Enter your height and weight to get your BMI, healthy weight range, BMI Prime, and Ponderal Index — plus a target/reverse BMI calculator, an optional two-person comparison, and a private BMI trend you can track over time.
Child/teen results show an estimated BMI-for-age percentile — a simplified approximation, not the official CDC/WHO growth chart. See the note below the calculator.
BMI is a screening measure, not a diagnosis — it does not directly measure body fat. These figures describe where a given BMI falls, not a recommended target for any individual.
BMI difference: —
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This shows how each person's own weight would need to change, at their own height, to numerically match the other's BMI — it is a like-for-like comparison, not a recommendation for either person.
| Date | Weight | BMI |
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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.
These fixed thresholds are used for adults aged 20 and over. They are not appropriate for children or teenagers — see the section below.
| 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 |
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.
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 = 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 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:
Because of this, BMI is best treated as one screening input among several — not a stand-alone measure of health.
| 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) |
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.
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.
For most adults, a BMI between 18.5 and 24.9 falls in the "normal weight" range used by major health bodies. BMI is a screening tool, not a diagnosis, so it works best alongside other measures rather than on its own.
BMI equals weight divided by height squared. In metric units it is kilograms divided by meters squared; in US customary units it is 703 times pounds divided by inches squared.
The standard adult BMI formula and category cutoffs are the same for men and women. Body composition still differs by sex on average, which is one reason BMI alone does not fully capture body fat or health risk for every individual.
No. BMI only uses height and weight, so it cannot distinguish muscle mass from fat mass. Very muscular people can show a high BMI while carrying relatively little body fat.
BMI Prime is your BMI divided by the upper boundary of the normal range (25). A BMI Prime of 1.0 sits exactly at that boundary, so a value below 1.0 falls within the normal range and above 1.0 falls above it.
The Ponderal Index is weight divided by height cubed, rather than height squared. Some researchers consider it a better fit for very tall or very short people, since it scales differently with height than BMI does.
Not reliably. Athletes and very muscular individuals often have a higher BMI than their body-fat percentage would suggest, because muscle is denser than fat. Waist circumference or body-fat measurement can be more informative for this group.
The BMI formula itself is identical, but interpreting the result is not — children's body fat changes as they grow, so their BMI is compared to other children of the same age and sex using BMI-for-age percentiles rather than the fixed adult categories.
Standard adult BMI categories are not designed for use during pregnancy, since weight gain during pregnancy is expected and healthy. Prenatal weight guidance should come from a healthcare provider rather than a general BMI calculator.
It depends on height: weight equals 25 multiplied by height in meters squared. Use the "Target BMI" section above with 25 entered as the target to see the exact figure for your own height.
Enter a target BMI in the calculator above and it will show the weight that corresponds to that BMI at your current height, along with the difference from your current weight.
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.