scale as a perfect square.
- Pregnancy, and children. Adult BMI categories do not apply during pregnancy at all. Children and adolescents are assessed against age- and sex-specific percentile charts, not these thresholds.
Waist measurement adds what BMI misses
Where fat sits matters more than how much there is in total. Fat around the abdomen is more strongly associated with cardiometabolic risk than fat on the hips and thighs. A waist-to-height ratio is a simple, useful complement:
Waist ÷ height < 0.5
Keeping your waist measurement below half your height is a widely used general guide, and it applies across heights and most populations. Measuring both BMI and waist gives a far better picture than either alone — and if the two disagree, the waist measurement is usually the more informative.
What a number outside the range actually means
A BMI outside 18.5–24.9 is a prompt to look further, not a diagnosis. It says nothing about your blood pressure, blood glucose, lipid profile, fitness, diet quality, sleep or strength — all of which influence health outcomes independently, and several of which can be excellent in someone whose BMI sits outside the range, or poor in someone whose BMI sits inside it. Fitness in particular is a strong independent predictor of health, and it is not visible in BMI at all.
Using it sensibly
BMI is most useful as one datapoint tracked over time in the same person, alongside measurements that describe composition and function. It is least useful as a single verdict on one day. If your result concerns you, or if it has changed substantially without a change in your habits, that is a reason to speak to a doctor — who can interpret it alongside your history, measurements and blood work.
This page is general information, not medical advice. Speak to a qualified healthcare professional about your own health, and before making significant changes to your diet or activity.