In this guide
Body mass index, or BMI, is a quick number that compares your weight with your height. Doctors use it as a screening tool because it is cheap and easy. It is not a diagnosis, and it has real limits, which this guide explains.
How BMI is calculated
In metric units, BMI is weight in kilograms divided by height in meters squared. A person who weighs 70 kg and is 175 cm tall has a BMI of 70 ÷ (1.75 × 1.75) = 22.9. In US units, multiply weight in pounds by 703 and divide by height in inches squared.
Adult BMI categories
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 to 24.9 | Healthy weight |
| 25 to 29.9 | Overweight |
| 30 or above | Obesity range |
Healthy weight range for your height
Working the formula backwards gives the weights that fall in the 18.5 to 24.9 range. At 170 cm that is about 53.5 to 72 kg. At 5 feet 6 inches (66 inches) it is about 115 to 154 pounds.
A short history
The idea comes from the 19th-century Belgian mathematician Adolphe Quetelet. The name "body mass index" was popularized in 1972 by the physiologist Ancel Keys, who wanted a simple measure for studying groups of people.
Where BMI falls short
- It cannot tell muscle from fat, so very muscular people can score as overweight.
- It ignores where fat is stored, and fat around the waist carries different risks from fat elsewhere.
- It can misjudge older adults, who may lose muscle without losing weight.
- Some health guidelines use different cut-offs for some ethnic groups.
Age and gender
For adults the formula and the categories are the same for men and women of any age. For children and teens aged 2 to 19, health authorities compare BMI with growth charts for the same age and sex, so an adult calculator is not suitable for them.
Other measures to consider
Waist circumference, body fat percentage, blood pressure, blood sugar and cholesterol give a fuller picture of health than BMI alone. A doctor can interpret them together.
A note on health advice
This guide is general information and is not medical advice. If you are worried about your weight or health, speak to a doctor or registered dietitian.
Try the calculators
- BMI Calculator: Body mass index and healthy range
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General information only, not professional advice. See our editorial policy.