BMI explained: adult categories, the healthy range and what it misses
Body mass index (BMI) is your weight in kilograms divided by your height in metres, squared. For adults, the World Health Organization groups the result as underweight below 18.5, healthy weight from 18.5 to 24.9, overweight from 25 to 29.9, and obesity from 30. It is a quick screening number, not a measure of health on its own.
BMI calculatorAm I a healthy weight?OpenThe formula
BMI = kg ÷ m². A person of 75 kg who is 1.80 m tall has a BMI of 75 ÷ (1.80 × 1.80) = 75 ÷ 3.24 = 23.1.
In imperial units the formula is BMI = 703 × pounds ÷ inches². For 160 pounds at 70 inches, that is 703 × 160 ÷ 70², which is 22.96, shown as 23.0. The 703 is a rounded conversion factor, so the imperial result can differ from the metric one in the last decimal place. The unit converter converts feet, inches, pounds and stone if you want to check a figure.
The WHO adult categories
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 to 24.9 | Healthy weight |
| 25.0 to 29.9 | Overweight |
| 30.0 and above | Obesity |
The World Health Organization also splits the obesity group into classes for clinical use. This tool shows the four groups above. It rounds the BMI to one decimal place before it sorts the result, so a figure shown as 25.0 is labelled overweight, and a figure shown as 24.9 is labelled healthy weight.
The healthy weight range for your height
The healthy range is the BMI band 18.5 to 24.9, turned into a weight for your height. Multiply your height in metres, squared, by 18.5 for the lowest weight and by 24.9 for the highest.
For a height of 180 cm the square is 3.24. The lowest weight is 18.5 × 3.24 = 59.9 kg, and the highest is 24.9 × 3.24 = 80.7 kg. In pounds that is about 132 to 178. For a height of 165 cm the range is about 50.4 to 67.8 kg.
The range is the set of weights that fall inside the BMI band. It is not a target, and being inside it does not guarantee good health.
What BMI leaves out
- Muscle and fat. BMI counts all weight the same way. A muscular athlete can be labelled overweight with little body fat, and a person with little muscle can be labelled healthy weight with a high body fat share.
- Where fat sits. Fat around the waist carries different risks from fat elsewhere, so a waist measurement is often used alongside BMI.
- Age. Adults tend to lose muscle as they get older, so the same BMI can mean something different at 70 than at 30. BMI does not adjust for age.
- Ethnic background. Some populations face higher risk at a lower BMI. The World Health Organization notes this for many Asian populations, so the cut-offs are not the same everywhere.
- Pregnancy. BMI is not designed to be read during pregnancy.
Children and teenagers
For people under 18, BMI is compared with age- and sex-specific percentiles on a growth chart, not with the adult cut-offs on this page. The same BMI can sit at a different percentile at age 8 and at age 15, so a child's result has to be read against the right chart. A doctor or nurse can do that comparison with the child present.
Using the number well
- Treat BMI as a first screen, not a diagnosis. It is one measure among several.
- Check the units first. Entering pounds where kilograms are expected, or inches where centimetres are expected, is a common error.
- Look at the trend over months rather than one weigh-in, because body weight changes through the day.
- If the result surprises you, or you have a health concern, talk to a clinician. This page is not medical advice.
Edge cases
- Very tall or very short people. The formula still works, but the healthy weight range scales with height squared, so it moves a lot for small changes in height.
- Clothes and shoes. Weigh without heavy clothing, and measure height without shoes, to match the usual method.
- Stone and pounds. One stone is 14 pounds. The calculator takes stone and pounds directly, so you do not need to convert first.
Quick answers
What BMI is healthy for an adult? The WHO healthy weight band is 18.5 to 24.9. It is a screening band, not a guarantee.
Why does my BMI say overweight when I am fit? Muscle weighs more than fat, and BMI cannot tell them apart.
Does BMI work for children? Not with the adult cut-offs. Use the age- and sex-specific percentile chart instead.
