BMI calculator

What is BMI?

Body mass index, or BMI, is a number that compares your weight to your height. Health organizations use it as a quick, low-cost screening tool to sort adults into weight ranges that are linked, on average, to different levels of health risk. It is a starting point for a conversation, not a measure of health on its own.

How BMI is calculated

The formula is your weight divided by the square of your height. In metric units that is weight in kilograms divided by height in meters squared (kg/m²). In imperial units it is weight in pounds divided by height in inches squared, multiplied by 703. This calculator does the arithmetic for you and rounds to one decimal place.

What the ranges mean

For adults, the World Health Organization and the U.S. Centers for Disease Control and Prevention use the same standard ranges:

BMI	Range
Below 18.5	Underweight
18.5 to 24.9	Healthy weight
25.0 to 29.9	Overweight
30.0 and above	Obesity

These are population ranges. Where you land is a prompt to look closer with a provider, not a verdict on your health.

What BMI can't tell you

BMI is useful because it is simple, but that simplicity is also its limit. It is worth being clear-eyed about what the number leaves out.

It does not measure body fat directly, and it cannot tell muscle from fat. A very muscular person can have a high BMI with low body fat, while someone at a healthy BMI can carry more fat than the number suggests. It says nothing about where fat sits on the body, which matters for health. It can be less accurate for older adults, who tend to lose muscle, and it is read differently across some groups: for many people of Asian descent, health risks can rise at a lower BMI, so some guidelines use lower cut-offs. It is not used the same way during pregnancy, and it does not apply to children and teens at all, who are assessed with age- and sex-specific percentiles instead.

Common questions

Is BMI accurate?

BMI is a reasonable screening tool across large groups, but for any one person it can misclassify, because it cannot separate muscle from fat or see where fat is stored. Treat it as one data point among several, alongside things like waist measurement, blood work, and how you feel.

What is a healthy BMI?

For most adults, the healthy-weight range is a BMI of 18.5 to 24.9. That said, a number inside or outside that band is not the whole story, and the right target for you is a conversation to have with a provider.

Does BMI work for athletes?

Often not well. Because muscle is denser than fat, athletes and very muscular people frequently land in the overweight or obesity range by BMI while carrying little body fat. Body-composition measures are more informative for them.

Can I use this for my child?

No. This calculator and these ranges are for adults aged 20 and over. Children and teens are assessed with BMI-for-age percentiles that account for growth, age, and sex, which a pediatric provider can interpret.