What BMI Measures
Body mass index, or BMI, is a calculation based on a person's weight relative to height. For adults, it is commonly used as a quick screening measure because it requires only two measurements and can help identify weight ranges that may warrant a broader health assessment.
Adult BMI Categories
| BMI | Standard adult category |
|---|
| Below 18.5 | Underweight |
| 18.5–24.9 | Healthy weight |
| 25.0–29.9 | Overweight |
| 30.0 or higher | Obesity |
How BMI Is Calculated
With metric measurements, BMI equals weight in kilograms divided by height in meters squared. With U.S. measurements, BMI equals weight in pounds divided by height in inches squared, multiplied by 703.
Why Height Matters So Much
Height is squared in the BMI formula. Two adults at the same weight can therefore have very different BMI values if their heights differ. Likewise, the weight interval corresponding to a given BMI range changes with height.
Important Limitations of BMI
BMI does not directly measure body fat. It also does not distinguish fat mass from muscle, bone, or other lean tissue. A muscular person can have a relatively high BMI without having the same body composition as another person with the identical BMI.
BMI also does not describe where body fat is carried, fitness, blood pressure, blood glucose, cholesterol, nutrition, mobility, or many other factors that can matter to health. For an individual, BMI is best understood as one screening measure rather than a complete health assessment.
Adults vs. Children and Teens
The standard adult category table on this page is intended for adults age 20 and older. BMI for children and teens is interpreted differently using age- and sex-specific growth charts and percentiles. This calculator should not be used to classify a child or teenager.
Pregnancy
Standard adult BMI interpretation is not designed to evaluate expected weight changes during pregnancy. Pregnancy-related weight questions should be discussed with an appropriate healthcare professional.
BMI Calculation Examples
These examples show the arithmetic and are not recommendations about what any individual should weigh.
| Example | Calculation | BMI |
|---|
| 150 lb, 5 ft 5 in | 150 ÷ 65² × 703 | About 25.0 |
| 180 lb, 5 ft 10 in | 180 ÷ 70² × 703 | About 25.8 |
| 70 kg, 1.75 m | 70 ÷ 1.75² | About 22.9 |
Frequently Asked Questions
Is BMI a diagnosis?
No. BMI is a screening measure. A BMI number by itself cannot diagnose a health condition or describe an individual's overall health.
Does BMI measure body fat?
No. BMI is calculated from height and weight. It does not directly measure body-fat percentage or distinguish fat from muscle.
Who are the adult categories for?
The categories shown here are for adults age 20 and older. Children and teens require age- and sex-specific interpretation.
Why does the calculator show a weight range?
The range shows the weights at your entered height that mathematically correspond to BMI values from 18.5 through 24.9. It is informational, not an individualized goal.
Should I make health decisions from BMI alone?
No. If a BMI result raises a health question, it can be discussed alongside other relevant information with a qualified healthcare professional.
Does BMICalculatorsHub store my weight or height?
The calculation runs in your browser. The site does not require an account or ask you to submit your height and weight to us.
About BMICalculatorsHub
BMICalculatorsHub is a free educational calculator designed to make the standard adult BMI calculation transparent and easy to understand. It supports both U.S. and metric measurements and explains what the result can—and cannot—tell you.