How this BMI calculator works
Body Mass Index (BMI) is a simple measure that relates your weight to your height. Enter your age, sex, height, and weight in either US (feet/inches, pounds) or metric (cm, kg) units to get your BMI.
BMI = weight (kg) ÷ height (m)²
For adults age 20 and older, the result is categorized using the standard adult BMI chart (Underweight, Healthy weight, Overweight, Obesity) from the World Health Organization and the National Institutes of Health.
For ages 2 through 19, this calculator instead reports a BMI-for-age percentile using the CDC's 2000 growth charts (the LMS method), since healthy BMI varies by age and sex during childhood and adolescence. A percentile below the 5th is Underweight, 5th up to the 85th is Healthy weight, 85th up to the 95th is Overweight, and 95th or above is Obesity. The same charts are also used to report weight-for-age and height-for-age percentiles for that age group.
What the result includes
- BMI and its category for the entered age, sex, height, and weight.
- BMI Prime (adults): BMI divided by the upper limit of the healthy range (25), where a value under 1.0 means a healthy weight for height.
- Healthy BMI range and the equivalent healthy weight for height, in the units you entered.
- Ponderal Index (weight ÷ height³), an alternative to BMI that scales better for very short or very tall people.
A note on limitations
BMI is a quick screening tool, not a diagnosis. It doesn't directly measure body fat and doesn't account for muscle mass, bone density, or body composition, so it can misclassify very muscular people or lose accuracy in older adults. Use it alongside other measurements and talk to a healthcare provider for a full picture of your health.
Frequently asked questions
What is a healthy BMI?
For adults, a BMI from 18.5 to 24.9 is considered a healthy weight, under 18.5 is underweight, 25 to 29.9 is overweight, and 30 or higher is obese. For children and teens, "healthy weight" is a BMI-for-age percentile from the 5th to the 85th percentile rather than a fixed number, since healthy BMI changes as kids grow.
Why does this BMI calculator ask for age for children?
A BMI that's healthy for a 6-year-old can be underweight or overweight for a 16-year-old, so the CDC evaluates children and teens (ages 2–19) against a percentile curve for their exact age and sex rather than the fixed adult cutoffs.
Does BMI account for muscle mass?
No. BMI is calculated from height and weight alone, so very muscular people can show a higher BMI without having excess body fat. Body fat percentage or waist circumference are better measures in that case.
What is BMI Prime?
BMI Prime is your BMI divided by 25 (the upper bound of the healthy adult range). A BMI Prime under 1.0 means your BMI falls within the healthy range for your height.
What's changed
- Age 20 now uses the adult BMI categories instead of the child and teen percentiles, matching the CDC.
- Launched.
References
- CDC — Adult BMI Calculator and Categories
the adult BMI categories used here: underweight below 18.5, healthy weight 18.5 to 24.9, overweight 25 to 29.9, and obesity 30 or higher
- CDC — Child and Teen BMI Calculator
the BMI-for-age percentile categories for children and teens: underweight below the 5th percentile, healthy weight from the 5th to below the 85th, overweight from the 85th to below the 95th, and obesity at the 95th or above
- CDC — Growth Chart Percentile Data Files (LMS parameters)
the L, M, and S values by sex and month of age for ages 2 to 20 that this calculator uses to compute the BMI-for-age, weight-for-age, and height-for-age percentiles
- NIH / NHLBI — Body Mass Index Calculator
the same adult BMI categories, calculated from height and weight in either US units (feet, inches, pounds) or metric units (centimeters, kilograms)