Free ToolLIVE

BMI Calculator.

Calculate BMI using metric or US customary units — for adults (20+) and children (2–19) alike. Adults get the standard CDC screening category; children get CDC's age- and sex-specific growth-chart percentile. This is a screening result, not a diagnosis.

Processed locally

Adults & children (2+)

Metric or US units

Calculate a BMI

Browser-only processing

MeasurementsAge, sex, height & weight

Age

Sex

Weight (kg)

Height (cm)

Processed locally in your browser. Values entered here are never transmitted to or stored by Carmedin.

ResultYour BMI and screening category

Adult BMI

kg/m²

Healthy weight range

Weight to healthy range

BMI Prime

Ponderal Index

How it's calculated: BMI = weight (kg) ÷ height (m)². Healthy weight range uses the same 18.5–25 kg/m² boundary applied to your height. BMI Prime is your BMI divided by 25, the upper bound of the normal range — a value of 1.0 sits exactly at that boundary. Ponderal Index uses height cubed instead of squared, which can be a steadier measure for people at the very tall or short end of the height range. None of these are diagnoses — they're standardized screening numbers meant to be discussed with a healthcare professional, not acted on alone.

Reference

Understanding your result

How BMI and BMI-for-age are calculated, classified, and what the categories mean.

Classification (CDC)

Classification	Percentile Range
Underweight	Less than 5th percentile
Healthy Weight	5th to less than 85th percentile
Overweight	85th to less than 95th percentile
Obesity	95th percentile or greater

Source: CDC BMI-for-age growth charts (ages 2–19).

Health Considerations at Higher Percentiles

CDC tracks a higher likelihood of several conditions among children with a BMI in the higher percentile ranges, including elevated blood pressure and cholesterol, type 2 diabetes, breathing difficulties such as asthma or sleep apnea, joint discomfort, and psychosocial effects like teasing or lower self-esteem. As with adults, these are population-level associations, not predictions for any individual child.

Health Considerations at Lower Percentiles

A BMI in the lower percentile ranges can be associated with nutrient deficiencies, slower growth, and reduced immune function in some children. As with adults, a low percentile can sometimes reflect an underlying condition rather than diet alone — this is worth raising with a pediatrician rather than addressing independently.

Limitations of BMI-for-Age

A single measurement is a snapshot, not a trend — the pattern of a child's growth over time, tracked at regular checkups, generally matters more than any one result. Percentile also doesn't account for individual differences in pubertal timing, body composition, or ethnicity. These reference curves come from CDC's 2000 growth charts; for children with a BMI above the 97th percentile, CDC recommends its separate 2022 Extended charts, which use additional reference data and a different statistical method for that range specifically — this tool currently reports "above the 95th percentile" for those cases rather than a more precise extended-range percentile.

Method Used

Percentiles are computed using CDC's published LMS parameters (a Box-Cox power, median, and coefficient of variation, by age in months and sex) for BMI-for-age, weight-for-age, and height-for-age, per CDC/NCHS methodology.

BMI Introduction

Body Mass Index estimates whether a person's weight falls in a typical range for their height. It's a screening measure, not a direct measurement of body fat — a wide range of factors (muscle mass, age, sex, and where fat is distributed) can shift what a given BMI actually means for a given person, but at a population level it remains a widely used first check for whether further evaluation is worth pursuing.

Classification (CDC, with WHO's underweight sub-categories)

Classification	BMI Range (kg/m²)
Underweight	Less than 18.5
— Severe Thinness (WHO)	Less than 16
— Moderate Thinness (WHO)	16 to less than 17
— Mild Thinness (WHO)	17 to less than 18.5
Healthy Weight	18.5 to less than 25
Overweight	25 to less than 30
Class 1 Obesity	30 to less than 35
Class 2 Obesity	35 to less than 40
Class 3 Obesity (Severe Obesity)	40 or greater

Primary categories: CDC. Underweight sub-categories: WHO — these are compatible, not competing, standards: WHO's overweight/obese thresholds match CDC's exactly; WHO simply divides the underweight range into three finer bands.

Health Considerations at Higher BMI

A BMI in the overweight or obese range is statistically associated with a higher likelihood of several conditions CDC tracks at a population level, including high blood pressure, elevated LDL cholesterol, type 2 diabetes, coronary heart disease, stroke, gallbladder disease, osteoarthritis, sleep apnea, certain cancers, and reduced quality of life. These are population-level associations, not individual predictions — many people with a higher BMI never develop these conditions, and many with a "healthy" BMI do.

Health Considerations at Lower BMI

A BMI below the healthy range carries its own associated considerations: nutrient deficiencies, reduced bone density, weakened immune response, and for some, disrupted menstrual cycles or increased risk during early pregnancy. In some cases a low BMI reflects an underlying condition rather than diet alone — this is worth raising with a healthcare professional rather than addressing through calorie changes on your own.

Limitations of BMI

BMI measures weight relative to height — it cannot distinguish muscle from fat. A muscular, highly-trained individual can show an "overweight" BMI while carrying very little body fat; conversely, an inactive older adult can have a "healthy" BMI while carrying more fat than the number suggests. Sex, age, and ethnicity also shift how well BMI tracks actual body composition. It remains a useful, low-cost first screen — just not the final word for any individual.

Formulas Used

BMI = weight (kg) ÷ height (m)²
BMI Prime = BMI ÷ 25
Ponderal Index = weight (kg) ÷ height (m)³

Beyond a screening number

We help healthcare organizations build the systems behind population health data.

From clinical data pipelines to reporting dashboards, tools like this are one small piece of a much larger health informatics picture. That's what we do.

Laboratory & Clinical Informatics →More Free Tools