Body Mass Index (BMI) is one of the most widely used numbers in health and fitness. Doctors use it as a quick screening tool, public health agencies use it to track population trends, and many fitness apps show it on the first screen.
It is also frequently misunderstood. This guide explains exactly how BMI is calculated, what the adult categories mean, and — just as importantly — what BMI cannot tell you about your health.
What BMI measures
BMI compares your weight with your height. The idea is simple: a taller person is expected to weigh more, so dividing weight by height squared gives a number that can be compared between people of different heights.
BMI does not measure body fat directly. It cannot tell the difference between muscle, fat, bone and water — it only knows your total weight and your height.
The BMI formula
Worked example (metric): 70 kg and 1.75 m tall. Height squared is 1.75 × 1.75 = 3.0625. BMI = 70 ÷ 3.0625 ≈ 22.9.
Worked example (imperial): 160 lb and 5 ft 6 in (66 in). Height squared is 4,356. BMI = 703 × 160 ÷ 4,356 ≈ 25.8.
Adult BMI categories
For adults aged 18 and over, the World Health Organization uses the following categories:
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Healthy weight |
| 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 |
BMI for children and teenagers
Adult categories should not be used for anyone under 18. Children’s bodies change as they grow, so their BMI is compared with other children of the same age and sex using growth charts, and the result is expressed as a percentile. A paediatrician or school nurse can help interpret a child’s BMI.
What BMI cannot tell you
- Muscle vs fat: muscular people can have a “high” BMI while carrying little body fat.
- Fat distribution: fat stored around the waist is linked with higher health risk than fat stored elsewhere, but BMI cannot see where fat is stored.
- Age and sex: at the same BMI, women tend to have more body fat than men, and older adults more than younger adults.
- Overall health: blood pressure, blood sugar, cholesterol, fitness, sleep and diet all matter and are not reflected in BMI.
Because of these limits, BMI is best used as a starting point — a screening number that can prompt a fuller conversation — rather than a verdict on your health. Pairing it with a waist measurement or a body-fat estimate gives a more rounded picture. We compare the two in BMI vs body fat percentage.
What to do with your BMI result
- If your BMI is in the healthy range, focus on habits that keep you well: regular activity, balanced meals and good sleep.
- If your BMI is outside the healthy range, it is worth discussing with a doctor, who can look at your wider health.
- If you want to change your weight, the healthy weight calculator shows the range for your height and the calorie calculator gives a starting calorie target.
Try the calculators
Put this guide into practice with a free estimate.
Frequently asked questions
What is a healthy BMI?
For most adults, a BMI from 18.5 to 24.9 is classed as a healthy weight by the World Health Organization. Some populations use lower cut-offs, so check with a healthcare professional.
Is BMI accurate?
BMI is a useful population-level screening tool but can misclassify individuals, especially muscular people and older adults, because it does not measure body fat or where fat is stored.
Why is height squared in the BMI formula?
Squaring height makes BMI less dependent on height, so people of different heights can be compared on the same scale. It is a statistical convention first proposed by Adolphe Quetelet in the 19th century.
Methods and sources
- World Health Organization — Obesity and overweight fact sheet (BMI classification) (opens in a new tab)
- U.S. Centers for Disease Control and Prevention (CDC) — Adult BMI categories and BMI-for-age growth charts