Body mass index

Before you rely on it

What to check

Read it beside something that says where fat is carried, like waist circumference.

The common error

Where people go wrong with body mass index

Taking the category label at face value. Two people at the same BMI can have completely different health pictures, and the number was never designed to distinguish them.

Worked through

Body mass index in plain numbers

A person who weighs 70 kg and is 1.75 m tall has a BMI of 70 ÷ (1.75 × 1.75). That is 70 ÷ 3.0625, or 22.9; the same person at 82 kg would score 26.8, and many guidelines treat 18.5 to 24.9 as the healthy-weight range for most adults and 25 to 29.9 as overweight, with 30 and above described as obesity. Those are screening bands. They say where a person sits on a population scale. They do not say whether that person is healthy, which depends on many things a height-and-weight ratio cannot see.

Easily confused

Body mass index and its neighbours

BMI is easy to confuse with body fat percentage, waist-to-hip ratio and ideal weight. Body fat percentage estimates the share of weight that is fat, which BMI does not measure. Waist-to-hip ratio and waist circumference say where fat sits, which matters for some health risks. Ideal body weight formulas are older estimates from height alone. BMI is a quick screening ratio, not a diagnosis; a muscular athlete can score high with little fat, and an older adult can score in range while carrying little muscle.

In real decisions

Where body mass index turns up

Clinicians use BMI as a first look because it costs nothing and needs only two measurements. Public health researchers use it to track populations. Insurers and programs sometimes set eligibility thresholds from it. Individuals use it as a rough check before discussing weight with a health professional. Because it is a ratio, it can be tracked over time, though the change in waist size and fitness often tells you more about progress than the number itself.

Know the edges

Where body mass index stops being reliable

BMI does not account for muscle, bone, age, sex or ethnicity, and standard cutoffs do not fit every group equally; it can misclassify very muscular people, older adults and some ethnic groups. Children and teenagers use age-specific percentiles instead of adult bands, and it is not medical advice, and no single number diagnoses a condition. If you have health concerns, a qualified clinician can assess you properly with tools that go beyond a height and weight ratio.

Quick answers

What people ask about body mass index

Is BMI accurate for athletes? It can mislead. Muscle weighs more than fat, so a muscular person may score as overweight with low body fat. Waist measurements and body composition tests add useful detail. A clinician can put the number in context.

What BMI counts as healthy? Many guidelines call 18.5 to 24.9 the healthy range for most adults. Bands differ for children and some populations. The number is a screen, not a verdict on health.