Body mass index is the most widely used body measurement in the world, and probably the most argued about. It is useful and limited at the same time. Understanding what it captures and what it misses lets you use it for what it is good at without letting it define you.
The formula
BMI = weight (kg) ÷ height (m)²
In pounds and inches, multiply by 703: BMI = 703 × weight (lb) ÷ height (in)².
Examples: 60 kg at 1.65 m gives 22.0. 90 kg at 1.80 m gives 27.8.
Where it comes from
The formula comes from the Belgian mathematician Adolphe Quetelet in the 1830s and 1840s, who studied the average man. It was named the "body mass index" and promoted as a population measure by physiologist Ancel Keys in a 1972 paper. It was designed to describe groups, not to diagnose individuals.
WHO categories for adults
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 to 24.9 | Healthy weight range |
| 25.0 to 29.9 | Overweight range |
| 30.0 to 34.9 | Obesity class I |
| 35.0 to 39.9 | Obesity class II |
| 40.0 and above | Obesity class III |
A "healthy" BMI range for your height is roughly 18.5 to 24.9. For a 175 cm adult that is about 57 to 76 kg.
What BMI is good for
- A quick screen. It is fast, free and needs only two numbers.
- Population trends. Across large groups, higher BMI is linked to higher rates of type 2 diabetes, hypertension, heart disease and some cancers, and very low BMI is linked to malnutrition, osteoporosis and frailty.
- Spotting trends in your own life. A steady climb or drop over years is a signal worth noticing.
Where BMI goes wrong
It cannot see muscle. Muscle is denser than fat. A muscular person can have a high BMI and low body fat. Many rugby players and lifters are technically "overweight" or "obese" by BMI.
It cannot see fat distribution. Two people with the same BMI can have very different levels of abdominal fat. That is why waist-based measures like waist-to-height ratio add information.
It varies with height. The formula squares height, which does not perfectly scale for very short or very tall people; BMI tends to overstate fatness in tall people and understate in short people.
It is ethnicity-dependent. Body composition at a given BMI differs between populations. For example, many Asian populations have higher body fat and greater metabolic risk at a lower BMI, which is why some guidelines use lower thresholds.
It changes with age. Older adults lose muscle and gain fat at the same weight. A BMI slightly above 25 is sometimes associated with the lowest mortality in older people, while a very low BMI is a risk.
It says nothing about fitness or blood markers. A person with a "healthy" BMI can have poor fitness and high blood pressure, and a person with a higher BMI can be fit and metabolically healthy.
What to use alongside BMI
- Waist-to-height ratio. Under 0.5 is a good sign; above 0.6 increases risk. See the WHtR guide.
- Body fat percentage. A direct composition measure. See the body fat chart.
- Fitness measures. Strength, walking pace, and how you feel doing daily activity predict health outcomes as strongly as BMI.
- Blood tests. Ask your clinician about blood pressure, glucose and lipids.
A note on weight, shame and health
BMI is not a moral measure. The goal of using it is to spot possible risk and prompt useful action, not to label anyone. If your BMI is in a higher band, waist size, fitness, sleep, diet quality and activity all matter, and improving them helps even if weight changes slowly. If your BMI is low, check you are eating enough and speak to a clinician if you are concerned about your weight or your relationship with food.
How to use BMI well
- Check it once or twice a year, not weekly.
- Read it with your waist and body fat, not alone.
- If you lift weights and BMI is high, check your waist-to-height ratio and body fat before drawing a conclusion.
- Speak to a clinician for decisions about your health, medication or eating.
The Body Report places your BMI next to the other measures and highlights which ones matter for you.