Body mass index (BMI) is one of the most widely used health numbers and one of the most misunderstood. It is quick and free to work out, which is why doctors, insurers and public health bodies use it, but it is a screening tool, not a diagnosis. This guide explains what BMI measures, what the categories mean, where it falls short, and what to use alongside it.
What BMI measures
BMI compares your weight with your height. In metric units it is your weight in kilograms divided by your height in meters squared. In pounds and inches it is 703 × weight ÷ height squared.
BMI = 703 × weight (lb) ÷ height (in)²
A person who is 175 cm (1.75 m) tall and weighs 70 kg has a BMI of 70 ÷ 1.75² = 22.9. Someone who is 5 ft 9 in (69 in) and weighs 154 lb has a BMI of 22.7. The idea goes back to the Belgian scientist Adolphe Quetelet in the 1830s, and the name “body mass index” was proposed by the physiologist Ancel Keys in 1972. Try your own numbers with the BMI calculator.
The adult categories
| BMI (kg/m²) | Category (WHO) |
|---|---|
| Below 18.5 | Underweight |
| 18.5 to 24.9 | Healthy weight |
| 25.0 to 29.9 | Overweight |
| 30.0 to 34.9 | Obesity, class 1 |
| 35.0 to 39.9 | Obesity, class 2 |
| 40.0 and above | Obesity, class 3 |
The World Health Organization’s categories for adults aged 18 and over.
Statistically, health risks such as type 2 diabetes and heart disease tend to rise as BMI moves above the healthy range, and some risks also rise when BMI is very low. That is why the ranges exist. They describe risk across large groups of people, not the health of any one person.
Where BMI falls short
- It cannot tell muscle from fat. A muscular athlete can have a high BMI and a low body fat percentage.
- It ignores where fat is carried. Fat around the waist carries more risk than fat on the hips and thighs, and BMI does not show the difference.
- Age and sex matter. Older adults tend to have more body fat at the same BMI, and women tend to have more than men.
- Ethnicity matters. A World Health Organization expert group reported in 2004 that in many Asian populations the risk of type 2 diabetes and heart disease begins at lower BMI values, and suggested extra action points at 23 and 27.5.
- Children are different. For under-18s, BMI is compared with growth charts for the child’s age and sex, so adult categories do not apply.
What to use alongside BMI
Waist measurement. Measured halfway between the lowest rib and the top of the hip bone, a waist of 94 cm (37 in) or more for men and 80 cm (31.5 in) or more for women is commonly used as a point at which health risk rises for people of European descent. Different thresholds apply to other groups. Waist-to-height ratio. A simple rule used in some health guidance is to keep your waist below half your height. Body fat percentage. An estimate from tape measurements, such as the U.S. Navy method in our body fat calculator, adds information that BMI cannot. Blood pressure, blood sugar and cholesterol tell you far more about your health than weight ever can, and only a doctor can interpret them.
How to use your BMI sensibly
- Treat it as a starting point for a conversation with a doctor, not as a verdict.
- Look at the trend over months, not a single reading.
- If you are very muscular, pregnant, elderly or under 18, ask a professional how to interpret it.
- Do not change your diet or exercise dramatically because of one number. If you want to lose or gain weight, our calorie calculator gives a realistic daily estimate, and a doctor or dietitian can tailor it to you.
This guide is general information and does not replace medical advice.
Sources and further reading
- WHO: Obesity and overweight (fact sheet)
- Body mass index (Wikipedia)
- Ideal body weight (Wikipedia)
- WHO expert consultation. “Appropriate body-mass index for Asian populations and its implications for policy and intervention strategies.” The Lancet, 2004
Numbers in worked examples are calculated by code and checked by automated tests. How we test.