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Understanding BMI

Body mass index is one of the most quoted health numbers and one of the most misunderstood. Here is what it actually measures, and where it falls short.

What BMI is

BMI compares your weight to your height. It does not measure body fat directly; it is a quick screen that sorts most adults into broad weight categories. Because it needs only two easy measurements, it became a standard first look at whether someone's weight might affect their health.

The categories

• Below 18.5: underweight
• 18.5 to 24.9: healthy weight
• 25 to 29.9: overweight
• 30 and above: obese

For example, someone 1.75 m tall weighing 70 kg has a BMI of about 22.9, which sits in the healthy range. The BMI calculator works in both metric and US units and shows your healthy weight range too.

What BMI gets right

At a population level, BMI tracks reasonably well with body fat and with the risk of weight-related conditions. It is cheap, fast, and consistent, which is why doctors still use it as an opening screen.

What BMI misses

BMI cannot tell muscle from fat, so a very muscular athlete can register as overweight despite low body fat. It also ignores where fat is stored (around the waist is riskier than the hips), and it does not account for age, sex, or ethnicity, all of which affect what a healthy weight looks like.

For a fuller picture, pair BMI with other measures. The body fat calculator estimates the share of your weight that is actually fat, and the ideal weight calculator shows a target range from several formulas.

How to use it sensibly

Treat BMI as one signal, not a verdict. If your number falls outside the healthy range, it is a prompt to look closer with a doctor, not a diagnosis on its own. Trends over time and how you feel matter more than a single reading.