BMI Explained: What It Means and What It Does Not

Body mass index, or BMI, is a single number that compares your weight with your height. It is quick to work out, which is why doctors and health surveys use it. It is also easy to over-read, so it helps to know what it can and can't tell you.

How it is calculated

BMI = weight in kilograms / (height in metres squared)

For example, a person who weighs 70 kg and is 1.75 m tall has a BMI of 70 / (1.75 x 1.75) = 70 / 3.0625, which is about 22.9. If you use pounds and inches, the formula multiplies by 703. If you mix units, the Unit Converter can switch them for you.

The standard categories for adults

• Under 18.5: underweight
• 18.5 to 24.9: healthy weight range
• 25 to 29.9: overweight
• 30 and above: obesity

These are the World Health Organization ranges for adults. Children and teenagers are judged differently, using charts that compare them with others of the same age and sex.

What BMI misses

• Muscle versus fat. Muscle is dense, so a very muscular person can have a high BMI and little body fat.
• Where fat sits. Fat around the waist carries more health risk than fat elsewhere, and BMI can't see it.
• Age, sex and ethnicity. Healthy ranges differ between groups, and older adults may need slightly higher values.
• Pregnancy. The standard ranges don't apply.

How to use the number sensibly

Treat BMI as one clue among several. Waist size, blood pressure, fitness, diet and family history all matter as much or more. If your result worries you, or you want to change your weight, talk to a doctor or dietitian instead of relying on a calculator.

You can work out your number with the BMI Calculator, and the Age Calculator is useful for checking exact ages. This site gives general information only and isn't medical advice.