What is BMI?
The body mass index puts your weight in relation to your height. It is a quick screening number used by doctors and health organizations to sort adults into weight ranges: underweight, healthy weight, overweight and obesity. It does not measure body fat directly, and it says nothing about where fat sits or how much muscle you carry, so it works best as a starting point for a conversation, not as a verdict.
How BMI is calculated
BMI is your weight in kilograms divided by your height in meters squared. In imperial units, the weight in pounds is divided by the height in inches squared and multiplied by 703. The calculator converts imperial values to metric first, so both ways give the same number.
BMI ranges for adults
| BMI | Range |
|---|---|
| below 18.5 | Underweight |
| 18.5 to 24.9 | Healthy weight |
| 25 to 29.9 | Overweight |
| 30 to 34.9 | Obesity, class I |
| 35 to 39.9 | Obesity, class II |
| 40 and above | Obesity, class III |
These are the World Health Organization ranges for adults aged 20 and over. They are the same for men and women. Some countries use lower cut-offs for overweight and obesity for people of Asian descent, typically 23 and 27.5.
Where BMI falls short
- Muscle counts as weight. Strength athletes often land in "overweight" with little body fat.
- Age and sex are ignored. Body composition changes over life, and women naturally carry more fat than men at the same BMI.
- Fat distribution is invisible. Fat around the waist carries more risk than fat on the hips. Waist circumference or waist-to-height ratio add that information.
If your BMI is outside the healthy range, or you are unsure what it means for you, talk to a doctor. This tool gives you the number, not medical advice.
How to use the BMI calculator
- 1Choose metric or imperial units.
- 2Enter your height and weight. The result updates as you type.
- 3Read your BMI, its range and the healthy weight range for your height. Copy the result or a link to it.
Frequently asked questions
What is a healthy BMI?
For adults, the World Health Organization counts a BMI from 18.5 to 24.9 as the healthy range. Below 18.5 is underweight, 25 to 29.9 is overweight and 30 or more is obesity. These ranges are the same for men and women.
Is BMI accurate for everyone?
No. BMI only compares weight with height, so it cannot tell muscle from fat. Athletes and very muscular people often get a high BMI without excess body fat, and older adults can have a normal BMI with little muscle. Some health bodies use lower cut-offs for people of Asian descent. Treat BMI as a first indicator, not a diagnosis.
Does BMI work for children and teenagers?
Not in this form. For people under 20, BMI is compared with growth charts for the same age and sex and expressed as a percentile. This calculator is meant for adults.
How is the healthy weight range calculated?
It takes your height squared in meters and multiplies it by the limits of the healthy zone, 18.5 and 24.9. Because the BMI is shown with one decimal, the range covers every weight whose BMI rounds to 18.5 through 24.9.
Is my data sent anywhere?
No. The calculation runs in your browser, nothing is stored or sent. If you copy a share link, your height and weight are part of that link, so share it only where you want.