Weight relative to height
BMI expresses body weight in relation to height using a standardized mathematical formula, making results comparable across adults.
Clinical context
What BMI can tell you, and what it cannot
BMI is useful because it is fast, standardized and inexpensive. It is most useful as a screening signal rather than a complete picture of an individual’s health.
A screening measure
CDC describes BMI as one potential health indicator that should be considered alongside medical history, health behaviors, physical findings and laboratory information.
Not body-fat measurement
BMI cannot distinguish muscle, bone and fat mass. Waist circumference and other clinical measurements can add important context.
Adult classification
BMI categories for adults
For adults age 20 and older, the CDC uses the following BMI categories. Obesity is divided into three classes.
| Category | BMI range | Interpretation |
|---|---|---|
| Underweight | Below 18.5 | Below the CDC healthy-weight category. |
| Healthy weight | 18.5 to <25 | Within the CDC healthy-weight category. |
| Overweight | 25 to <30 | Above the healthy-weight category and below obesity. |
| Class 1 obesity | 30 to <35 | First obesity class. |
| Class 2 obesity | 35 to <40 | Second obesity class. |
| Class 3 obesity | 40 or greater | Highest CDC adult obesity class, also called severe obesity. |
Methodology
How this BMI calculator works
The calculation uses the standard BMI formula. US customary inputs are converted through the equivalent 703-factor equation.
Metric formula
Weight in kilograms divided by height in meters squared.
BMI = weight (kg) / height² (m²)
US customary formula
Weight in pounds divided by height in inches squared, multiplied by 703.
BMI = 703 × weight (lb) / height² (in²)
Evidence & references
Primary health references
The calculator’s adult classification and interpretation language are aligned with public guidance from major health authorities.
Frequently asked questions
About BMI and your result
What is BMI?
Body Mass Index is a calculated measure of weight relative to height. It is commonly used as a quick screening measure in adults.
What is considered a healthy BMI?
For adults age 20 and older, CDC adult categories define 18.5 to less than 25 as the healthy-weight range.
Is BMI accurate for athletes?
BMI can be less informative when a person has unusually high muscle mass because it does not distinguish muscle from body fat.
Does BMI diagnose obesity or disease?
BMI is a screening measure. Clinical assessment can include history, physical examination, laboratory findings and additional measurements.
Should children use this calculator?
No. This calculator is designed for adults age 20 and older. Children and teens require age- and sex-specific BMI interpretation.
Why does the calculator show a healthy weight range?
It applies the adult healthy BMI interval of 18.5 to 24.9 to the height you entered. It is a mathematical reference range, not an individual target or treatment recommendation.
Editorial standard
Updated August 2026
This page distinguishes screening information from medical diagnosis, identifies the population the calculator is intended for, and links directly to the public-health references used for BMI thresholds. It does not provide individualized medical diagnosis or treatment advice.
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