How to Use the BMI Calculator
- Choose Adult or Child/Teen, and select sex.
- Pick metric or US units, then enter height and weight.
- Read your BMI, category, healthy weight range, BMI Prime, and Ponderal Index.
- Optionally enter a target BMI to see the matching weight at your height, or turn on comparison to see a second person's BMI alongside yours.
What Is BMI?
Body Mass Index (BMI) is a simple ratio of weight to height, used as a population-level screening tool to flag whether someone's weight may fall outside a typical range for their height. It was developed as a quick statistical measure, not as a direct measurement of body fat or an individual health diagnosis.
Because it only needs height and weight, BMI is inexpensive and easy to calculate consistently, which is why it remains widely used by health organizations worldwide as a first-pass screening measure — usually alongside other information such as waist circumference, activity level, and clinical history.
BMI Formula
- Metric: BMI = weight (kg) ÷ height (m)²
- US customary: BMI = 703 × weight (lb) ÷ height (in)²
BMI Categories for Adults
These fixed thresholds are used for adults aged 20 and over. They are not appropriate for children or teenagers.
| BMI (kg/m²) | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Normal weight |
| 25.0 – 29.9 | Overweight |
| 30.0 – 34.9 | Obese — Class I |
| 35.0 – 39.9 | Obese — Class II |
| 40.0 and above | Obese — Class III |
Healthy Weight Range, Based on Height
The calculator's "healthy weight range" is simply the weight that produces a BMI of 18.5 at the low end and 24.9 at the high end, for the height you entered. Rearranging the BMI formula gives: weight = BMI × height (m)².
This range describes where the standard adult BMI band falls for a given height — it is not a personalized target, since factors like frame size, muscle mass, and overall health are not captured by height and weight alone.
BMI for Children and Teenagers
The BMI formula is the same for children as for adults, but the fixed adult categories (underweight, normal, overweight, obese) do not apply. Children's body fat naturally changes as they grow and differs by age and sex, so pediatric BMI is interpreted using BMI-for-age percentiles — comparing a child's BMI to a reference population of children the same age and sex, rather than to a fixed number.
The percentile shown in this calculator's Child/Teen mode is a simplified statistical estimate, built to approximate the general shape of standard growth curves. It is not the official CDC or WHO growth-chart calculation, which uses detailed reference tables (LMS parameters) published by those organizations.
For anything clinically important — tracking a child's growth, discussing weight with a pediatrician, or making decisions about a child's health — use the official CDC or WHO BMI-for-age growth charts, or consult a pediatrician directly.
BMI Prime and Ponderal Index
BMI Prime = BMI ÷ 25. It expresses BMI as a ratio to the upper boundary of the normal range, so a BMI Prime under 1.0 sits within the standard normal range and a value over 1.0 sits above it — useful for quickly seeing how far above or below that boundary someone falls.
Ponderal Index = weight ÷ height³ (using height in meters). Because it divides by height cubed rather than height squared, some researchers find it scales more consistently across a wider range of heights than BMI does, particularly at the extremes.
BMI Limitations
BMI does not distinguish muscle, bone, or fat mass, and it does not describe where fat is distributed on the body — both of which matter for health. Common groups where BMI can be misleading include:
- Muscular people and athletes — higher muscle mass can push BMI up without a corresponding increase in body fat.
- Older adults — muscle mass typically decreases with age, so BMI can understate body-fat percentage in older people.
- Pregnant people — expected, healthy weight gain during pregnancy makes standard adult BMI categories inappropriate.
- Children and teenagers — require age- and sex-specific percentiles rather than fixed adult thresholds.
- Differences in body composition generally — two people with the same BMI can have very different amounts of fat, muscle, and fat distribution.
Because of this, BMI is best treated as one screening input among several — not a stand-alone measure of health.
BMI vs. Body Fat Percentage
| Measure | What it captures | Main limitation |
|---|---|---|
| BMI | Weight relative to height | Can't tell muscle from fat |
| Body Fat % | Proportion of body weight that is fat | Needs specialized measurement (skinfold, DEXA, bioimpedance) |
BMI vs. Waist Circumference
Waist circumference adds information BMI cannot provide: where fat is carried on the body. Fat concentrated around the waist is associated with different health considerations than fat distributed elsewhere, even at the same BMI — which is why some health guidance pairs a BMI reading with a waist-circumference measurement rather than relying on BMI alone.
How to Interpret a BMI Result
A BMI outside the 18.5–24.9 range means the calculated value sits above or below the standard adult reference band — it does not, by itself, diagnose a health condition or prescribe a course of action. If you have questions about your own weight, body composition, or health, a doctor, dietitian, or other qualified healthcare provider can take your full history and circumstances into account in a way a formula cannot.
This tool is for general educational and screening purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. BMI does not directly measure body fat and does not account for muscle mass, bone density, fat distribution, age, sex differences, or pregnancy in every case. The child/teen percentile shown is an approximation, not the official CDC/WHO calculation. Always seek the advice of a physician or other qualified health provider with any questions regarding your health or a child's growth.